The application of chemistry to the development of the principles of agriculture, though it has only of late years attracted general attention, is by no means new. It dates as far back as the period at which agriculture, after remaining for ages almost stationary, received that stimulus which has led to its recent progress. The earliest reference to the composition of vegetables, and the source of their food in an agricultural point of view, we owe to Jethro Tull; and though his conclusions are no doubt often erroneous, as indeed they could scarcely fail to be, in the then imperfect state of our knowledge of vegetable chemistry and physiology, and with his limited acquaintance with what was even then known, yet some of his observations are unquestionably both ingenious and valuable. They were, however, incidental merely, for he attributed the benefits of his improved method of culture to mechanical, and not to chemical principles, and deliberately denies the statement put forth by Van Helmont, and some of the chemists of his own time, that plants derive their food from the air.
While we thus fix the work of Jethro Tull as the earliest in which allusion is made, though only indirectly, to the chemical principles of agriculture, its effect was rather to turn attention from, than direct it to the investigation of the subject; and it was not till the close of the last century that its importance was brought distinctly before the agricultural public by the publication of Lord Dundonald's Treatise on the intimate connexion between Chemistry and Agriculture. Almost simultaneously with its publication appeared the earlier researches of De Saussure, which, extending over a long series of years, have the merit of laying the foundation of all that has been recently done, and of opening up the field which has been since so successfully cultivated. Saussure investigated, in every point of view, and with a care and accuracy which have never been surpassed, the principal phenomena of the life of plants, and directed attention to the important bearings which many of the facts he substantiated had upon the practice of Agriculture. Neither Saussure's investigations nor the work of Lord Dundonald, appear to have excited that attention which they deserved, or to have produced any immediate effects in the progress of agriculture; but a lively interest was excited by a course of lectures on agricultural chemistry given by Sir Humphry Davy, in the year 1812, at the instance of the Board of Agriculture, and afterwards published. These lectures, written with all the elegance and precision which characterised their author's style, brought prominently before the public the results of Saussure's experiments, and contained a number of useful practical suggestions, many of which have been adopted into our everyday practice, and become so thoroughly incorporated with it, that their scientific origin has been altogether forgotten. The interest which Sir Humphry Davy's work awakened was only temporary: it soon died out, and the whole subject lay in abeyance for a considerable number of years. The truth is, that at that time agriculture was not ripe for science, nor science ripe for agriculture. The necessities of a rapidly increasing population had not then begun to compel the agriculturist to use such means as would increase the amount of production to its utmost limit; and that branch of chemistry which treats of the nature and constitution of the various components of animals and vegetables was still entirely uncultivated, and the whole science in a comparatively imperfect state. We cannot be surprised, therefore, that matters remained with but little change during the comparatively long period of nearly thirty years. Indeed, with the exception of the investigation of soils by Schübler, and some other inquiries of minor importance, and which, in this country at least, excited no attention on the part of the agriculturist, nothing was done until the year 1840, when Liebig published his treatise on Chemistry, in its application to Agriculture and Physiology. Few works have ever created a more powerful impression. Written in a lively and attractive style, and essentially a popular work, dealing with scientific truths in a bold and original manner, and impressing, by its earnestness and the importance of its conclusions, it was at once received by the agricultural public, with the full conviction that the application of its principles was to be immediately followed by the production of immensely increased crops, and by a rapid advance in every branch of practical agriculture. The disappointment of those extravagant expectations, which chemists themselves foresaw, and for which they vainly attempted to prepare the agriculturist, was followed by an equally rapid reaction; and those who had embraced Liebig's views, and landed them as the commencement of a new era, but who had absurdly expected an instantaneous effect, changed their opinion, and contended, as strongly as they had before supported, the application of chemistry to agriculture. Taking all things into account, this effect is scarcely to be wondered at, and part of it is even to some extent attributable to Liebig himself; for in place of bringing always prominently before his reader the fact that agricultural chemistry was still in its early youth, and burdened with all the faults and errors of youth, he treated it too much as if it were already perfect in all its parts. Conclusions, ingenious, but founded on insufficient evidence, and sometimes altogether hypothetical, were stated as if they were fully demonstrated scientific truths; and when these proved, as they sometimes did, to be at variance with practice, it is not surprising that they should have produced a feeling of distrust on the part of persons incapable, from an imperfect and still oftener from no knowledge of science, of drawing the line of demarcation, which Liebig frequently omitted to do, between the positive fact and the ingenious hypothesis founded upon it. This omission, which would be of no consequence with scientific men, becomes a source of serious misapprehension in a work addressed to persons unacquainted with science, and who adopt indiscriminately both the facts and the hypotheses of the author. Be this as it may, Liebig's work, though rather too much of a popular character, has had perhaps all the more on that account a very powerful influence in awakening attention to the improvement of agriculture through means of science.
Liebig's work was followed, in the year 1844, by the publication of Boussingault's Économie Rurale, a work which, though it excited at the time infinitely less interest than Liebig's, is really quite as important a contribution to scientific agriculture, and in some respects even surpasses it; for it contains the accumulated results of a large number of investigations, both in the laboratory and the field, by the author himself, which have served to establish a great many important truths. Boussingault possesses the qualification, at present somewhat rare, of combining a knowledge of practical agriculture with extended scientific attainments; and his investigations, which have been made with direct reference to practice, and their results tested in the field, must be considered as the foundation of a large part of our correct knowledge of scientific agriculture.
The same year was marked by an event of equal importance in the history of scientific agriculture, namely, the foundation of the Agricultural Chemistry Association of Scotland, which was instituted through the exertions of a small number of practical farmers, for the purpose of pursuing investigations in agricultural chemistry, and affording to its members assistance in all matters connected with the cultivation of the soil. That institution has formed the model of similar associations in London, Dublin, Belfast, and in Germany, and it is peculiarly creditable to the intelligence, energy, and zeal of the practical farmers of Scotland, that with them commenced a movement, which has already found imitators in so many quarters, and has conferred so many benefits on agriculture. Within the last six or eight years, and mainly owing to the exertions of these associations, great progress has been made in accumulating facts on which to found an accurate knowledge of the principles of agricultural chemistry, and the number of chemists who have devoted themselves to agriculture, has considerably increased, though still greatly less than the exigencies of the subject require. Even now, we are but on the threshold of the subject, and are only, as the result of numerous and laborious investigations, becoming acquainted with the path which may be most advantageously followed in elucidating the applications of chemistry to agriculture. Much still remains to be done, and it behoves the agricultural public to adopt such measures as shall be most likely to advance the study of the principles of their art. What these means are will be afterwards indicated. Meanwhile it admits of no question, that with all the faults and errors of a science still in its infancy, the progress which has been made is sufficiently encouraging to induce practical men to turn their attention towards it.
THE ORGANIC CONSTITUENTS OF PLANTS.
When a plant, or any of its parts, is exposed to a high temperature, it catches fire, burns, and is gradually consumed, until at length there is left behind a quantity of a white earthy matter, which undergoes no further change, however long the heat be continued. The action of heat thus divides the constituents of plants into two great classes, the organic constituents, contained in that part which is volatilised by burning, and the inorganic constituents which are found in the residual white matter or ash. All plants contain both classes of substances, and though their relative proportions vary considerably, the former greatly exceed the latter, and invariably form the principal part of the plant. The organic constituents are four in number:
Carbon. Hydrogen. Nitrogen. Oxygen.
The inorganic constituents, though smaller in amount, are much more numerous, not less than twelve having been observed as essential to the plant, while one or two others have been detected under certain circumstances, although they appear to be only accidentally present, and form no part of its living tissues. Those which have been clearly ascertained to be essential constituents are:
Potash. Silicic Acid. Soda. Phosphoric Acid. Lime. Sulphuric Acid. Magnesia. Chlorine. Peroxide of Iron.
and more rarely:
Manganese. Iodine. Fluorine.
None of these substances occur in the plant in their elementary or uncombined state, but exist as compounds of greater or less complexity formed by the union of two or more of them; which compounds are extremely varied in their properties, and are especially adapted for performing the various functions of the plant.
The Organic Constituents of Plants.—It would be out of place to enter here into full details regarding the properties of the organic elements, which fall to be considered under the head of pure chemistry, but a few words regarding their more important characteristics, are essential to a right understanding of what is to follow.
Carbon, in the pure state, is found only in the diamond. It is left as charcoal in a less pure condition when vegetable and animal substances are heated in close vessels, and is then obtained as a black substance insoluble in water, which burns in the air, and, by its union with oxygen, is converted into a transparent and colourless gas called carbonic acid. Carbon is the largest constituent of plants, and forms, in round numbers, nearly 50 per cent. of their dry substance.
Hydrogen is only met with in nature in combination with other substances, and its principal compound is water, of which it forms one-ninth, the remaining eight-ninths being oxygen. It is separated from water by a well-known process, and then appears as a transparent and colourless gas, remarkable for its lightness. It catches fire in the air, burns with a pale flame, and is converted into water by combining with oxygen. It is a constituent of all plants, and of all their parts, and though in much smaller quantity than carbon, is equally essential. Dry plants rarely contain more than 3 or 6 per cent. of hydrogen, and sometimes considerably less.
Nitrogen, like hydrogen, is a gas, but unlike it, is found in great abundance in an uncombined state. It exists in large quantities in atmospheric air, of which it forms nearly four-fifths, or more correctly 79 per cent., of its volume. When separated from the oxygen with which it is mixed in the air, it forms a transparent gas, which is incombustible and extinguishes flame. It is a remarkably inert substance, and is incapable of directly entering into combination with the other elements, although it can be made to do so by indirect processes, a peculiarity which has very important bearings on many points which we shall afterwards have to consider. Nitrogen is found in plants to the extent of from 1 to 4 per cent.
Oxygen is one of the most widely distributed of all the elements, and, from its powerful affinities, is the most important agent in almost all natural changes. It is found in the air, of which it forms 21 per cent., and in combination with hydrogen and various other substances. When obtained in the pure state it possesses very remarkable properties. All substances burn in it with greater brilliancy than they do in atmospheric air, and its affinity for most of the elements is extremely powerful. It supports the respiration of animals, but only for a short time, for it excites a violent inflammation in the system, which proves fatal after the lapse of an hour or two. It is found in plants, in quantities varying from 30 to 36 per cent.
Now, in order that a plant may grow, all its four organic constituents must be presented to and absorbed by it; and that this absorption may take place, it is essential that they be presented to it in suitable forms. A seed may be planted in pure carbon, and supplied with unlimited quantities of hydrogen, nitrogen, oxygen, and inorganic substances, and it will not germinate; and a plant, under similar circumstances, will not only not increase, but will soon die. These substances cannot then be absorbed when in the elementary state, but when they have entered into certain forms of combination, they acquire the property of being readily taken up, and assimilated by the organs of the plant.
It has been ascertained by numerous experiments, that the forms of combination in which these elements must exist for this purpose, are by no means numerous; and though great difference of opinion formerly existed, it is now generally admitted that the most important compounds are—for the carbon, carbonic acid; for hydrogen, water; for nitrogen, ammonia and nitric acid; and for oxygen, water and carbonic acid. The properties and general chemical relations of these substances are fully treated of in the article Chemistry, and need not be specially discussed here; it will be enough to remind the reader that carbonic acid is a compound of carbon and oxygen, water a compound of hydrogen and oxygen, ammonia of hydrogen and nitrogen, and nitric acid of nitrogen and oxygen; and that all these substances, with the single exception of ammonia, may supply the plant with oxygen as well as with that element of which it is the particular source.
There are only two sources from which these substances can be obtained by the plant, viz. the atmosphere, and the soil, and it is necessary that we should here consider the mode in which they may be obtained from these sources.
The Atmosphere as a source of the organic constituents of Plants.—Up to the middle of the last century the atmosphere was universally considered to be one of the great chemical elements, but at that time suspicions of its complex nature began to be entertained, which were afterwards substantiated by the experiments of Priestley, Rutherford, and other observers. It has been clearly established that the main bulk of the atmosphere is formed of a mechanical mixture of oxygen and nitrogen, along with certain other ingredients which, though in extremely minute proportion, are as essential to it as its larger constituents. It has been shown by many experiments that 100 volumes of air deprived of moisture and minor constituents, contain
| Substance | Volume | |-----------|--------| | Oxygen | 21 | | Nitrogen | 79 |
Though only in mechanical mixture, the proportion of these ingredients is fixed and invariable. Experiments, entirely corresponding in their results, have been made by Humboldt, Gay-Lussac, and Dumas at Paris, by Saussure at Geneva, and by Lewy at Copenhagen; and similar results have also been obtained from air collected by Gay-Lussac during his ascent in a balloon at the height of 21,430 feet, and by Humboldt on the mountain of Antisano in South America at a height of 16,640 feet. In short, under all circumstances, and in all places, the relation subsisting between the oxygen and nitrogen is constant; and though no doubt many local circumstances exist which may tend to modify their proportions, these are so slow and partial in their operations, and are so counterbalanced by others operating in an opposite direction, as to retain a uniform proportion between the main constituents of the atmosphere, and to prevent any undue accumulation of one or other of them at any one point. It is different with the minor constituents, which are liable to have their proportion varied to some extent under different circumstances. Of these minor constituents the most important is carbonic acid.
Carbonic Acid.—The proportion of carbonic acid in the air has been carefully investigated by Saussure. From his experiments it appears that the mean quantity in 10,000 volumes of air, at the village of Chambeisy, near Geneva, amounts to 4·15 volumes; and this quantity was not constant, but varied between 3·15 and 5·75 volumes. The variations in question are dependent on different circumstances. It was found that the quantity was greater during the night than during the day, the mean quantity in the former case being 4·32, in the latter 3·88. The largest quantity found during the night was 5·74, during the day 5·4. Heavy and continued rain had the effect of diminishing the quantity of carbonic acid, by dissolving and carrying it down with it into the soil. Saussure found that in the month of July 1827, during the time when nine millimetres of rain fell, the ave- Agricultural Chemistry.
rage quantity of carbonic acid amounted to 5-18 volumes in 10,000; while in September 1829, when 254 millimetres fell, it was only 3-57. On the other hand, continued frosts, by retaining the atmosphere and soil in a dry state, had an opposite effect. High winds increase the carbonic acid to a small extent. It was also found to be greater over the cultivated lands than over the lake of Geneva; greater at the tops of mountains than at the level of the sea; in towns than in the country. The differences observed in all these cases are but small, but they are beyond the limits of error in the experiment, and have been confirmed by subsequent experimenters.
Ammonia is also an invariable constituent of the atmosphere, but in extremely minute quantity. Its amount appears to vary within wider limits than any of the other components; but it admits of question whether the very variable, and indeed almost conflicting, statements of different observers may not be to some extent dependent on the mode of experimenting, and the care which has been devoted to it. Our observations on this constituent of the air are less numerous than on any other; for though it has long been known that ammonia exists there, it is only recently that it has been ascertained to be invariably present, and that the recognition of its importance has led to the determination of its quantity. Dr Kemp determined the quantity present in 1,000,000 parts of air to be 3-68; Graeger found in the same quantity no more than 0-323; while Fresenius obtained only 0-098 parts by day, and 0-169 by night. The experiments of the latter observer appear to have been conducted with great care; those of Kemp probably give too high a result; but the number of experiments we at present possess is too small to permit us to draw any general conclusions as to the average proportion of ammonia, although they all concur in proving its invariable presence. It is easy to prove that it is present in the air, by collecting the first few drops of a shower, and applying to them the well-known tests for ammonia; but the accurate determination of its quantity is extremely difficult and tedious.
Water.—The air always contains a quantity of watery vapour, which varies greatly at different times and places, and is dependent to a great extent upon the atmospheric temperature, being largest in hot weather and least in cold. It is increased also by the proximity of the sea, of lakes and rivers, evaporation from which moistens the superincumbent air, and is diminished in dry districts. It is deposited on the surface of the earth in the form of rain and dew, and is connected with many very important natural changes. One thousand volumes of air contain, on the average, about eight volumes of watery vapour; but under certain circumstances this quantity may be greatly increased or diminished.
Carburetted Hydrogen.—Gay-Lussac, Humboldt, and Boussingault have shown, that when the whole of the moisture and carbonic acid have been removed from the air, it still contains a small quantity of carbon and hydrogen; and Saussure has rendered it probable that they exist in a state of combination as carburetted hydrogen gas. No definite proof of this position has, however, as yet been adduced, and the function of the compound is entirely unknown. It is possible that the presence of carbon and hydrogen may be due to a small quantity of organic matter; but, whatever be its source, its amount is certainly extremely small.
Nitric Acid is sometimes, but not invariably, found in the atmosphere. It has been detected after thunder-storms, during which it is apparently formed by the electric spark causing the combination of its elements. Its proportion is extremely minute.
Sulphuretted Hydrogen and Phosphoretted Hydrogen.—The proportion of these substances is almost infinitesimal; but they are pretty general constituents of the atmosphere, and are apparently derived from the decomposition of animal and vegetable matters.
From the preceding statements, it is apparent that the atmosphere may prove a source of all the organic constituents of plants; for not only does it contain nitrogen and oxygen in a pure state, but likewise in those forms of combination in which they are most readily absorbed; and it affords also a supply of carbon in the form of carbonic acid. No doubt the quantity of these substances appears trifling, but it is only relatively and not absolutely small; for, if we take into account the enormous mass of air surrounding the globe, it will be at once apparent, that even the minute fraction of ammonia, amounting, according to Fresenius, to less than a ten-millionth of the atmosphere, corresponds to a very large total quantity. It has been found, by a simple calculation, that the atmosphere must weigh in round numbers
\[5,050,000,000,000,000\text{ tons},\]
and it must consequently contain
Carbonic acid, \[3,300,000,000,000\text{ tons},\]
Ammonia, \[50,000,000\text{ tons},\]
quantities sufficiently large to afford an abundant supply of these elements to all the plants on the surface of the earth.
The Soil as a Source of the Organic Constituents of Plants.—When a portion of soil is subjected to heat, it is found that it, like the plant, consists of a combustible and an incombustible part; but while in the plant the incombustible part or ash forms only a small proportion of the whole, the reverse is the case with the soil, which rarely contains more than 5 or 6 per cent. of organic matter, and sometimes much less. The organic matter exists in the state of what has been called humus, a substance which must be considered here as a source of the organic constituents of plants, independently of the general composition of the soil, which will be afterwards discussed.
The term humus is generic, and is applied by chemists to a rather numerous group of substances, very closely allied in their properties, of which several are generally present in all fertile soils. They have been submitted to examination by various chemists, but by none more accurately than by Mulder and Herman, to whom, indeed, we owe almost all the precise information we possess on the subject. The organic matters of the soil may be divided into three great classes; the first, containing those substances which are soluble in water; the second, those which are extracted by means of caustic potash; and the third, those which are insoluble in all menstrua. When a soil is boiled with a solution of caustic potash, a deep brown fluid is obtained, from which acids precipitate a dark brown flocculent substance, consisting of a mixture of at least three different acids, to which the names of humic, ulmic, and geic acids have been applied. The fluid from which they have been precipitated contains two substances, crenic and apocrenic acid, while the soil still retains what has been called insoluble humus.
The chemical characters of all the acids above named are pretty closely allied. They have, however, been divided into three groups, the humic, geic, and crenic groups, which present some differences in properties and composition. They are all compounds of carbon, hydrogen, and oxygen, and are characterised by their affinity for ammonia, which is so great that they are with difficulty obtained free from that substance, and generally exist in the soil in combination with it. They are all products of the decomposition of vegetable matters in the soil, and are produced in succession by the gradual progress of their decay. This decomposition may be easily traced by observing what takes place when a piece of wood, or any other vegetable substance is exposed to air and moisture. It first acquires a dark brown, and finally a black colour, and is then converted into two substances, named ulmic and humin. These are insoluble in alkalies, and are apparently identical with the insoluble humus of the soil. As the decay advances, the products become soluble in alkalies, and then contain humic, ulmic, and geic acids, and finally, by a still further progress, crenic and apocrenic acids are formed by a process of oxidation, which goes on during the decay.
In fact, these substances are representatives of the different stages of decomposition of plants; and that this is actually the source of all the humus compounds, is obvious from the fact that they are only found in the soil itself, that is to say, in the upper foot or two of the earth's surface, and only in those parts of it on which plants grow. Numerous analyses of the different substances already mentioned have been made, and have served to establish a number of minor differences in the composition even of those to which the same name has been applied; and these differences are manifestly attributable to the fact, that, as their production is the result of a gradual decomposition, at no time can it be possible to extract from the soil one pure substance, but only a variable mixture of several. For this reason it is that such discrepancies exist in the statements of the most careful observers. As far as the composition of these substances is concerned, little need be said, as we shall immediately see that it has no very direct bearing upon agricultural questions. It will suffice, therefore, to give the names and chemical formulae of those which have been analysed and described,— - Ulmic acid from long Frisian turf........... C_{16} H_{15} O_{16} - Humic acid from hard turf.................... C_{16} H_{15} O_{15} - Humic acid from arable soil.................. C_{16} H_{16} O_{16} - Humic acid from a pasture field............. C_{16} H_{14} O_{14} - Geic acid...................................... C_{16} H_{15} O_{17} - Apocrenic acid................................. C_{16} H_{12} O_{24} - Crenic acid..................................... C_{16} H_{13} O_{16}
Humus was formerly considered a much more important constituent of the soil than chemists are now inclined to suppose. It was believed to be the exclusive, or at all events the chief source of the organic constituents of plants, and by absorption through the roots to yield to them the greater part of their nutriment. This view is still supported by some chemists and vegetable physiologists, among whom Mulder is the most distinguished; but notwithstanding their authority, there is little doubt that humus is not a direct source of the organic constituents of plants, and is not absorbed as such by their roots; but it is so indirectly, in as far as the decomposition which it is constantly undergoing in the soil yields carbonic acid, which can be absorbed. The older opinion is refuted by many well-ascertained facts. As regards the exclusive origin of the carbon of plants from humus, it is easy to see that this at least cannot be true. The humus, as we have already stated, is itself derived solely from the decomposition of vegetable and animal matters; and if the plants on the earth's surface were to be supported by it alone, the whole of their substance would have to return to the soil as humus, in order to supply the generation which succeeds them. We know, however, that this is not the case; for the respiration of animals, the combustion of fuel, and many other processes, are annually converting a large quantity of these matters into carbonic acid; and if there were no other source of carbon but the humus of the soil, the amount of vegetable life would gradually diminish, and at length become entirely extinct. Schleiden, who has discussed this subject in full, has made an approximate calculation of the total quantity of humus on the earth's surface, and of the quantity of carbon annually converted into carbonic acid by the respiration of man and animals, the combustion of wood for fuel, and other minor processes; and he draws the conclusion that, if there were no other source of carbon except humus, the quantity of that substance existing in the soil would only support vegetation for a period of sixty years. So far from humus being the only source of carbon, it is obvious that a great part of it must be derived from other sources; for Boussingault has shown that cultivated crops carry off, on the average of years, about one ton more carbon than they receive in the manure applied to them, and without any corresponding diminution on the quantity of humus. A still more convincing evidence of the same nature is given by Humboldt. He states that an acre of land, planted with bananas, yields annually about 155,000 pounds weight of fruit, containing about 32,000 pounds, or upwards of 14 tons of carbon; and as this production goes on during a period of twenty years, there must be withdrawn in that time no less than 280 tons of carbon. But the soil on an acre of land weighs, in round numbers, 1000 tons, and supposing it to contain 4 per cent. of humus, the total weight of carbon in it would amount to little more than 20 tons.
It is manifest from these facts that the influence of humus must be very small, and while no one now supposes it to be the sole source of carbon, as was once believed, it has been contended, that there really is a certain, though small, absorption. Numerous facts are, however, at variance even with this opinion. It is found that the conditions which insure the solubility of the humus are by no means the most suitable to vegetation, though we should expect them to be so were humus absorbed. Peat soils, for instance, which contain large quantities of it in solution, so far from being favourable, are positively injurious to most plants. On the other hand, innumerable examples are found of plants growing luxuriantly in soils and places where no humus exists. The sands of the sea-shore, and the most barren rocks, have their vegetation, and the red-hot ashes which are thrown out by active volcanoes, are no sooner cool than a crop of plants springs up on them.
The conclusions to be drawn from these considerations have been further confirmed by the direct experiments of different observers. Boussingault sowed peas, which weighed 15-60 grains, in a soil composed of a mixture of sand and clay, which had been heated red-hot, and consequently contained no humus, and after 99 days growth, during which they had been watered with distilled water, he found the crop to weigh 68-72 grains, so that there had been a fourfold increase. Similar experiments have been made by Salm Horstmar, on oats and rape. He sowed them in a soil which had been previously ignited, and found that they grew readily and arrived at complete maturity. One oat straw grew to a height of three feet, and bore 78 grains; another bore 47; and a third, 28, in all 153. These when dried at 212° weighed 46-302 grains, and the straw 45-6 grains. The most satisfactory experiments, however, are those of Weigman and Polstorff, who found that, provided care were taken to produce an artificial soil without humus, but having the physical characters of a fertile soil, it was possible to obtain a two-hundred-fold produce of barley. They prepared a mixture of six parts of sand, two of chalk, one of white bole, and one of wood charcoal; to this was added a small quantity of felspar, which had been fused up with marble and some soluble salts, so as to imitate as closely as possible the inorganic parts of a soil, and in it they planted twelve barley plants. The plants grew luxuriantly, reaching a height of three feet, and each bearing nine ears; the ears gave 22 pickles each. The grain of the twelve plants weighed 2040 grains, the straw 2449 grains. On the other hand, experiments have been made which show that even when present, humus is not absorbed. The first experiments of this sort we owe to Saussure, who allowed plants of the common bean and the Polygonum Persicaria to grow in solutions of humate of potash, and found a very trifling diminution in the quantity of humic acid present; but the value of his experiments is invalidated by his having omitted to ascertain whether the diminution of humic acid which he observed were really due to absorption by the plant. This omission has been supplied by Weigman and Polstorff. They grew plants of mint (Mentha pulegium) and of Polygonum Persicaria in solutions of humate of potash, and placed beside the glass containing the plant, another perfectly similar, and containing only the solution of humate of potash. The solution, which contained in every 100 grains, 0·148 grains of solid matter, consisting of humate of potash, &c., was found to become gradually paler, and at the end of a month, during which time the plants had increased by 6½ inches, the quantity of solid matter in 100 grains had diminished to 0·132. But the solution contained in the other glass, and in which no plant had grown, had diminished to 0·136, so that the absorption could not have amounted to more than 0·004 grains for every 100 grains of solution employed. This quantity is so small as to be within the limits of error of experiment, and we are consequently entitled to draw the conclusion that humus, even under the most favourable circumstances, is not absorbed by plants.
While it appears, then, that humus is not directly a food of plants, it must not be supposed that it is altogether devoid of importance. The decompositions which it is constantly undergoing in the soil, make it a source of carbonic acid, which may be absorbed by the plants; and it consequently has indirectly an important bearing on their nutrition. Its functions in the soil are also important, but these we leave for future consideration.
Carbonic acid, ammonia, and water, are the great organic foods of plants. But while the plant has afforded to it an inexhaustible supply of the last, the quantities of the two former, both in the atmosphere and the soil, which are available as food, are limited, and insufficient to sustain its life for a prolonged period. It has been shown by Chevandrier, that an acre of land under beech wood accumulates annually about 1650 lbs. of carbon. But the column of air resting upon an acre of land contains only about 15,500 lbs. of carbon, and the soil may be estimated to contain 1 per cent., or 22,400 lbs. per acre, and the whole of this carbon would therefore be removed, both from the air and the soil, in the course of little more than 23 years. But it is a familiar fact, that plants continue to grow with undiminished luxuriance year after year in the same soil, and they do so because neither their carbon nor their nitrogen are permanently absorbed; they are there only for a period, and when the plant has finished its functions and dies, they sooner or later return into their original state. Either the plant decays, in which case its carbon and nitrogen pass more or less rapidly into their original state, or it becomes the food of animals, and by the processes of respiration and secretion, the same change is effected. In this way a sort of balance is sustained; the carbon which at one moment is absorbed by the plant, passes in the next into the tissues of the animal, only to be again expired in that state in which it is fitted to commence again its round of changes.
But while there is thus, as it were, a continuous circulation of these constituents through both plants and animals, there are various changes which tend to diminish the quantities of carbonic acid and ammonia at the earth's surface, carbon being separated from plants and animals under certain circumstances in the elementary state, and the decomposition of nitrogenous matters yielding nitrogen, which is incapable of returning into the state of ammonia except in small quantity and by very circuitous processes. The elements carbon and nitrogen being, as we have already mentioned, incapable of direct absorption by plants, a gradual though slow diminution in the amount of vegetable life would necessarily occur were it not that nature has provided against this by establishing sources of carbonic acid and ammonia so as to sustain their quantity. The most important of these sources is perhaps volcanic action, both ammonia and carbonic acid being evolved from active volcanoes to an extent which may appear trifling when superficially examined, but is really very large. The production of nitric acid during thunder-storms, apparently by the combination of the nitrogen and oxygen of the atmosphere, which either directly or indirectly ministers to the growth of plants, is another mode in which the supply of these substances is sustained, and the small annual loss of the available food of plants counterbalanced.
Source of the Inorganic Constituents of Plants.—The nature of the inorganic constituents of plants, their being solid and fixed substances, sufficiently indicate that there can be but one source from which they may be derived. That source is the soil, which, as we shall afterwards see, contains all these substances in greater or less abundance, and has always been admitted to be the only substance capable of supplying them. The older chemists and physiologists, however, attributed no importance to these substances, and looking to the small quantities in which they are found in plants, imagined that they were there present merely as accidental impurities which had been absorbed from the soil along with the humus, which was at that time considered to be their organic food. This opinion, sufficiently disproved by the constant occurrence of the same substances, and in the same proportions, in the ash of each individual plant, has been further refuted by the experiments of Prince Salm Horstmar, who has established the fact of the origin of all these substances from the soil, and of their importance to vegetation, by experiments upon oats grown on artificial soils, in each of which one inorganic constituent was omitted. He found that, without silica, the grain vegetated, but remained small, pale in colour, and so weak as to be incapable of supporting itself; without lime, it died when it had produced its second leaf; without potash and soda, it grew only to the height of three inches; without magnesia, it was weak and incapable of supporting itself; without phosphoric acid, it was weak but upright; and without sulphuric acid, it was weak though normal in form, but produced no fruit.
Manner in which the Constituents of Plants are Absorbed.
Water.—The absorption of water by plants takes place in great abundance, and is connected with many of the most important phenomena of vegetation. It is absorbed by the roots alone, and passing into the tissues of the plant, a part of it is decomposed, and goes to the formation of certain of its organic compounds; but by far the larger quantity does not remain in the plant, but is again exhaled by the leaves. The extent to which this takes place is very large. Hales found that a sunflower exhaled in twelve hours about 1 lb. 5 oz. of water, but this quantity was liable to considerable variation, being greater in dry, and less in wet weather, and was greatly diminished during the night. Saussure made similar experiments, and found that the quantity of water exhaled by a sunflower amounted to about 220 lbs. in four months. The subject of the exhalation of water by plants has recently been examined with great accuracy by Lawes. His experiments were made by planting single plants of wheat, barley, beans, peas, and clover, in large glass jars capable of holding about 42 lbs. of soil, and covered with glass plates, furnished with a hole in the centre for the passage of the stem of the plant. Water was supplied to the soil at certain intervals, and the jars were carefully weighed. The result of the experiments, continued during a period of 172 days, is given in the following table, which shows the total quantity of water exhaled in grains:
- Wheat ........................................... 113,527 - Barley ............................................ 120,025 - Beans ............................................. 112,231 - Pease .............................................. 109,082 - Clover, cut 23rd June ......................... 55,093
It was found, further, that the exhalation was not uniform, but increased during the period of active growth of the plant, and diminished again when that period was passed. These variations are shown by the subjoined tables, of which the first gives the total exhalation during certain periods, and the second the average daily loss of water during the same periods.
**Table I. — Showing the Number of Grains of Water given off by the Plants during stated divisional Periods of their Growth.**
| Description of Plant | 9 Days | 11 Days | 21 Days | 34 Days | 30 Days | 14 Days | 27 Days | |----------------------|--------|---------|---------|---------|---------|---------|---------| | Wheat | 129 | 1268 | 4,385 | 40,030 | 46,060 | 15,420 | 6235 | | Barley | 129 | 1867 | 12,029 | 37,480 | 45,060 | 17,046 | 6414 | | Beans | 88 | 1854 | 4,846 | 30,110 | 58,950 | 12,626 | 3657 | | Pease | 101 | 1332 | 2,873 | 36,715 | 62,780 | 5,281 | | | Clover | 400 | 1645 | 2,948 | 50,100 | | | |
**Table II. — Showing the average daily Loss of Water (in Grains) by the Plants, within several stated divisional Periods of their Growth.**
| Description of Plant | 9 Days | 11 Days | 21 Days | 34 Days | 30 Days | 14 Days | 27 Days | |----------------------|--------|---------|---------|---------|---------|---------|---------| | Wheat | 14-3 | 409 | 162-4 | 1177-4 | 1535-3 | 1101-4 | 230-9 | | Barley | 14-3 | 602 | 445-5 | 1102-3 | 1502-0 | 1217-6 | 237-5 | | Beans | 9-7 | 59-8 | 179-5 | 885-6 | 1965-0 | 901-8 | 135-4 | | Pease | 11-2 | 42-9 | 106-4 | 1079-8 | 2092-7 | 377-2 | | | Clover | 4-4 | 53-0 | 109-2 | 1473-5 | | | |
Similar experiments were made with the same plants in soils to which certain manures had been added, and with results generally similar. If, now, a calculation be made from these results of the quantity of water exhaled by the plants growing on an acre of land, it will be found greatly to exceed the annual fall of rain; but we know that of all the rain which falls only a small proportion can be absorbed by the plants growing on the soil, for a large quantity is carried off by the rivers, and never reaches their roots. It has been calculated, for instance, that the Thames carries off in this way at least one-third of the annual rain that falls in the district watered by it, and the Rhine nearly four-fifths. The exhalation which takes place to so great an extent must therefore be dependent on the repeated absorption of the same quantity of water, which, after being exhaled, is again deposited on the soil in the form of dew, and passes repeatedly through the plant. This constant percolation of water is of immense importance to the plant, as it forms the channel through which some of its other constituents are carried to it.
**Carbonic Acid.**—While thus the whole of the water which a plant requires is absorbed by its roots, exactly the reverse is the case with carbonic acid, of which only an inconsiderable quantity is so absorbed. It passes into the plant by the leaves, as has been clearly shown by Boussingault, by a very simple experiment. He took a large glass globe having three apertures, through one of which he introduced a branch of a vine, with twenty leaves on it. With one of the side apertures a tube was connected, by means of which the air could be drawn slowly through the globe, and into an apparatus in which its carbonic acid could be accurately determined. He found, in this way, that while the air which entered the globe contained 0·0004 of carbonic acid, that which escaped contained only 0·0001, so that three-fourths of the carbonic acid had been absorbed.
**Ammonia.**—The absorption of ammonia, so far as we at present know, takes place entirely by the roots; and although a quantity of it no doubt exists in the air, which is important to the plant, there is little doubt that even that reaches it through the root, being carried down by the rain, and absorbed in that way. The greater part of the ammonia, being derived from the organic matter of the soil, is undoubtedly absorbed by the roots.
**Inorganic Constituents.**—These are likewise absorbed by the roots; and it is as a solvent for these substances that the large quantity of water that passes through the plants is so important. The inorganic constituents exist in the soil in particular states of combination, in which they are only sparingly soluble; so much so, indeed, that many of them are considered to be almost absolutely insoluble in water. But in the soil their solubility is increased by the presence of carbonic acid, which being absorbed by the water causes it to dissolve, to some extent, substances otherwise insoluble. It is in this way that lime, which occurs in the soil principally as the insoluble carbonate, is dissolved and absorbed. Phosphate of lime is dissolved in water containing carbonic acid, or even common salt in solution; and generally we have some solvent substance always present. The amount of solubility produced by these substances is extremely small; but it is sufficient for the purpose of supplying to the plant as much of its mineral constituents as are required, for the quantity of water which, as we have already seen, passes through a plant is very large when compared with the amount of inorganic matters absorbed. It has been shown by Lawes that about 2000 grains of water pass through a plant for every grain of mineral matter fixed in it, so that there is no difficulty in understanding how the absorption takes place. There is no doubt that the substances, before they can pass into the plant, must be dissolved, experiment having distinctly shown that the spongioles or apertures through which this absorption takes place are too minute to admit even the smallest solid particle.
**The Proximate Constituents of Plants.**
The substances which are absorbed by the plant undergo within it a series of complicated changes, and produce a number of complex organic compounds, of which the mass of the plant is composed. These substances may be divided into three great classes, of widely different properties, composition, and functions.
1st, **The Saccharine and Amylaceous Constituents.**—These substances are compounds of carbon, hydrogen, and oxygen, and all possess a certain degree of similarity in composition. The quantities of hydrogen and oxygen which they contain are always in the proportion to form water, so that they may be considered as compounds of carbon and water; not that it can be asserted that they actually do contain water, as such, for of that we have no evidence, but only that its constituents are there in the proportion to form it.
2nd, **Cellulose.**—This substance forms the fundamental part of all plants. It is the principal constituent of woody fibre, and is found in a state of purity in the fibre of cotton and flax, and in the pith of plants; but in wood it is generally contaminated with another substance, which has been called incrusting matter, as it is deposited in and around the cells, of which the plant is in part composed. Cellulose is insoluble in all menstrua, but when boiled for a long time with sulphuric acid, is converted into a substance called dextrine. Cellulose consists of—
| From pith of Elder-tree | Spongiales of roots | |-------------------------|---------------------| | Carbon | 45-37 | | Hydrogen | 60-4 | | Oxygen | 50-59 |
100-00
It is represented chemically by the formula, \( \text{C}_{24} \text{H}_{21} \text{O}_{21} \), which shows it to be a compound of 24 atoms of carbon with 21 of hydrogen and 21 of oxygen.
**Incrusting matter.**—Of this substance, large quantities enter into the composition of all plants; but of its chemical nature little is known, as it cannot be obtained separate from cellulose. It is, however, of analogous composition, and probably contains hydrogen and oxygen in the proportion to form water.
**Starch.**—When a quantity of the flour of wheat, or of many other seeds, is exposed to a gentle stream of water, there is separated from it a fine white powder, which is common starch. This powder, when examined by the microscope, is found to be composed of minute grains, formed of concentric layers deposited on one another. These grains vary considerably in size and structure in different plants; but in the same plant they are generally so much alike as to admit of their recognition by a practised observer. These grains were formerly believed to be composed of an external coating of a substance insoluble in water, and containing in their interior a soluble kernel. This opinion has, however, been refuted, and distinct evidence been brought to show that the exterior and interior of the globules are identical in chemical properties. If boiled with water, starch dissolves to a thick fluid; and if heated in the dry state, to a temperature of about 390° Fahr., it becomes soluble in cold water. It is distinguished by giving a brilliant blue compound with iodine. Starch contains—
| Carbon | 44-47 | | Hydrogen | 6-28 | | Oxygen | 49-25 |
100-00
and its composition is represented by the formula \( \text{C}_{12} \text{H}_{10} \text{O}_{10} \). It differs, therefore, but little from cellulose in composition, but its chemical functions in the plant are extremely different. It is connected with some of the most important changes which occur in the growing plants, and by a series of remarkable transformations is converted into sugar and other important compounds.
**Lichen Starch** is found in most species of lichens, and is distinguished from common starch by producing a green colour with iodine. Its composition is the same as that of ordinary starch.
**Iodine.**—The species of starch to which this name is given is characterised by its dissolving in boiling water, and giving a white pulverulent deposit in cooling. It is found in the tuber of the dahlia, in the dandelion, and some other plants. Its composition is identical with that of cellulose, and its formula is \( \text{C}_{24} \text{H}_{21} \text{O}_{21} \).
**Gum** is exuded from various plants in the form of a thick fluid, which dries up into a resinous mass. Its composition is the same as that of starch. It differs considerably in its properties when derived from different plants, but in all cases its chemical composition is the same.
**Dextrine.**—When starch is exposed to a heat of about 400°, or when treated with sulphuric acid, or with a substance extracted from malt called diastase, it is converted into dextrine. The same substance may also be obtained from cellulose by a similar treatment. The dextrine so obtained has the same composition as the starch from which it is obtained, but in its properties more nearly resembles gum. It is a highly important constituent of all plants, and may be converted into sugar on the one hand, and into starch on the other.
**Sugar.**—Under the general name of sugar are included four or five different substances which chemists have distinguished. Of these the most important are cane sugar, grape sugar, and an uncrystallisable sugar found in most plants.
**Cane Sugar** is met with in the sugar-cane, the maple, and many other plants. It is extremely soluble in water, and may be obtained in large crystals, as in common sugar-candy. Its composition is—
| Carbon | 42-22 | | Hydrogen | 6-60 | | Oxygen | 51-18 |
100-00
and its chemical formula is \( \text{C}_{12} \text{H}_{11} \text{O}_{11} \).
**Grape Sugar** is met with in the grape, and most other fruits. But it is also produced when starch is boiled for a long time with sulphuric acid, or treated with a large quantity of diastase. It is less soluble in water than cane sugar, and crystallises in small round grains. Its composition, when dried at 284°, is—
| Carbon | 40-00 | | Hydrogen | 6-66 | | Oxygen | 53-34 |
100-00
and its formula is \( \text{C}_{12} \text{H}_{12} \text{O}_{12} \).
The uncrystallisable sugar of plants is closely allied to grape sugar, and so far as we know, its composition is the same.
**Mucilage** is the name applied to the substance existing in linseed, and in many other seeds, and which communicates to them the property of swelling up and becoming gelatinous when treated with water. It is found in a state of considerable purity in gum tragacanth, and some other gums. Its composition is not known with absolute certainty, but it is either \( \text{C}_{24} \text{H}_{19} \text{O}_{19} \) or \( \text{C}_{12} \text{H}_{19} \text{O}_{10} \); and in the latter case it must be identical with starch and gum.
All the substances belonging to this class are obviously very closely related in chemical composition, some of them, indeed, as starch and gum, though easily distinguished by their properties, are identical in constitution, and the others only differ in the quantity of water, or of its elements which they contain. In fact, all these substances may be considered as compounds of carbon and water, and their relations are, perhaps, more distinctly seen when their formula are written so as to show this, as is done in the following table, in which they are all supposed to contain 24 equivalents of carbon, so as to make them comparable with cellulose:
| Grape sugar, \( \text{C}_{12} \text{H}_{12} \text{O}_{12} \) | \( \text{C}_{24} \text{H}_{24} \text{O}_{24} \) | \( \text{C}_{24} + 24 \) | | Cane sugar, \( \text{C}_{12} \text{H}_{11} \text{O}_{11} \) | \( \text{C}_{24} \text{H}_{22} \text{O}_{22} \) | \( \text{C}_{24} + 22 \) | | Cellulose, \( \text{C}_{24} \text{H}_{21} \text{O}_{21} \) | \( \text{C}_{24} \text{H}_{21} \text{O}_{21} \) | \( \text{C}_{24} + 21 \) | | Inulin, \( \text{C}_{24} \text{H}_{22} \text{O}_{21} \) | \( \text{C}_{24} \text{H}_{22} \text{O}_{21} \) | \( \text{C}_{24} + 21 \) | | Starch, \( \text{C}_{24} \text{H}_{21} \text{O}_{19} \) | \( \text{C}_{24} \text{H}_{20} \text{O}_{20} \) | \( \text{C}_{24} + 20 \) | | Dextrine, \( \text{C}_{12} \text{H}_{19} \text{O}_{19} \) | \( \text{C}_{24} \text{H}_{20} \text{O}_{20} \) | \( \text{C}_{24} + 20 \) | | Gum, \( \text{C}_{12} \text{H}_{19} \text{O}_{19} \) | \( \text{C}_{24} \text{H}_{20} \text{O}_{20} \) | \( \text{C}_{24} + 20 \) | | Mucilage, \( \text{C}_{12} \text{H}_{19} \text{O}_{19} \) | \( \text{C}_{24} \text{H}_{20} \text{O}_{20} \) | \( \text{C}_{24} + 20 \) | The differences being thus so slight, it will not be difficult to understand how one of these compounds may pass into the other. Thus, for instance, cellulose has only to absorb an atom of water to become cane sugar, or to lose an atom in order to be converted into starch; changes which, we have every reason to believe, actually do occur in the plant.
**Pectine and Pectic Acid.**—These substances are met with in many fruits and roots, as, for instance, in the apple, the carrot, and the turnip. They differ from the starch group of substances, in containing a larger quantity of oxygen than is required to form water along with their hydrogen; but their exact composition is still uncertain, and they undergo numerous changes during the ripening of the fruit.
2d. Fatty Matters.—The fatty constituents of plants form a rather extensive group of substances all closely allied, but distinguished by minor peculiarities in properties and differences in constitution. Some of them are of very frequent occurrence, but others are almost peculiar to individual plants. They are all compounds of carbon, hydrogen, and oxygen, and are at once distinguished from the preceding class, by containing oxygen in greatly less quantity than is required to form water with their hydrogen. The principal constituents of the fatty matters and oils of plants are three substances, called stearine, margarine, and oleine, the two former solids, the latter a fluid. These substances rarely if ever occur alone, but are mixed together in variable proportions, and the fluidity of the oils is due principally to the quantity of the last which they contain. If a quantity of olive oil be exposed to cold, it is seen partially to congeal; and if it be then pressed, a fluid flows out, and a crystalline solid remains, the former is oleine, though not absolutely pure, and the latter margarine. The separation of these substances involves a variety of troublesome chemical processes; and when it has been effected, it is found that each of them is a compound of a peculiar acid with another substance called glycerine, or the sweet principle of fats. Glycerine, as it exists in the fats, appears to be a compound of $C_3H_2O$, and its properties are the same from whatever source it is obtained. The acids separated from it are known by the names of margaric, stearic, and oleic acids.
**Margaric and Stearic Acids.**—These substances are white crystalline solids insoluble in water, and fusing at a low temperature. They were formerly believed to be different in composition, but the more accurate analyses of later chemists have shown, that they have both the following composition:
- Carbon ........................................... 75·64 - Hydrogen ........................................... 12·71 - Oxygen ............................................. 11·65
100·00
and are both represented by the formula $C_{34}H_{54}O_4$.
**Oleic Acid.**—Under this name two different substances appear to be included. It has been applied generally to the fluid acids of all oils, while it would appear that the drying and non-drying oils actually contain substances of different composition. The acid extracted from olive oil appears to have the formula $C_{34}H_{54}O_4$, while that from linseed oil is $C_{48}H_{80}O_6$, but this is still doubtful.
Other fatty acids have been detected in palm oil, cocoanut oil, &c., &c., but they are of minor importance, and so closely resemble margaric and stearic acids, as to be easily confounded with them, although their composition is undoubtedly different.
**Wax** is a substance closely allied to the fats. It consists of two substances, cerine and myricine, each of which is extremely complex in its composition. The former consists principally of an acid similar to the fatty acids, called cerotic acid, and containing $C_{34}H_{54}O_4$. The latter has the formula $C_{32}H_{52}O_4$. These substances are separated from one another by boiling with alcohol, in which the former is more soluble. The wax found in the leaves of the lilac and other plants appears to consist of myricine, while that extracted from the sugar-cane is said to be different, and to have the formula $C_{48}H_{80}O_2$. It is probable that other plants contain different sorts of wax, but their investigation is still so imperfect, that nothing definite can be said regarding them. Wax and fats appear to be produced in the plant from starch and sugar; at least it is unquestionable that the bee is capable of producing the former from sugar, and we shall afterwards see that a similar change is most probably produced in the plant.
3d. Nitrogenous or Albuminous Constituents of Plants and Animals.—The nitrogenous constituents of plants and animals are so closely allied, both in properties and composition, that they may be most advantageously considered together.
**Albumen.**—Vegetable albumen is found dissolved in the juices of most plants, and is abundant in that of the potato, the turnip, and wheat. In these juices it exists in a soluble state, but if its solution be heated to about 150° it coagulates into a floccy insoluble substance. It is also thrown down by acids and alcohol. Coagulated albumen is soluble in alkalies and in nitric acid. Animal albumen exists in the white of eggs, the serum of blood, and the juice of flesh; and from all these sources is scarcely distinguishable by its properties from vegetable albumen. The composition of both varieties is the same—
| From Wheat | From Potatoes | From Blood | From White of Egg | |------------|---------------|------------|------------------| | Carbon | 53·7 | 53·1 | 53·4 | 53·0 | | Hydrogen | 7·1 | 7·2 | 7·0 | 7·1 | | Nitrogen | 15·6 | ... | 15·5 | 15·6 | | Oxygen | ... | ... | 22·1 | 22·9 | | Sulphur | 23·6 | 0·97 | 1·6 | 1·1 | | Phosphorus | ... | ... | 0·4 | 0·3 |
100·00 100·00 100·00
Closely allied to vegetable albumen is the substance known by the name of glutin, which is obtained by boiling the gluten of wheat with alcohol. It appears to be a sort of coagulated albumen, and its composition is the same as that given above.
**Vegetable Fibrine.**—If a quantity of wheat flour be tied up in a piece of cloth, and kneaded for some time under water, the starch it contains is gradually washed out, and there remains a quantity of a glutinous substance called gluten. When this is boiled with alcohol, glutin above referred to is extracted, and vegetable fibrine is left. It dissolves in dilute potash, and on the addition of acetic acid is deposited in a pure state. Treated with hydrochloric acid, diluted with ten times its weight of water, it swells up into a jelly-like mass. When boiled or preserved for a long time under water, it cannot be distinguished by its properties from coagulated albumen.
**Animal Fibrine** exists in the blood and the muscles, and agrees in all its characters with vegetable fibrine, as is seen by the subjoined analyses—
| Wheat Flour | Blood | Flesh | |-------------|-------|------| | Carbon | 53·1 | 52·5 | 53·3 | | Hydrogen | 7·0 | 6·9 | 7·1 | | Nitrogen | 15·6 | 15·5 | 15·3 | | Oxygen | 23·2 | 24·0 | 23·1 | | Sulphur | 1·1 | 1·1 | 1·2 |
100·00 100·00 100·00 Caseine.—Vegetable caseine is abundantly found in most plants, and is met with in the juice from which albumen has been precipitated by heat, and may be separated from it in flocks by the addition of an acid. It has been obtained for chemical examination, principally from peas and beans, and from the almond and oats. That prepared from the pea has been called legumine, that from almonds emuline, and that from oats arenine; but they are all three identical in their properties, although formerly believed to be different, and distinguished by these names. Vegetable caseine is best obtained by treating peas or beans with hot water, and straining the fluid. On standing, the starch held in suspension is deposited, and the caseine is retained in solution in the alkaline fluid; by the addition of an acid it is precipitated as a thick curd. Caseine is insoluble in water, but dissolves readily in alkalies; its solution is not coagulated by heat, but, on evaporation, becomes covered with a thin pellicle, which is renewed as often as it is removed.
Animal Caseine is the principal constituent of milk, and is obtained by the cautious addition of an acid to skimmed milk, by which it is precipitated as a thick white curd. It is also obtained by the addition of rennet, and the process of curdling milk is simply the coagulation of its caseine. It is soluble in alkalies, and is precipitated from its solution by acids, and in all other respects agrees with vegetable caseine.
The composition of animal caseine has been well ascertained, but considerable doubt still exists as to that of vegetable caseine, owing to the difficulty of obtaining it absolutely pure. The analyses of different chemists give rather discordant results, but we have given those which appear most trustworthy—
| From Pease. | |-------------| | Carbon | 50-6 | | Hydrogen | 6-8 | | Nitrogen | 16-5 | | Oxygen | 23-8 | | Sulphur | 0-5 | | Phosphorus | 2-3 |
100-0
Other results differ considerably from these, and some observers have even obtained as much as eighteen per cent. of nitrogen and fifty-three of carbon.
The composition of animal caseine differs from this principally in the amount of carbon. Its composition is—
| Carbon | 53-6 | | Hydrogen | 7-1 | | Nitrogen | 15-8 | | Oxygen | 22-5 | | Sulphur | 1-0 |
100-0
It will at once manifest that a very close relation subsists between the different substances just described. Indeed, with the exception of vegetable caseine, they may be said all to present the same composition; and, as has been mentioned above, there are analyses of it which would class it completely with the others. While, however, the quantities of carbon, hydrogen, nitrogen, and oxygen are the same, differences exist in the small quantities of sulphur and phosphorus they contain, and which are indubitably essential to them. Much importance has been attributed to these constituents by various chemists, and especially by Mulder, and he has endeavoured to make out that all the albuminous substances are compounds of a substance to which he has given the name of proteine, with different quantities of sulphur and phosphorus. The composition of proteine, according to his newest experiments, is—
| Carbon | 54-0 | | Hydrogen | 7-1 | | Nitrogen | 16-0 | | Oxygen | 21-4 | | Sulphur | 1-5 |
100-0
and is exactly the same from whatever albuminous compound it is obtained. Although the importance of proteine is probably not so great as Mulder supposed, it affords an important illustration of the close similarity of the different substances from which it is obtained. There is every reason to believe that the different albuminous compounds are capable of changing into one another, just as starch and sugar are mutually convertible; and the possibility of this change throws much light on many of the phenomena of nutrition in plants and animals. Indeed, it would seem probable that these compounds are formed from their elements by plants only, and are merely assimilated by animals to produce the nitrogenous constituents they contain.
Diastase is the name applied to a substance existing in malt, and obtained by macerating that substance with cold water, and adding a quantity of alcohol to the fluid, when the diastase is immediately precipitated in white flocks. It is produced during the malting process, and is not found in the unmalted barley. Its chemical composition is unknown, but it is nitrogenous, and is believed to be produced by the decomposition of gluten. If a very small quantity of diastase be mixed with starch suspended in hot water, the starch is found gradually to dissolve, and to pass first into the state of dextrine, then into that of sugar. The change thus effected takes place also in a precisely similar manner in the plant, for diastase is produced during the process of germination of all seeds and tubers, for the purpose of effecting this change, and to fulfil other functions less understood, but no doubt equally important. Diastase is found in the seeds only during the period when the starch they contain is passing into sugar; as soon as that change has taken place, its function is ended, and it disappears.
THE CHANGES WHICH TAKE PLACE IN THE FOOD OF PLANTS DURING THEIR GROWTH.
1st, Changes which occur during germination.—When a seed is placed in the soil under favourable circumstances, it becomes the seat of an important and remarkable series of chemical changes, which result in the production of the young plant. Experiment and observation have shown that heat, moisture, and air, are necessary to the production of these changes. In all instances these three requisites must be combined, with the further addition, that at the earliest period the seed must be protected from the light. The elevation of temperature required for germination is very different with different seeds; some germinate at a few degrees above the freezing point, and others require a tolerably high temperature. The presence of oxygen is also essential, for it has been shown that if seeds are placed in a soil exposed to an atmosphere containing no oxygen, or if they be buried so deep that the air does not reach them, they may lie for an unlimited period without sprouting; but so soon as they are exposed to the air, germination immediately takes place. This phenomenon is frequently observed where earth has been thrown up from a considerable depth, when it is often covered by plants unknown in the neighbourhood, and which have sprung from buried seeds.
When all the necessary conditions are fulfilled, the seed first absorbs moisture, swells up, and sends out a shoot which rises to the surface, and a rootlet which descends, to form the organ by which nourishment is by and by to be absorbed. Until this takes place, however, the young plant derives its whole nutriment from the seed. When the seed begins to swell, oxygen is absorbed, and re-acting upon the gluten of the seed, causes its conversion into diastase. The diastase in its turn acts upon the starch, and converts it first into dextrine, and then into cellulose, which being deposited in the form of organised cells, produces the first little shoot of the plant. The germ continues to derive the whole of its nutriment from the seed until leaves are produced, and during this time the substances laid up in it undergo a series of complicated changes. From the first moment of growth oxygen is absorbed, and carbonic acid evolved, and at the same time water also is formed from the organic constituents of the seed, which gradually diminishes in weight. The amount of this diminution is different with different plants, but is always considerable. Boussingault found that the loss of dry substance in the pea amounted in 26 days to 52 per cent., and in wheat to 57 per cent. in 51 days. Against this, of course, is to be put the weight of the young plant produced, but this is always much less than the loss of weight of the seed, for Saussure found that a horse bean and the plant produced from it weighed, after 16 days, less by 29 per cent. than the seed before germination. The same phenomenon is seen in the process of malting, which is in fact the artificial germination of barley; the malt produced always weighing considerably less than the grain from which it was obtained. It was believed by Saussure, and the older investigators, that the carbonic acid evolved was entirely produced from starch and sugar. These substances, as we have already stated, may be looked upon as compounds of carbon and water, and the carbon was supposed to be simply oxidised to carbonic acid, and its water eliminated. The action cannot, however, be so simple as this, for the woody fibre contains more carbon than the sugar from which it is produced; and we have, moreover, every reason to believe that the nitrogenous substances are also oxidised. In fact, all the constituents of the seed appear to take part in this change, and the process of germination may in some respects be compared to decay or putrefaction, which, like it, is attended by the absorption of oxygen and evolution of carbonic acid; but while in the latter case the residual substances remain in a useless state, in the former they at once become part of a new organism.
Changes which occur during the after-growth of the plant.—So soon as the plant has exhausted the store of materials laid up for it in the seed, it begins to derive its subsistence from the surrounding air, and to absorb carbonic acid, water and ammonia, and to decompose and convert them into the different constituents of its tissues. Each of these substances, of course, undergoes a different series of changes, which we shall separately consider.
Decomposition of Carbonic Acid.—Carbonic acid is absorbed by the leaves and stems of plants, and in their interior is entirely decomposed, its carbon being retained, while its oxygen is again evolved in the gaseous state. For the production of this change, the influence of the sun's rays is essential, and the stronger the light the greater is the amount of oxygen exhaled, and consequently of carbonic acid decomposed. The separation of oxygen is observed only in the green parts of plants; the flowers and roots, and the fruits when approaching ripeness, produce an exactly opposite effect, absorbing oxygen and evolving carbonic acid. The absorption of carbonic acid and escape of oxygen has been proved by numerous direct experiments by Saussure and others, in which both atmospheric air and artificial mixtures containing an increased quantity of carbonic acid have been employed. Saussure allowed seven plants of periwinkle (Vinca minor) to vegetate in an atmosphere containing 7·5 per cent. of carbonic acid for 6 days, during each of which the apparatus was exposed for 6 hours to the sun's rays. The air was analysed both before and after the experiment, and the results obtained were—
| Volume of the Air | Nitrogen | Oxygen | Carbonic Acid | |------------------|----------|--------|---------------| | Before the experiment | 5746 | 4199 | 1116 | | After | 5746 | 4338 | 1408 | | Difference | 0 | +139 | +292 |
There were therefore absorbed 431 volumes of carbonic acid, and 292 volumes of oxygen evolved. Had the whole oxygen of the carbonic acid been evolved, its volume would have been equal to that of the acid, or 431; so that the deficiency of 139 volumes of oxygen must have been retained in the organs of the plant. Similar results have been obtained with other plants, and in all of them it was observed that after the experiment the nitrogen was increased in amount. It might be supposed that this nitrogen had been produced from the nitrogenous constituents of the plant, but Saussure has effectually disproved this, and rendered it probable that it had simply been retained in the interstices of the plant, and dissolved in the water in which they grew. The absorption of carbonic acid takes place only during the day; at night or in the dusk, exactly the opposite occurs, oxygen being absorbed and carbonic acid evolved from all parts of the plant. Saussure found that the oak, the horse-chestnut, and other plants, absorb oxygen and give off carbonic acid in less volumes than the oxygen, while the house-leek, the cactus, and other plants, absorb oxygen without evolving carbonic acid. While this action is exactly the reverse of that occurring during the day, it must not be supposed that it is due to the carbon which has been fixed during the day being again eliminated in absence of light. On the contrary, it appears to be a purely mechanical and not a chemical action. The carbonic acid is absorbed along with the moisture which day and night is passing into the plant by the roots, but in the absence of the sun it passes through the plant and is exhaled by the leaves without undergoing any change.
The absorption of oxygen during the night, however, appears to be a true chemical process; were it mechanical it would not be confined to oxygen alone, but would take place also with the other gases in contact with the plant. Moreover, the absorption is extremely different in different plants, in some scarcely appreciable, in others very abundant. The plants containing volatile oils, which pass readily into resins by absorption of oxygen, and those which contain tannine and other readily oxidisable substances, taking up the largest quantity. Thus the leaves of the Agave americana after 24 hours' exposure in the dark, have absorbed only 0·3 of their volume of oxygen. The leaves of the fir which contain abundance of volatile oil have absorbed 10 times as much, and those of the oak which contain much tannine, 18 times as much oxygen.
In the flowers, both by day and night, there is a constant absorption of oxygen and evolution of carbonic acid. In fact, an active oxidation is going on attended by the evolution of heat, which in the Arum maculatum, and some other plants, is so great as to raise the temperature 10° or 12° above that of the surrounding air.
Decomposition of water in the Plant.—In addition to the function which water performs in the plant, as the solvent of the different substances which form its food, and hence as the medium through which they pass into its organs, it serves also as a direct food, undergoing decomposition, and yielding hydrogen to the organic substances. Its constituents, along with those of the carbonic acid absorbed, undergo a variety of transformations, and form the principal part of the non-nitrogenous constituents. We have already stated that starch, sugar, and the other allied substances may be considered as compounds of carbon with water, and their formation might be conceived to be effected by the carbonic acid losing all its oxygen, and then direct combination ensuing between the residual carbon, and a certain proportion of water. This, of course, would imply that no change takes place in the water, and though probably the simplest view of the case, it is by no means the most probable. It is much more likely that the carbonic acid is only partially decomposed, half its oxygen being removed, and, at the same time the oxygen being separated from a quantity of water, its hydrogen takes the place of the oxygen which has been removed from the carbonic acid. Thus, for instance, sugar may be produced from twelve equivalents of carbonic acid and twelve equivalents of water, twenty-four equivalents of oxygen being eliminated as thus represented:
\[12 \text{ equivalents of Carbonic Acid, } C_{12} O_{12} O_{12}\] \[12 \text{ Water, } H_{12} O_{12}\] \[1 \text{ Sugar, } 24\text{ of Ox. } C_{12} H_{12} O_{12} + O_{24}\]
It cannot, of course, be positively asserted that sugar is really produced exactly in the manner here shown, but there are many facts which point to the probability of its occurring in a somewhat similar method. That water must be decomposed is evident from the fact established by analysis, that the hydrogen of the plant is generally larger than is required to form water with its oxygen, so that this excess at least must be produced by the decomposition of water. The hydrogen of the volatile oils, many of which contain no oxygen, and that of the fats, which contain only a small quantity of oxygen, must manifestly be obtained in a similar manner.
**Decomposition of Ammonia.**—The nitrogenous or albuminous compounds of vegetables must of necessity obtain their nitrogen from the decomposition of ammonia, experiment having distinctly shown that they are incapable of absorbing it in the free state from the atmosphere. It has been clearly ascertained, that the albuminous substances do not contain ammonia, and it is hence apparent that a complete decomposition of that substance must take place in the plant. In fact, carbonic acid, water, and ammonia, must simultaneously take part in these changes, which must of necessity be complicated; so much so, indeed, that in the present state of our knowledge we cannot attempt any distinct explanation of them.
It must be clearly understood, that while such changes as those described manifestly must take place, the explanations of them which have been attempted by various chemists are not to be accepted as determinately established facts; they are at present no more than hypothetical views which have been expressed chiefly with the view of presenting some definite idea to the mind, and are unsupported by absolute proof; they are only inferences drawn from the general bearings of known facts, and not facts themselves. Although, therefore, they are to be received with caution, they have advantages in so far as they present the matter to us in a somewhat more tangible form than the vague general statements which are all that could otherwise be made.
### THE INORGANIC CONSTITUENTS OF PLANTS
The examination of the inorganic constituents or ash of plants has, of late years, formed the subject of a large number of laborious investigations which have served to give us pretty full information on this subject, and to refute several errors at one time prevalent. The proportion in which the inorganic constituents exist in many plants is so small that they were believed by the older chemists to be entirely fortuitous components, which were present merely because they had been dissolved along with the humus, which was then supposed to enter the roots in solution, and to form the chief food of the plant. This supposition which could only be sustained at a time when analysis was imperfect, has been long since disproved and abandoned, and it has been distinctly shown by repeated experiment that not only are these inorganic substances necessary to the plant, but that every one of them, however small its quantity, must be present if it is to grow luxuriantly and arrive at a healthy maturity. The experiments of Prince Salim Horstmar, before alluded to, have established beyond a doubt, that while a seed may germinate, and even grow, to a certain extent, in absence of one or more of the constituents of its ash, it remains sickly and stunted, and is incapable of producing either flower or seed.
While the necessity for a certain quantity of mineral matters is thus certain, it nevertheless appears that their relative and absolute quantities may vary within very wide limits. The total quantity of ash in different plants and parts of plants is extremely different, and the extent of this difference may be best seen from the table given below, which gives the quantity of ash in 100 parts of the different substances in a dry state.
#### Table showing the quantity of inorganic matters in 100 parts of different plants dried at 212 degrees:
| SEEDS | Hemp | 4:14 | |-------|------|------| | Wheat | 1:97 | Gold of Pleasure | 6:05 | | Barley | 2:48 | Rape | 4:41 | | Oats (with husk) | 3:80 | Potato | 14:90 | | Oats (without husk) | 2:06 | Jerusalem Artichoke | 4:40 | | Rye | 2:00 | ENTIRE PLANT | | Millet | 3:60 | Potato | 17:70 | | Rice | 0:37 | Spurry | 10:06 | | Maize | 1:20 | Red Clover | 8:79 | | Pease | 2:88 | White Clover | 8:72 | | Beans | 3:22 | Yellow Clover | 8:56 | | Kidney Beans | 4:09 | Crimson Clover (T. incarnatum) | 10:81 | | Lentils | 2:51 | Tares | 2:60 | Cow Grass (T. medium) | 11:31 | | Buckwheat | 2:13 | Sainfoin | 6:51 | | Linseed | 4:40 | Ryegrass | 6:42 | | Hemp seed | 5:60 | Meadow Foxtail (Alopecurus pratensis) | 7:81 | | Rape seed | 4:35 | Sunflower | 2:95 | Sweet-scented Vernal Grass (Anthoxanthum odoratum) | 6:32 | | Guinea Corn | 1:99 | Grass (Anthoxanthum odoratum) | 6:32 | | Gold of Pleasure | 4:10 | White Mustard | 4:15 | Downy Oat Grass (Arenaria pubescens) | 5:22 | | Black Mustard | 4:31 | Poppy | 6:56 | Bromus erectus | 5:21 | | Horse-chesnut | 2:81 | Bromus mollis | 5:82 | | Grape | 2:76 | Cynosurus cristatus | 6:38 | | Clover | 6:19 | Dactylis glomerata | 5:31 | | Turnip | 3:98 | Festuca duriaucula | 5:42 | | Carrot | 10:03 | Holcus lanatus | 6:37 | | Sainfoin | 5:27 | Hordeum pratense | 5:67 | | Italian Ryegrass | 6:91 | Lolium perenne | 7:54 | | Mangold-Wurzel | 6:58 | Poa annua | 2:83 | | STRAWS AND STEMS | Poa pratensis | 5:94 | | Wheat | 4:54 | Pea | 8:33 | | Barley | 4:99 | Phleum pratense | 5:29 | | Oat | 7:24 | Plantago lanceolata | 8:68 | | Winter Rye | 5:15 | Poterium Sanguisorba | 7:97 | | Summer Rye | 5:78 | Yarrow | 13:45 | | Millet | 8:32 | Rape Kale | 8:00 | | Maize | 3:60 | Cow Cabbage | 10:00 | | Pea | 4:81 | Asparagus | 6:40 | | Bean | 6:59 | Parsley | 1:10 | | Tares | 6:00 | Furze | 3:11 | | Lentil | 5:38 | Chamomile (Anthemis arvensis) | 9:66 | | Buckwheat | 4:50 | Wild Chamomile (Matricaria Chamomilla) | 9:10 | | Hops | 4:42 | Flax straw | 4:25 | AGRICULTURAL CHEMISTRY.
An examination of this table indicates, that though great differences exist in the proportion of ash in different plants, some general relations may be traced. It appears that the grains of the cerealia of all other seeds contain the smallest quantity of ash, and that the proportion is nearly the same in all, and may be stated in round numbers at two per cent. In the leguminous plants (pease, beans, &c.), the quantity is larger, amounting to about three per cent, while in rape-seed, linseed, and the other oily seeds, it reaches four per cent. In the stems and straws less uniformity exists, but if we except a few extreme cases, the quantity of ash in most of them approaches pretty closely to five per cent. Still more diversified are the results obtained from the entire plants; but this diversity is probably much more apparent than real, and must be, in part at least, dependent on the proportion existing between the stem and leaves; for, the leaves, as we observe under the next head, are peculiarly rich in ash, and a leafy plant must, of course, indicate a higher total per-cent-age of ash, although, if stems and leaves were separately examined, they might not show so conspicuous a difference.
The leaves of all parts of plants are richest in ash; the table shows that in some instances, as in the maple, the inorganic constituents exceed one-fourth of the whole of the weight of the dry matter. In other instances, and particularly in the coniferous plants, the quantity is much smaller. The average proportion of ash in the leaves amounts to about thirteen per cent., but where such variations exist, little value is to be attached to an average such as this, except as an indication of their general abundance.
Roots and tubers likewise show some variety, but, with the exception of the turnip and potato, all approach pretty closely to seven per cent.
The wood is that part of the plant which contains the smallest quantity of ash. In one case only does it reach five per cent, while the average scarcely exceeds one per cent, and in the fir the quantity amounts to no more than one six-hundredth of the dry matter. In the bark the quantity is much larger, and may be stated at seven per cent.
We have thus, in round numbers, the following proportions of ash in different parts of plants:
| Parts of Plant | Proportion of Ash | |---------------|------------------| | Wood | 1 | | Seeds | 3 | | Stems and straws | 5 | | Roots and tubers | 7 | | Bark | 7 | | Leaves | 13 |
The differences which exist in the proportion of ash, are much more strikingly seen when we examine different parts of the same plant. In few instances, however, have analyses been sufficiently multiplied to give much information on this point. The oat, the orange tree, and the horse-chesnut, are the only plants in which it has been done. The results obtained with the oat are given in the following table:
| Part of Oat | Mean | |-------------|------| | Grain | 2·14 | | Husk | 6·47 | | Chaff | 16·53| | Leaves | 8·44 | | Upper part of straw | 4·95 | | Middle part of straw | 6·11 | | Lower part of straw | 5·33 |
from which it will be seen that though considerable variations occur, the relative proportions of ash in different parts of the plant are pretty constant.
The proportion of ash which a plant contains varies greatly at different periods of its growth, but the changes which it undergoes seem, so far as we at present know, to be governed by no general laws. It appears, however, generally, that during the period of active growth the quantity of ash is largest. Thus, it has been found that in early spring the wood of the young shoots of the horse-chesnut contains 9·9 per cent. of ash. In autumn this has diminished to 3·4, and the last year's twigs contain only 1·1 per cent., while in the old wood the quantity does not exceed 0·5. Sanssure has also observed that the quantity of ash diminishes in certain plants when the seed has ripened. Thus, he found that the percentages of ash, before flowering, and after seeding, were as follows:
| Plant | Before flowering | With ripe seed | |-----------|-----------------|----------------| | Sunflower | 14·7 | 9·3 | | Wheat | 7·9 | 3·3 | | Maize | 12·2 | 4·6 | On the other hand, the quantity of ash in the leaves of trees increases considerably in autumn, as shown by this table:
| Per-centages of ash in May and September | |-----------------------------------------| | Oak leaves | 5-3 | 5-5 | | Poplar | 6-6 | 9-3 | | Hazel | 6-1 | 7-0 | | Horse-chesnut | 7-2 | 8-6 |
In other cases, the proportion of ash appears to increase as the plant reaches maturity, and this is particularly seen in the oat, of which we have very complete analyses, at different periods of its growth:
**Proportion of Ash in different parts of the Oat at different periods of its growth.**
| Date | Stalks | Leaves | Chaff | Grain with husk | |---------------|--------|--------|-------|-----------------| | 2d July | 7-83 | 11-35 | 4-91 | | | 9th July | 7-80 | 12-20 | 4-36 | | | 16th July | 7-94 | 12-61 | 6-00 | 3-38 | | 23d July | 7-99 | 16-45 | 9-11 | 3-62 | | 30th July | 7-45 | 16-44 | 12-28 | 4-22 | | 5th August | 7-63 | 16-05 | 13-75 | 4-31 | | 13th August | 6-62 | 20-47 | 18-68 | 4-07 | | 20th August | 6-66 | 21-14 | 21-07 | 3-64 | | 27th August | 7-71 | 22-13 | 22-46 | 3-51 | | 3d September | 8-35 | 20-90 | 27-47 | 3-65 |
Here a rapid increase takes place in the quantity of ash in the leaves and chaff. In the stalks it remains nearly uniform at all periods of the growth. In the grain, again, there is a decided diminution; but this diminution is apparent, not real, and is due to the determination of the ash being made on the grain, with its husk, and the rapid increase in weight of the grain, which is poor in ash, while the husk remains nearly unchanged, causes an apparent diminution in its proportion.
The quantity of ash contained in a plant is also dependent upon the nature of the soil on which it grows. Of this an interesting illustration is given in the following table:
**Table of the Composition of the Ash of different Plants in 100 parts.**
Note.—Alumina and oxide of manganese are of so rare occurrence that separate columns have not been introduced for them, but when they occur their quantity is stated in a note at the end of the table.
| Plant | Potash | Soda | Chloride of Potassium | Chloride of Sodium | Lime | Magnesia | Oxide of Iron | Phosphoric Acid | Sulphuric Acid | Carbonic Acid | Silica | |------------------------|--------|------|-----------------------|--------------------|------|----------|--------------|----------------|---------------|---------------|-------| | Wheat, grain | 30-02 | 3-82 | | | 1-15 | 13-39 | 0-91 | 46-79 | | | 3-89 | | straw | 17-98 | 2-47 | | | 7-42 | 1-94 | 0-45 | 2-75 | 3-09 | | 63-89 | | chaff | 9-14 | 1-79 | | | 1-88 | 1-27 | 0-37 | 4-31 | | | 81-22 | | Barley, grain | 21-14 | 5-65 | 1-01 | | 1-65 | 7-26 | 2-13 | 28-53 | 1-91 | | 30-68 | | straw | 11-22 | | | | 2-14 | 5-79 | 2-70 | 1-36 | 7-20 | 1-09 | 68-50 | | Oats, grain | 20-63 | 1-03 | | | 10-28| 7-82 | 3-85 | 50-44 | | | 4-40 | | straw | 19-46 | 1-93 | 2-71 | 4-27 | 7-01 | 3-79 | 1-49 | 5-07 | 3-35 | 1-96 | 49-56 | | chaff | 6-33 | 3-93 | | | 0-24 | 1-95 | 0-38 | 1-58 | 1-04 | 9-61 | 72-85 | | Rye, grain | 33-83 | 0-39 | | | 2-61 | 12-51 | 1-04 | 39-92 | 0-17 | | 9-22 | | straw | 17-29 | 0-30 | 0-60 | | 9-10 | 2-40 | 1-40 | 3-80 | 0-50 | | 64-50 | | Maize, grain | 28-37 | 1-74 | | | trace| 13-60 | 0-47 | 53-69 | | | 1-55 | | stalks and leaves | 35-26 | | | | 2-29 | 10-53 | 5-52 | 2-28 | 8-09 | 5-16 | 2-57 | 27-98 | | Rice, grain | 20-21 | 2-49 | | | 7-18 | 4-26 | 2-12 | 62-23 | | | 1-37 | | Pease (gray), seed | 41-70 | | | | 4-78 | 5-73 | 0-18 | 36-50 | 4-47 | 0-82 | 0-68 | | straw | 21-30 | 4-22 | | | 37-17| 7-17 | 1-07 | 4-65 | 8-08 | 12-48 | 3-23 | | Beans (common field), grain | 51-72 | 0-54 | | | 5-20 | 6-90 | | | 2-72 | 3-05 | 3-42 | 0-42 | | straw | 32-85 | 2-77 | | | 11-54| 19-85 | 2-53 | 0-61 | 0-49 | 1-40 | 25-32 | 2-61 | | Taro, straw | 39-82 | 3-27 | 4-93 | | 20-73| 5-31 | 0-65 | 10-59 | 2-32 | 13-73 | 1-23 | | straw | 31-72 | | | | 7-41 | 4-55 | 15-71 | 1-65 | 10-34 | 4-67 | 20-37 | 3-57 |
1 Oxide of Manganese, 0-42. 2 Oxide of Manganese, 0-22. | Plant | Potash | Soda | Chloride of Potash | Chloride of Sodium | Lime | Magnesium | Oxide of Iron | Phosphate Acid | Sulfate Acid | Carbonic Acid | Silica | |---------------|--------|------|--------------------|-------------------|------|-----------|--------------|---------------|--------------|---------------|-------| | Flax, seed | 34-17 | 1-69 | 0-36 | 8-40 | 13-11| 0-50 | 38-54 | 1-56 | 0-22 | 1-45 | | straw | 21-53 | 3-68 | 9-21 | 21-20 | 4-20 | 5-58 | 7-53 | 3-39 | 15-75 | 7-92 | | Rape, seed | 16-33 | 0-34 | 0-96 | 8-30 | 8-80 | 1-79 | 31-90 | 5-38 | 5-44 | 19-98 | | straw² | 16-63 | 10-57| 2-53 | 21-51 | 2-92 | 1-30 | 4-68 | 3-90 | 23-04 | 11-80 | | Spurry | 26-12 | 1-14 | 8-90 | 14-46 | 8-88 | ... | 10-20 | 1-79 | 27-38 | 1-14 | | Red clover | 25-60 | ... | 9-08 | 6-02 | 21-37| 8-47 | 1-26 | 4-09 | 2-96 | 18-05 | 1-95 | | Cow grass, *Trifolium medium* | 22-78 | ... | 12-30 | 1-86 | 24-42| 8-86 | 1-09 | 4-94 | 2-66 | 20-16 | 1-12 | | Yellow clover | 27-48 | ... | 11-72 | 8-16 | 17-26| 8-39 | 1-40 | ... | 4-82 | 4-31 | 1-76 | | Alsike clover | 29-72 | ... | 6-29 | 1-05 | 26-83| 4-01 | 0-71 | 5-64 | 3-25 | 20-74 | 1-73 | | Lucerne | 27-56 | ... | 11-64 | 1-91 | 20-60| 5-22 | 2-23 | 6-47 | 4-80 | 15-94 | 2-63 | | Anthoxanthum odoratum | 32-03 | ... | 7-03 | 4-90 | 9-21 | 2-63 | 1-18 | 10-09 | 3-39 | 1-26 | 28-35 | | Alopecurus pratensis | 37-03 | ... | 9-50 | ... | 3-90 | 1-28 | 0-47 | 6-25 | 2-16 | 0-65 | 38-75 | | Arena pubescens | 31-21 | ... | 4-05 | 5-66 | 4-72 | 3-17 | 0-72 | 10-82 | 3-37 | ... | 36-28 | | Bromus erectus | 20-33 | ... | 10-63 | 1-33 | 10-38| 4-99 | 0-26 | 7-53 | 5-46 | 0-55 | 38-48 | | Bromus mollis | 30-09 | 0-33 | 3-11 | 6-64 | 2-60 | 0-28 | 9-62 | 4-91 | 9-07 | 33-34 | | Cynosurus cristatus | 24-99 | ... | 11-60 | ... | 10-16| 2-43 | 0-18 | 7-24 | 3-29 | ... | 40-11 | | Daetilis glomerata | 22-52 | ... | 17-56 | 3-09 | 5-82 | 9-92 | 0-59 | 8-60 | 3-62 | 2-69 | 26-65 | | Festuca duriaucula | 31-84 | ... | 8-17 | 0-62 | 10-31| 2-83 | 0-78 | 12-07 | 3-45 | 1-38 | 28-53 | | Holcus lanatus | 34-83 | ... | 3-91 | 6-66 | 8-31 | 3-41 | 0-31 | 8-02 | 4-41 | 1-82 | 28-31 | | Lolium perenne | 24-67 | ... | 13-80 | 7-25 | 9-64 | 2-85 | 0-21 | 8-73 | 5-20 | 0-49 | 27-13 | | Annual ryegrass | 25-99 | 0-87 | 5-11 | 6-82 | 2-59 | 0-28 | 10-67 | 3-45 | ... | 41-79 | | Poa annua | 41-86 | ... | 0-47 | 3-35 | 11-69| 2-44 | 1-57 | 9-11 | 10-18 | 3-29 | 16-03 | | Poa pratensis | 31-17 | ... | 11-25 | 1-31 | 5-63 | 2-71 | 0-28 | 10-02 | 4-26 | 0-40 | 32-03 | | Poa trivialis | 29-40 | ... | 6-90 | ... | 8-80 | 3-22 | 0-29 | 9-13 | 4-47 | 0-29 | 37-50 | | Phleum pratense | 31-09 | ... | 0-70 | 3-24 | 14-94| 5-30 | 0-27 | 11-29 | 4-86 | 4-02 | 31-09 | | Plantago lanceolata | 33-26 | ... | 4-53 | 8-50 | 19-01| 3-51 | 0-90 | 7-08 | 6-11 | 14-40 | 2-57 | | Poterium Sanguisorba | 30-26 | ... | 3-27 | 1-35 | 24-82| 4-21 | 0-86 | 7-51 | 4-84 | 21-72 | 0-83 | | Achillea Millettia | 30-37 | ... | 20-49 | 3-63 | 13-40| 3-01 | 0-21 | 7-13 | 2-44 | 9-36 | 9-92 | | Potato, tuber | 43-18 | 0-09 | 7-92 | 1-80 | 3-17 | 1-17 | 0-86 | 8-61 | 15-24 | 18-29 | 1-94 | | stem | 39-53 | 3-95 | 20-43 | 14-85 | 1-10 | 1-34 | 6-68 | 6-56 | ... | 2-56 | | leaves | 17-27 | ... | 4-95 | 11-37 | 27-69| 7-78 | 4-50 | 13-60 | 6-37 | ... | 6-47 | | Jerusalem Artichokes | 55-89 | ... | 4-88 | ... | 3-34 | 3-30 | 0-45 | 16-99 | 3-77 | 11-80 | 1-52 | | stem | 38-40 | 0-69 | 4-68 | 20-31 | 1-91 | 0-88 | 2-97 | 3-23 | 25-40 | 1-51 | | leaves | 8-81 | 3-72 | 1-82 | 40-15 | 1-85 | 1-14 | 6-61 | 2-21 | 24-31 | 17-25 | | Turnip, seed | 21-91 | 1-23 | ... | 17-46 | 8-74 | 1-95 | 40-17 | 7-10 | 0-82 | 0-67 | | bulb | 23-70 | 14-75| 7-05 | 11-82 | 3-28 | 0-47 | 9-31 | 16-13 | 10-74 | 2-69 | | leaves | 11-56 | 12-43| 12-41 | 28-49 | 2-62 | 3-02 | 4-85 | 10-36 | 6-18 | 8-04 | | Mangold-Wurzel, root | 21-68 | 3-13 | 49-51 | 1-90 | 1-78 | 0-52 | 1-60 | 3-14 | 15-23 | 1-40 | | leaves | 8-34 | 12-21| 37-66 | 8-72 | 9-84 | 1-46 | 5-89 | 6-54 | 6-92 | 2-35 | | Carrot, root | 42-73 | 12-11| 5-64 | ... | 2-29 | 0-51 | 12-31 | 4-26 | 18-00 | 1-11 | | leaves | 17-10 | 4-85 | 3-62 | 24-05 | 0-58 | 3-43 | 6-21 | 5-08 | 23-15 | 11-61 | | Kohl-rabi, bulb | 36-27 | 2-84 | 11-90 | 10-20 | 2-36 | 0-38 | 13-45 | 11-43 | 10-24 | 0-83 | | leaves | 9-31 | ... | 5-99 | 6-66 | 3-81 | 3-62 | 5-50 | 9-43 | 10-63 | 8-97 | 9-57 | | Cow cabbage, head | 40-86 | 2-43 | ... | 15-01 | 2-39 | 0-77 | 12-53 | 7-27 | 16-68 | 1-66 | | stalk | 40-93 | 4-05 | 2-08 | 10-61 | 3-85 | 0-41 | 19-57 | 11-11 | 6-33 | 1-04 | | Poppy seed | 9-10 | ... | 7-15 | 1-94 | 33-36| 9-49 | 0-41 | 31-38 | 1-92 | 3-24 | | leaves | 56-37 | ... | 2-50 | 2-51 | 30-24| 6-47 | 2-14 | 3-28 | 5-09 | 11-40 | | Mustard seed (white) | 25-78 | 0-33 | ... | 19-10 | 5-90 | 0-39 | 44-97 | 2-19 | 1-31 | | Radish root | 21-16 | ... | 1-29 | 7-07 | 8-78 | 3-33 | 1-19 | 41-09 | 7-71 | 8-17 |
A simple inspection of this table leads to various interesting conclusions. It is to be observed that two at least of the constituents of the ash, viz. alumina, and oxide of manganese, occur rarely, and in small quantity, and must be of very little importance. They have indeed been altogether excluded by some chemists from the list of the true constituents of the ashes of plants, and their presence considered as purely fortuitous. Oxide of iron, of which the proportion is also very small, has sometimes been classed along with these substances as a fortuitous component; but it is invariably present, and the experiments of Prince Salm Horstmar leave no doubt that it is essential to the plant. Its function is unknown, but it is an important constituent of the blood of herbivorous animals, and may be present in the plant, less for its own benefit than for that of the animal of which it is destined to become the food.
Soda would appear also to be a comparatively unimportant constituent, being absent in some cases, and found in most plants in but small quantity. It is only in the cruciferous plants (turnip, rape, &c.) that it is found abundantly, and in them it appears to be an indispensable element; but in most other plants it admits of replacement by potash, and this replacement is probably in some instances the result of cultivation. It has at least been found that the proportions of the two alkalies vary greatly in cultivated and uncultivated specimens of the same plant, the proportion of soda being greatest in the uncultivated. This is conspicuously seen in the asparagus plant, which gave the following quantities of alkalies and chlorine:
Wild Cultivated Potash 18-8 50-5 Soda 16-2 traced Chlorine 16-5 8-3
¹ Alumina, 1-02. ² Alumina, 0-63. The soda having almost entirely disappeared in the cultivated plant, while a corresponding increase had taken place in the quantity of potash.
Potash is a most important constituent of plants, and generally forms a considerable proportion of their ash. It is most abundant in the roots and tubers, and sometimes forms more than half of their mineral constituents. It is also abundant in the seeds, while its proportion is small in the straws and stems, and particularly in the chaff, of our common grains. In general, it may be said to constitute a third part of the ash of most plants.
The proportions of lime and magnesia are liable to very great variations. As a general rule, the proportion of the former greatly exceeds that of the latter. An exception to this is found in the cereals, the grains of which are remarkably rich in magnesia; but, generally speaking, the quantity of magnesia is small, and rarely exceeds 4 per cent. Lime is most abundant in the leguminous plants, exceeding, in some instances, 30 per cent. of the ash. In the cereals it is remarkably small.
Chlorine is by no means an invariable constituent of the ash, although it is most commonly met with, and sometimes in considerable quantity. It appears to have some relation to the quantity of soda, and is always largest when that element is most abundant. A reference to the analyses of the wild and cultivated asparagus will render this obvious, and the same conclusions may be drawn from the table of the composition of ashes, where it will be seen that the chlorine exists to a great extent, as chloride of sodium, and conspicuously so in the ash of mangold-wurzel, where it amounts to almost exactly half of the mineral matter.
Sulphuric Acid is an essential constituent of the ash. But it is to be observed that it is in some instances entirely, and in all partially, a product of the combustion to which the plant has been submitted in order to obtain the ash. It is partially derived from the albuminous compounds, which all contain a certain quantity of sulphur, and which, being oxidised to sulphuric acid during the burning, remains in the ash. It is certain, however, that we thus obtain an imperfect estimate of the whole quantity of sulphur which the plant contains in its natural state; and for this reason experiments have been made, by treating the plants with nitric acid, so as effectually to oxidise the whole of their sulphur, and admit of its being accurately determined. From such experiments, the following table, showing the total amount of sulphur contained in 100 parts of different crop plants, has been constructed.
| Plant | Sulphur (parts) | |------------------------|-----------------| | Poa palustris | 0·165 | | Lolium perenne | 0·310 | | Italian Ryegrass | 0·329 | | Trifolium pratense | 0·107 | | repens | 0·099 | | Lucerne | 0·336 | | Vetch | 0·178 | | Potato tuber | 0·082 | | tops | 0·206 | | Carrot, root | 0·092 | | tops | 0·745 | | Mangold-Wurzel, root | 0·058 | | tops | 0·502 | | Swede, root | 0·435 | | tops | 0·458 | | Rape | 0·448 |
Phosphoric Acid is found principally in the seeds of plants in which it amounts to from 30 to 50 per cent. The straws of the cereal plants contain it only in very small proportion, and the stems and leaves of other plants afford intermediate proportions.
Silica is an invariable constituent of the ash. It is only, however, in the grasses that it is abundant, and that principally in their stems. It there contributes to the strength of the straw, and by giving it additional rigidity prevents its being broken or injured by the weight of the ear.
The knowledge of the composition of the ash of plants leads to many important practical deductions. It enables us to explain why some plants will not grow upon particular soils on which others flourish. Thus, for instance, a plant which contains a large quantity of lime, such as the bean or turnip, will not grow in a soil in which that element is deficient, although wheat or barley, which require but little lime, may yield excellent crops. Again, if the soil be deficient in phosphoric acid, those plants only will grow luxuriantly which require but a small quantity of that element. An extension of this principle leads us to the conclusion that even where a soil contains a proper quantity of all its ingredients, the repeated cultivation of a plant which removes a large quantity of any one substance, may, in the course of time, so far reduce its amount, that the soil becomes incapable of any longer producing that plant, but if it be replaced by another which requires but little of the element thus removed, it may again produce an abundant crop. On this principle the rotation of crops is founded, and its success is dependent on the cultivation in successive years of plants which remove preponderating quantities of different substances.
It may be observed by a minute inspection of the table of ashes, that some plants are peculiarly rich in alkalies, others in lime, and others again in silica; and it would, of course, be the object of the farmer to employ, in succession, crops containing these elements in different proportions. Practical experience in this matter has led to conclusions in all respects identical with those of science, and the successive crops of a good rotation always belong to these different classes. It has been attempted to classify different plants under the heads of silica plants, lime plants, and potash plants, and the following table, extracted from Liebig's Agricultural Chemistry, in which the constituents of the ash are grouped under the three heads of salts of potash and soda, lime and magnesia, and silica, gives such a classification as far as it can be done:
| Plant | Salts of Potash and Soda | Salts of Lime and Magnesia | Silica | |------------------------|--------------------------|---------------------------|-------| | Oat straw with seeds | 34·00 | 4·00 | 62·00 | | Wheat straw | 22·50 | 7·20 | 61·50 | | Barley straw with seeds| 19·00 | 25·70 | 55·30 | | Rye straw | 18·65 | 16·52 | 63·89 | | Good hay | 6·00 | 34·00 | 60·00 | | Tobacco | 24·34 | 67·44 | 8·30 | | Pea straw | 27·82 | 68·74 | 7·81 | | Potato plant | 4·20 | 59·40 | 36·40 | | Meadow Clover | 39·20 | 56·00 | 4·90 | | Maize straw | 72·45 | 6·50 | 18·00 | | Turnips | 81·60 | 18·40 | — | | Beet root | 88·00 | 12·00 | — | | Potatoes | 85·81 | 14·19 | — | | Jerusalem Artichoke | 84·30 | 15·70 | — |
The special application of these facts must be left till we come to treat of the subject of the rotation of crops in full, for which a knowledge of the composition of soils is required. It must be manifest that, as the crops which we remove from the soil contain a greater or less amount of inorganic matters, the quantity of these must, under any circumstances, be undergoing diminution. In many cases the soil contains an almost inexhaustible supply of those substances, but in other instances, where the quantity is small, a system of reckless cropping may reduce a soil to a state of absolute sterility. A remarkable illustration of this fact is found in the virgin soils of certain parts of America. The early settlers there reaped from these soils almost unheard-of crops, but, by repeated cultivation, they were soon exhausted and abandoned, new tracts being brought in and cultivated only to be in their turn exhausted and abandoned. The knowledge of the composition of the ash of plants shows us how this exhaustion may be avoided, and indicates the mode in which such soils may be preserved in a fertile state.
THE SOIL.—ITS CHEMICAL AND PHYSICAL CHARACTERS.
No department of agricultural chemistry is surrounded with such difficulties and uncertainties as that relating to the properties of the soil. When chemistry began to be applied to agriculture it was from the determination of the composition of the soil that its principal advantages were anticipated, and it was certainly from it that the most striking results were at first obtained; for when analysis revealed, as it occasionally did, the absence of one or more of the essential constituents of the plant in a barren soil, it indicated at once the cause and the cure of the defect. The expectations naturally formed from the facts then observed have been but very partially fulfilled; for, as our knowledge has advanced, it has become apparent that it is only in rare instances that it is possible satisfactorily to connect together the composition and the properties of a soil, and with each advancement in the accuracy and minuteness of our analysis the difficulties have been rather increased than diminished. It has become more and more obvious that the question of the composition of a soil is one of extreme complexity, and we are now convinced that it will be necessary to commence again almost de novo, and, discarding many of the observations hitherto made, endeavour to determine the fundamental principles on which the fertility of a soil depends. It has been found that while in some instances it is possible to predicate with certainty that a particular soil is barren, in numerous others a barren and a fertile soil may approach so closely in composition that it is scarcely possible to distinguish them from one another; and so much is this the case that the analysis of a soil must, at the present moment, be considered as in many instances of comparatively little practical value. No doubt practical deductions of importance may occasionally be drawn from the careful analysis of a soil, but the great majority of those hitherto made fail to give the desired information. This may be partly owing to the imperfect analyses which have too often been made, but it is certainly mainly due to imperfect knowledge of the chemical conditions requisite for fertility; and until these are clearly known we cannot expect to derive from the analysis of a soil the important conclusions which it ought to, and at some future period certainly will, yield. Under the present circumstances, therefore, we can only detail such limited facts as are at present known, and of these we shall endeavour to give as clear and succinct an account as possible.
Origin of Soils.—The constituents of the soil, like those of the plant, may be divided into the great classes of organic and inorganic. The origin of the former of these we have already discussed. We have pointed out that they are derived from the decay of plants which have already grown upon the soil, and which, in various stages of decomposition, form the numerous class of substances grouped together under the name of humus. The organic substances may therefore be considered as in a manner secondary constituents of the soil, which have been accumulated in it as the consequence of the growth and decay of successive generations of plants, while the primeval soil consisted of inorganic substances only.
The inorganic constituents of the soil are obtained as the result of a succession of chemical changes going on in the rocks which protrude through the surface of the earth. We have only to examine one of these rocks to observe that it is constantly undergoing a series of important changes. Under the influence of air and moisture it is seen to become soft, to disintegrate, and to fall to powder, and is finally washed away by the rains. These actions, minute and trifling as they may at first sight appear, acting throughout many thousand years, are the source of the inorganic matters of all our soils. Geology points to a period at which the earth's surface must have been altogether devoid of soil, and have consisted entirely of hard crystalline rocks, such as granite and trap, by the disintegration of which, slowly proceeding from the creation down to the present time, all the soils which now cover the surface have been produced. But they have been produced as the result of very complicated processes; for these disintegrated rocks being washed away in the form of fine mud, or at least of minute particles, and being deposited at the bottom of the primeval seas, have there hardened into what are called sedimentary rocks, which, being raised above the surface by volcanic action or other great geological forces have been again disintegrated to yield different soils. Thus, then, all soils are directly or indirectly derived from the crystalline rocks, those soils which overlie them being formed immediately by their decomposition, while those which overlie the sedimentary rocks may be traced back through them to the crystalline rocks from which they were originally formed. Such being the case, the composition of a soil must manifestly be dependent on the crystalline rocks from which it is derived. When we inquire into the matter, we find that these crystalline rocks are by no means numerous, and that they are made up of but a small number of different minerals. The great mass of our different rocks is made up of mixtures, in variable proportions, of quartz, felspar, micas, hornblende, augite, and zeolites. With the exception of quartz and augite, these names are, however, representatives of different classes of minerals. There are, for instance, not less than four different sorts of felspar, which have been distinguished by mineralogists by the names of orthoclase, albite oligoclase, and Labrador felspar; and there are, at least, two sorts of mica and hornblende, and many varieties of zeolites. The composition of those different minerals, with the exception of quartz, which is pure silica, is as follows, Thomsonite being given as a general illustration of the zeolites, which is a very numerous family:
| Minerals | Potash Orthoclase | Soda Albite | Lime Soda Oligoclase | Lime Labrador | |-------------------|-------------------|-------------|----------------------|--------------| | Silica | 65-72 | 67-99 | 62-70 | 54-66 | | Alumina | 18-37 | 19-61 | 23-50 | 27-87 | | Peroxide of iron | traces | 0-70 | 0-62 | | | Oxide of manganese| traces | | | | | Lime | 0-34 | 0-66 | 4-60 | 12-01 | | Magnesia | 0-10 | | 0-02 | | | Potash | 14-02 | | 1-05 | | | Soda | 1-25 | 11-12 | 8-00 | 5-46 |
100-00 100-08 100-79 100-00 It is very obvious that soils produced from the disintegration of those minerals must be very different in their composition and fertility. Potash felspar, for instance, must yield a soil of much greater value than albite or Labrador, as it contains abundance of potash which, in the foregoing section, we have seen to be so important a constituent of the ash of plants; in the same way mica ought to produce a valuable soil. On the other hand, Labrador, hornblende, and augite supply lime and magnesia, and in this respect may be said to surpass the other felspars. But the value of these different minerals is not dependent on their chemical constitution alone; the facility with which they disintegrate, and undergo decomposition so as to liberate their constituents in a state in which they may become available to the plant, is an important element of their value, and in this respect very marked differences are observable. The disintegration of these minerals occurs in two different ways: in those rich in the alkalies it depends on their gradual separation into a silicate of alumina, and an alkaline silicate; in those which contain little alkali, and much protoxide of iron it depends on the gradual absorption of oxygen by that substance, which causes the breaking up of the mineral into new compounds. These changes take place with very different degrees of rapidity. In potash and soda felspar they proceed apace, but in Labrador, owing to the absence of alkalies, they are extremely slow.
The changes which take place in felspar may be easily traced through all their stages. We observe, in the first place, that after a certain time the mineral loses its peculiar lustre, acquires a dull and earthy appearance, absorbs water, becomes gradually soft, and at length falls into a more or less white and soft powder, presenting the characters of common clay. The nature of this change will be best seen by the following analysis of the clay produced by the decomposition of felspar, which is employed in the manufacture of porcelain under the name of kaolin,
| Silica | 46·80 | |--------|-------| | Alumina | 36·83 | | Peroxide of Iron | 3·11 | | Carbonate of Lime | 0·55 | | Potash | 0·27 | | Water | 12·44 |
In this instance the decomposition of the felspar had reached its limit, a mere trace of potash being left, but if taken at different stages of the process, variable proportions of that alkali are met with. This decomposition of felspar is the source of the great deposits of clay which are so abundantly distributed over the globe; and it takes place with nearly equal rapidity with potash and soda felspar. It is only in rare instances complete, and the soils produced from it frequently contain a considerable proportion of undecomposed felspar, which continues for a long period to yield a supply of alkalies to the plants which grow on them.
Mica undergoes decomposition with extreme slowness, as is at once illustrated by the fact that its shining scales may frequently be met with entirely unchanged in the soil. Its persistence is dependent on the small quantity of alkaline constituents which it contains; and for this reason it is observed that the magnesian micas undergo decomposition less rapidly than those containing the larger quantity of potash. Eventually, however, both varieties become converted into clay, their magnesia and potash passing gradually into soluble forms. Hornblende and augite, as already mentioned, owe their decomposition to another cause, oxygen is absorbed, the protoxide of iron being converted into peroxide; lime and magnesia being separated, and a ferruginous clay produced. The zeolites are all rapidly decomposable minerals; like the felspars they yield a clay, while lime and alkalies are separated.
It is obvious from what has just been stated that all of these minerals may yield soils, but most of them would be devoid of many essential ingredients, while not one of them would yield either phosphoric acid, sulphuric acid, or chlorine. It has, however, been recently ascertained that certain of these minerals, or at least the rocks formed from them, contain minute, but distinctly appreciable traces of phosphoric acid, although in too small quantity to be detected by ordinary analysis; and small quantities of chlorine and sulphuric acid may also in most instances be found. Still it will be observed that most of these minerals would yield a soil containing only two or three of those substances, which, as we have already learned, are essential to the plant. Thus potash felspar, while it would give abundance of potash, would be but an inefficient source of lime and magnesia; and Labrador, which contains abundance of lime, is altogether deficient in magnesia and potash.
Nature has, however, provided against this difficulty, for she has so arranged it that these minerals rarely occur alone, the rocks which form our great mountain masses being composed of intimate mixtures of two or more of them, and that in such a manner that the deficiencies of the one compensate those of the other. We shall shortly mention the composition of these rocks.
Granite is a mixture of quartz, felspar, and mica in variable proportions. The quality of the soil it yields is dependent on the variety of felspar present, for both orthoclase and albitte occur in it. When the former is the constituent, granite yields soils of tolerable fertility provided their climatic conditions be favourable; but it frequently occurs in high and exposed situations which are unfavourable to the growth of plants. Gneiss is a similar mixture, but characterised by the predominance of mica, and by its banded structure. Owing to the small quantity of felspar which it contains, and the abundance of the difficultly decomposable mica, the soils formed by its disintegration are generally inferior. Mica slate is also a mixture of quartz, felspar, and mica, but consisting almost entirely of the latter ingredient, and consequently presenting an extreme infertility. The position of the granite gneiss and mica slate soils in this country is such that very few of them are of much value; but in warm climates they not unfrequently produce abundant crops of grain. Syenite is a rock similar in composition to granite, but having the mica replaced by hornblende, which by its decomposition yields supplies of lime and magnesia more readily than they can be obtained from the less easily disintegrated mica. For this reason soils produced from the syenitic rocks are frequently possessed of considerable fertility.
The series of rocks of which greenstone is the type, and which are among the most widely distributed, are very different in composition from those already mentioned. They are divisible into two great classes, which have received the names of diorite and dolerite, the former a mixture of albite and hornblende, the latter of augite and Labrador, sometimes with considerable quantities of a sort of oligoclase containing both soda and lime, and of different kinds of zeolitic minerals. Generally speaking, the soils produced from diorite are superior to those from dolerite. The albite which the former contains undergoes a rapid decomposition, and yields abundance of soda along with some potash, which is seldom altogether wanting, while the hornblende supplies both lime and magnesia. Dolerite, when composed entirely of augite and labrador, produces rather inferior soils; but when it contains oligoclase and zeolites, and comes under the head of basalt, its disintegration is the source of soils remarkable for their fertility; for these latter substances undergoing rapid decomposition furnish the plants with abundant supplies of alkalis and lime, while the more slowly decomposing hornblende affords the necessary quantity of magnesia. In addition to these the basaltic rocks are found to contain appreciable quantities of phosphoric acid, so that they are in a condition to yield to the plant almost all its necessary constituents.
The different rocks now mentioned, with a few others of less general distribution, constitute the whole of our great mountain masses; and while their general composition is such as I have stated, they frequently contain disseminated through them quantities of other minerals which, though in trifling quantity, nevertheless add their quota of valuable constituents to the soils. Moreover, the exact composition of the minerals of which the great masses of rocks are composed is liable to some variety. Those which we have taken as illustrations have been selected as typical of the minerals; but it is not uncommon to find albite containing 2 or 3 per cent. of potash, Labrador with but 2 per cent., the remainder being replaced by magnesia, and zeolitic minerals containing several per cent. of potash, the presence of which must of course considerably modify the properties of the soils produced from them. The properties of the soils are also greatly altered by the mechanical influences to which the rocks are exposed. Situated for the most part in elevated positions, they are no sooner disintegrated than they are washed down by the rains. A granite, for instance, as the result of disintegration, has its felspar reduced to an impalpable powder, while its quartz and mica remain, the former entirely, the latter in great part, in the crystalline grains which existed originally in the granite. If such a disintegrated granite remains on the spot, it is easy to see what its composition must be; but if exposed to the action of running water, by which it is washed away from its original site, a process of separation takes place, the heavy grains of quartz are first deposited, then the lighter mica, and lastly the felspar. Thus there may be produced from the same granite soils of very different nature and composition, from a pure and barren sand to a rich clay formed entirely of the debris of the felspar.
The sedimentary rocks are too numerous and too varied in their composition to admit of much detail being given regarding their relations to the soil. Being derived, however, from the crystalline rocks, the observations which we have already made regarding the latter will apply in some sort to the former. In fact, the sedimentary rocks may to all intents and purposes be classed under the head of clays (embracing the different sorts of clay slates, shales, &c.), and sandstone, the former derived from felspathic, the latter from quartzose minerals. To these must be added limestone, which in various forms is one of the most important of the stratified rocks. Each of these forms many subdivisions, partly dependent on chemical differences, and partly on their position in the geological series. The purest clays, such as the fire clay of the coal formation, have manifestly been produced by the thoroughly complete decomposition of felspar, and in them almost nothing but its silica and alumina have been left; such clays yield almost absolutely barren soils. But when the decomposition has not proceeded so far, different sorts of clay slates and shales are produced, which, though of considerable hardness, disintegrate sometimes with great rapidity, and often produce soils of much value. As an illustration of the general composition of such rocks the following analyses of the fire clay of the coal formation, and of transition clay slate are given.
| Transition Clay Slate | Fire Clay | |----------------------|-----------| | Silica | 60-03 | 54-77 | | Alumina | 14-91 | 28-61 | | Peroxide of iron | 8-94 | 4-92 | | Lime | 2-08 | 0-58 | | Magnesia | 4-22 | 1-14 | | Potash | 3-87 | 1-00 | | Soda | | 0-24 | | Carbonic acid | 5-67 | 8-24 | | Water | | | | | 99-72 | 99-50 |
The sandstones require little mention; many of them consist of nearly pure silica, and these produce mere sandy soils, incapable of supporting vegetable life; but in other instances silica is only the principal constituent, and is mixed with a certain proportion of clay, and such sandstones may yield soils of better quality, but they are always light and poor. Where such sandstones occur interstratified with clays, still better soils are produced, the mutual admixture of the disintegrated rocks producing a soil of intermediate properties, and in which the heaviness of the clay is tempered by the lightness of the sandstone.
Limestone is one of the most widely distributed of the stratified rocks, and in different localities occurs of very different composition. Limestones are divided into two classes, common and magnesian; the former a nearly pure carbonate of lime, the latter a mixture of that substance with carbonate of magnesia. But while these are the principal constituents, it is not uncommon to find small quantities of phosphate and sulphate of lime, which, however trifling their proportions, are not unimportant in an agricultural point of view. The following analyses will serve to illustrate the general composition of these two sorts of limestone:
| Common | Magnesian | |--------|-----------| | Mid-Lothian | Sutherland | Sutherland | Dumfries | | Silica | 2-00 | 7-42 | 6-00 | 2-31 | | Peroxide of iron and alumina | 0-45 | 0-76 | 1-57 | 2-00 | | Carbonate of lime | 93-61 | 84-11 | 50-21 | 58-81 | | Carbonate of magnesia | 1-62 | 7-45 | 41-22 | 36-41 | | Phosphate of lime | 0-56 | ... | ... | ... | | Sulphate of lime | 0-92 | ... | ... | 0-10 | | Organic matter | 0-20 | ... | ... | ... | | Water | 0-50 | ... | 0-69 | ... | | | 99-86 | 99-74 | 99-69 | 99-68 |
These are pure limestones; but there occurs yet another sort, which is a mixture of carbonate of lime with variable quantities of clay. Limestone and chalk, when disintegrated, produce light and open soils; but when mixed with clay, they give rise to soils of high fertility. This is parti- cularly the case with chalk; on which are found some of the most valuable of all soils. But it is true only of the common limestones, for experience has shown that those which contain magnesia in large quantity are generally prejudicial to vegetation, and yield barren or at best very inferior soils.
Such are the general characters of the three great classes of stratified rocks; any attempt to particularise the numerous varieties of each would lead us far beyond the limits of the present article. It is necessary, however, to remark, that in many instances the one variety passes into the other, or, more correctly speaking, sedimentary rocks occur, which are, so to speak, mixtures of two or more of the three great classes. Thus we have sandstones which contain much clay, clay slates and shales, which are rich in lime, limestone rocks with a large intermixture of clay. Such mixtures usually produce better soils than either of their constituents separately, and accordingly, in those geological formations in which they are abundant, the soils are generally of excellent quality. The same effect is produced where numerous thin beds of members of the different classes are interstratified, the disintegrated portions being gradually intermixed, and valuable soils formed. It may be stated generally that the soils of the clay slates are for the most part cold, heavy, and very difficult and expensive to work; those of sandstone light and poor, and of limestone generally poor and thin. These statements must, however, be considered as very general; for individual cases occur in which some of these substances may produce good soils. Such is the case with the lower chalk, and with some of the shales of the coal formation. Little is at present known regarding the peculiar nature of these rocks, or their composition; and the cause of the differences in the fertility of the soil produced from them is a subject worthy of minute investigation.
Chemical Composition of the Soil.—We have already referred to the division of the constituents of the soil into the two great classes of organic and inorganic. When treating of the sources of the organic constituents of plants, we entered with some degree of minuteness into the composition and relations of the different members of the former class, and expressed the opinion that they did not admit of being directly absorbed by the plant. As a direct source of these substances, humus is unimportant; but it has other functions to perform which render it an essential constituent of all fertile soils. These functions are dependent on the power which it has of absorbing and entering into chemical composition with ammonia, and with certain of the soluble inorganic substances of the soil. Its effects in this way are strikingly seen in the manner in which ammonia is absorbed by peat soils; it suffices merely to pour upon some dried peat a small quantity of a dilute solution of ammonia to find its smell immediately disappear. This peculiar absorptive power extends also to the fixed alkalies, potash and soda, as well as to lime and magnesia, and has an important effect in preventing these substances being washed out of the soil—a property which, as we shall afterwards see, is possessed also by the clay contained in greater or less quantity in most soils.
In examining into the inorganic constituents of the soil, we find that it is not sufficient merely to inquire into the nature of these substances, but that the states of combination in which they exist is of the very highest importance. Two soils may, for instance, be found on analysis to yield exactly the same results, and yet the one may prove in practice to be fertile, the other barren; and these differences may be entirely dependent on the conditions in which their individual elements exist. To pass into the plant, these substances must be soluble in water, and, unless they are so, it matters not in what quantity the soil contains them; if they are insoluble, they are locked up from use, and the soil is left to hopeless infertility. Accordingly, it must be at once apparent that the determination of the total amount of each of the elements of the soil is not sufficient to establish its value. If the analysis is to be of any use, it ought to indicate also the conditions in which they exist, so that we may ascertain the case or difficulty with which they may be absorbed. For this purpose it is necessary to determine, 1st, The substances soluble in water; 2d, The substances insoluble in water, but soluble in acids; 3d, The substances insoluble both in water and acids. If to these we add the organic constituents, we have four separate heads, under which the components of a soil ought to be classified. This classification is accordingly adopted in the most careful and minute analyses; but the difficulty and labour attending such analyses has hitherto precluded the possibility of making them except in a few instances; and, generally speaking, chemists have been contented with treating the soil with an acid, and determining in the solution all that is dissolved. Such analyses are at times useful for practical purposes, as, for example, when they show the absence of lime, or any other individual substance, by the addition of which we may rectify the deficiency of the soil; but they are of comparatively little scientific value, and throw but little light on the true constitution of the soil, and the sources of its fertility. Nor is it likely that we shall arrive at much satisfactory information until the number of minute analyses is so far extended as to enable us to establish the fundamental principles on which the various properties of the soil depends.
The separation of the constituents of a soil into the four great groups already mentioned is effected in the following manner:—A given quantity of the soil is boiled with three of four successive quantities of water, which dissolves out all the soluble matters. These soluble matters generally amount to about one-half per cent. of the whole soil, and consist of nearly equal proportions of organic and inorganic substances. In very light and sandy soils, it occasionally happens that not more than one or two-tenths per cent. dissolve in water, and in peaty soils, on the other hand, the proportion is sometimes considerably increased, principally owing to the abundance of soluble organic matters.
When the residue of this operation is heated with dilute hydrochloric acid, the portion soluble in acids is obtained in the fluid. This portion of the soil is liable to very great variations. In some soils of excellent quality, and well adapted to the growth of wheat, it does not exceed three per cent., while in calcareous soils, such as those of the chalk formation, it may reach as high as 50 or 60 per cent. In general, however, it amounts to about 10 per cent. The organic constituents are also very variable in amount; ordinary soils of good quality containing from 2 to 10 per cent., while in peat soils it is no uncommon thing for them to exceed 30 per cent., and in some instances to reach as high as 50. Such soils, however, cannot be considered fertile soils. The insoluble constituents are likewise subject to great variations. In the ordinary clay and sandy soils of this country, they generally form from 80 to 90 per cent. of the whole, but they are occasionally as low as 30, especially in such soils as are very rich in lime.
The distribution of the constituents under these different heads will be best illustrated by a few analyses of soils of good quality. The following are the analyses of two noted for the excellent crops of wheat they produce, and for their general fertility. The analyses were made from the upper 10 inches, and a quantity of the 10 inches immediately subjacent was analysed as subsoil. The first is the ordinary wheat soil of the county of Mid-Lothian, the other the alluvial soil of the Carse of Gowrie in Perthshire, so celebrated for the abundance and luxuriance of the crops it produces. ### Agricultural Chemistry
#### Substances soluble in water.
| Substance | Mid-Lothian | Perthshire | |--------------------|-------------|------------| | Silica | 0·0149 | 0·0104 | | Lime | 0·0300 | 0·0072 | | Magnesia | 0·0097 | 0·0016 | | Chloride of magnesium | ... | ... | | Potash | 0·0034 | 0·0037 | | Soda | 0·0065 | 0·0049 | | Chloride of potassium | ... | ... | | Chloride of sodium | 0·0088 | 0·0080 | | Sulphuric acid | 0·0193 | 0·0124 | | Chlorine | trace | trace | | Organic matters | 0·1481 | 0·2228 |
#### Soluble in acids.
| Substance | Mid-Lothian | Perthshire | |--------------------|-------------|------------| | Silica | 0·1490 | 0·0680 | | Peroxide of iron | 5·1730 | 3·4820 | | Alumina | 2·1540 | 1·8130 | | Lime | 0·4470 | 0·3810 | | Magnesia | 0·4120 | 0·2850 | | Potash | 0·0650 | 0·1650 | | Soda | 0·0050 | 0·0560 | | Sulphuric acid | 0·0250 | 0·0850 | | Phosphoric acid | 0·4300 | 0·1970 | | Carbonic acid | ... | 0·0500 |
#### Insoluble in acids.
| Substance | Mid-Lothian | Perthshire | |--------------------|-------------|------------| | Silica | 71·3890 | 82·5090 | | Alumina | 4·7810 | 3·5120 | | Peroxide of iron | trace | trace | | Lime | 0·7520 | 0·5500 | | Magnesia | 0·6610 | 0·5500 | | Potash | 0·2860 | 0·2400 | | Soda | 0·4220 | 1·3100 |
#### Organic matters.
| Substance | Mid-Lothian | Perthshire | |--------------------|-------------|------------| | Insoluble organic matter | 8·8777 | 4·2370 | | Humine | 0·8850 | 0·3450 | | Humic acid | 0·1340 | 0·0310 | | Apocroenic acid | 0·1533 | 0·0520 | | Water | 2·6840 | 1·7670 |
#### Sum of all the constituents.
| Substance | Mid-Lothian | Perthshire | |--------------------|-------------|------------| | Carbon | 4·510 | 1·3060 | | Hydrogen | 0·550 | 0·3324 | | Nitrogen | 0·220 | 0·0973 | | Oxygen | 4·918 | 3·1001 |
#### Amount of carbon, hydrogen, nitrogen, and oxygen contained in 100 parts of each soil.
| Substance | Carbon | Hydrogen | Nitrogen | Oxygen | |--------------------|--------|----------|----------|--------| | | 10·198 | 4·8358 | 8·55 | 6·82 |
From an examination of these analyses, it is apparent that certain of the inorganic constituents of the soil are met with in each of the three heads under which they are arranged, while others are confined to one or two. Silica and the alkalies occur generally, though not invariably in all three. Chlorine is met with only in the part soluble in water, phosphoric acid only in that soluble in acids, while sulphuric acid occurs in both the last-named divisions. The part soluble in water is composed entirely of salts of the alkalies, lime, and magnesia; the acids being sulphuric acid, silica, and chlorine, the latter in very small quantity. All the substances met with in solution are important constituents of the ash of plants. It is different, however, with those soluble in acids, of which the larger proportion consists of alumina and oxide of iron, both of which are comparatively unimportant to the plant, but very important, as we shall afterwards see, in relation to the physical properties of the soil. The remainder of the substances soluble in acids, amounting to from 1 to 2 per cent., is composed of some of the most essential constituents of plants. Lime, magnesia, potash, and soda, appear again in larger quantity than in the soluble part, and along with them we have the phosphoric acid to the amount of from 0·2 to 0·4 per cent. of the whole soil, and sulphuric acid in much smaller quantity. The insoluble matters show a striking difference in the two soils, in regard to the proportion of alkalies they contain, the Mid-Lothian soil showing only 0·28 per cent. of potash, the subsoil none, while the Perthshire contains 2·45 and 2·00 respectively.
As a contrast with these soils, we have the following analysis of a soil from the island of Antigua, from which very large crops of sugar-cane are obtained. The soil is of great depth, and analyses of the subsoil at the depth of 18 inches and 5 feet are given. These last analyses are not so minute as that of the soil itself, the soluble matters not having been separately determined, but included in that soluble in acids.
#### Soluble in water.
| Substance | Surface | 18 inches | 5 feet | |--------------------|---------|-----------|--------| | Lime | 0·07 | | | | Magnesia | trace | | | | Potash | 0·06 | | | | Soda | 0·04 | | | | Chlorine | 0·05 | | | | Organic matter | 0·15 | | |
#### Soluble in acids.
| Substance | 0·74 | | | |--------------------|---------|-----------|--------| | Peroxide of iron | 2·22 | 1·67 | 1·87 | | Protioxide of iron | 0·77 | 0·95 | 3·10 | | Alumina | 1·90 | 2·52 | 4·21 | | Lime | 10·43 | 3·04 | 25·75 | | Magnesia | 0·20 | 0·54 | 0·51 | | Potash | 0·03 | 0·29 | 0·28 | | Soda | 0·02 | 0·11 | 0·16 | | Sulphuric acid | trace | 0·02 | 0·13 | | Phosphoric acid | 0·14 | trace | 0·04 | | Carbonic acid | 7·38 | 0·82 | 20·23 |
#### Insoluble in acids.
| Substance | 23·83 | 18·06 | 56·28 | |--------------------|---------|-----------|--------| | Silica | 41·44 | 51·24 | 27·67 | | Protioxide of iron | 8·24 | 0·26 | 1·40 | | Alumina | 9·00 | 1·50 | 1·00 | | Lime | 0·08 | 0·88 | trace | | Magnesia | 0·80 | 0·54 | trace | | Potash | 0·74 | | | | Soda | 0·25 | | |
#### Organic matters.
| Substance | 54·56 | 55·41 | 30·07 | |--------------------|---------|-----------|--------| | Humine | 1·58 | | | | Humic acid | 1·15 | 12·05 | 7·49 | | Insoluble organic matters | 7·66 | | | | Water | 11·13 | 14·69 | 6·06 |
#### Sum of all the constituents.
| Substance | 21·52 | 26·74 | 13·55 | |--------------------|---------|-----------|--------| | | 100·28 | 100·21 | 99·90 |
In this soil there is a general resemblance in the compo- sition of the portion soluble in water to those of the wheat soils. But the part soluble in acids is distinguished by the great abundance of carbonate of lime.
The soil of Holland from the neighbourhood of the Zuider Zee, which is an alluvial deposit from the waters of the Rhine, and produces large crops, gave the results which follow.
| | Surface | 15 inches deep | 30 inches deep | |----------------|---------|---------------|---------------| | Insoluble silica | 57-646 | 51-706 | 55-372 | | Soluble silica | 2-340 | 2-496 | 2-286 | | Alumina | 1-830 | 2-900 | 2-888 | | Peroxide of iron | 9-039 | 10-305 | 11-864 | | Protioxide of iron | 0-350 | 0-563 | 0-200 | | Oxide of manganese | 0-288 | 0-354 | 0-284 | | Lime | 4-092 | 5-096 | 2-480 | | Magnesia | 0-130 | 0-140 | 0-128 | | Potash | 1-026 | 1-430 | 1-521 | | Soda | 1-972 | 2-069 | 1-937 | | Ammonia | 0-060 | 0-078 | 0-075 | | Phosphoric acid | 0-466 | 0-324 | 0-478 | | Sulphuric acid | 0-896 | 1-104 | 0-576 | | Carbonic acid | 6-085 | 6-940 | 4-775 | | Chlorine | 1-240 | 1-302 | 1-418 | | Humic acid | 2-798 | 3-991 | 3-428 | | Crenic acid | 0-771 | 0-731 | 0-037 | | Apocrenic acid | 0-107 | 0-160 | 0-152 | | Other organic matters | 8-324 | 7-700 | 9-348 | | Combined water | 0-540 | 0-611 | 0-753 |
It is unnecessary to multiply analyses of fertile soils, those now given being sufficient to shew their general composition. They are all characterised by the presence in considerable quantity of all the essential constituents of plants, and by their presence in a state in which they may be readily absorbed. The absence of one or more of these substances immediately diminishes or altogether destroys the fertility of the soil; and the extent to which this occurs is illustrated by the following analysis of a soil from Pampherton, Mid-Lothian, forming a small patch in the lower part of a field, and on which nothing would grow. Being naturally wet, it had been drained and sowed with oats, which died out about six weeks after sowing, and left a bare soil on which weeds did not show the slightest disposition to grow.
Soluble in acids.
| | | |----------------|---------| | Soluble silica | 0-173 | | Peroxide of iron | 6-775 | | Alumina | 1-150 | | Oxide of manganese | trace | | Carbonate of lime | 0-856 | | Magnesia | 0-099 | | Potash | 0-132 | | Soda | 0-123 | | Phosphoric acid | trace | | Chlorine | trace |
Silica... 73-096 Peroxide of iron... 1-371 Alumina... 4-263 Lime... 0-858 Magnesia... 0-520
Organic matter... 8-012 Water... 2-391
In this instance the barrenness of the soil is distinctly traceable to the deficiency of phosphoric acid, sulphuric acid, and chlorine. There is also a remarkably large quantity of oxide of iron, which, when dissolved by the humic acid, is well known to be highly prejudicial to vegetation. That this took place was shown by the fact that the drains, a couple of months after being laid, were almost stopped up by humate of iron. Still more striking are the following analyses:
| | Moorland soil near East Freetland, Wigtownshire. | Sandy soil near East Freetland, Wigtownshire. | Salt soil near Muirhares, Banffshire. | |----------------|--------------------------------------------------|---------------------------------------------|-------------------------------------| | Silica and sand | 70-575 | 96-000 | 77-780 | | Alumina | 1-050 | 0-500 | 9-490 | | Oxide of iron | 0-252 | 2-000 | 5-900 | | Oxide of manganese | trace | trace | 0-105 | | Lime | trace | trace | 0-001 | | Magnesia | 0-012 | | 0-728 | | Potash | trace | | trace | | Soda | trace | | 0-003 | | Phosphoric acid | trace | | trace | | Sulphuric acid | trace | | 0-200 | | Carbonic acid | trace | | trace | | Chlorine | trace | | trace | | Humic acid | 11-910 | 0-200 | 0-732 | | Insoluble humus | 16-200 | 1-299 | 0-200 | | Water | trace | | 4-095 |
The results contained in these analyses are peculiarly remarkable, indicating as they do the almost total absence of all those substances which the plant requires. These must, however, be considered as in a great measure exceptional cases, as it is no doubt but rarely that so large a number of constituents is absent, and far more frequent to find the deficiency restricted to one or two substances. They are illustrations of barrenness dependent on different circumstances. The first shows the unimportance of the organic matters of the soil, which are here unusually abundant, without in any way counteracting the unfertility dependent on the absence of the other constituents. The second is that of a nearly pure sand; and the third, though it contains a greater number of the essential ingredients of the ash, is still rendered unfruitful by the deficiency of alkalis, sulphuric acid, and chlorine.
An examination of the foregoing analyses indicates pretty clearly the conditions of fertility of the soil. It must obviously contain all the constituents of the plants which are to grow upon it, and in a soluble state, so that they may admit of absorption by the plant. It is clear, however, that the part directly soluble in water embraces only a certain number of the constituents of the plant, and that only in small quantity. This becomes still more apparent if we estimate the quantities contained in an acre of soil. It is calculated that the soil on an imperial acre of land 10 inches deep weighs in round numbers about 1000 tons; and calculating from this, we find that the quantity of potash soluble in water in the Mid-Lothian wheat soil amounts only to about 70 lb. per acre. But a crop of hay carries off from the soil about 38 lb. of potash, and one of turnips, including tops, no less than 200 lb. Manifestly, therefore, if only the matters soluble in water could be taken up by the plant, such soils could not possess the amount of fertility which they actually do. But the soil is not an inert unchangeable substance; it is the theatre of an important series of chemical changes effected by the action of air and moisture, and producing a continued liberation of its constituents. The oxygen of the air acts upon the organic matters of the soil, and produces a constant though slow evolution of carbonic acid, which is absorbed by the moisture contained in the soil, and exerts a solvent action on its constituents. In fact, though a very feeble acid, carbonic acid, by continuous action, is constantly effecting the solution of new quantities of the constituents of the soil; and this action goes on so slowly that at any one moment, the quantity in a soluble condition is not sufficient to supply the total amount required by the plant; but by a beautiful provision of nature, they are brought into the soluble state only in proportion as they are required for the plant, and thus the loss which would take place by the rain falling upon a soil charged with soluble matters and washing them out, is effectually prevented.
Carbonic acid is therefore a most important agent in producing the chemical changes in the soil, and the organic matters are valuable, as affording a supply of that substance within the soil itself; but the carbonic acid of the atmosphere will itself effect these changes, although with different degrees of rapidity according to the character of the soil. In light soils of open texture it will act easily, but in stiff clay soils its action is very slow. The solvent action of the carbonic acid is, no doubt, principally exerted on the substances soluble in acids, but not entirely, for we know that the part insoluble in acids is gradually being disintegrated and made soluble; and hence it is that the composition of that part of the soil which resists the action of acids, and which at first sight might appear of no moment, is really important. We observe, in fact, that this circumstance must at once confer on the soil of the Carse of Gowrie a great superiority over those of Mid-Lothian and most other districts; for it contains in its insoluble part a quantity of alkalies which must necessarily form a source of continual fertility. Accordingly, experience has all along shown the great superiority of that soil, and of alluvial soils generally, which are all more or less similar to it. The facility with which these matters are attackable by carbonic acid is also an important element of the fertility of a soil, and it is to the existence of compounds which are readily soluble that we attribute the high fertility of the trap soils.
By a further examination of the analyses of fertile soils, it is at once apparent that the most essential constituents of plants are by no means the most abundant in the soil. In fact, phosphoric and sulphuric acids, lime, magnesia, and the alkalies, which in most instances make up nine-tenths of the ash of plants, form but a small portion of even the most fertile soils; while silica, which except in the grasses occurs in small quantity, oxide of iron which is a limited, and alumina a rare, constituent of the ash, form by far their larger part. Thus the total amount of potash, soda, lime, magnesia, phosphoric and sulphuric acids and chlorine contained in the Mid-Lothian wheat soil amounts only to 3·5888 per cent., and in the Perthshire to 6·4385, the entire remainder being substances which enter into the plant for the most part in much smaller quantity. Now, as these small quantities of the more important substances are capable of supplying the wants of the plant, it must be obvious that a very small fraction of the silica, oxide of iron, and alumina, which the soils contain, would afford to it the whole quantity of these substances it requires, and that the rest of these constituents must have some other functions to perform in the soil. Hitherto we have looked upon a soil merely as the source of the inorganic food of plants, but it has to act also as a support for the plant while growing, and to retain a sufficient quantity of moisture to support its life; and unless it possess the properties which fit it for doing so, it may contain all the elements of the food of plants, and yet be nearly or altogether barren.
If a quantity of a soil be shaken up with water and allowed to stand for a few minutes, we find that it rapidly deposits a quantity of grains which we at once recognise as common sand. If the water be now poured off into another vessel and allowed to stand for a longer time, there is deposited a quantity of a fine soft powder, having the properties and composition of common clay, while the clear fluid now contains the soluble matter. By a more careful treatment we can likewise distinguish and separate humus, and in soils lying on chalk or limestone, calcareous matter or carbonate of lime. We perform in this way a sort of mechanical analysis, and classify the components of the soil into four groups, a mixture of two or more of which in variable proportions is found in all soils.
The relative proportions in which these substances exist in a soil are intimately connected with its mechanical and physical properties, which have as important an influence in its fertility as its chemical composition; for a soil may contain all the necessary elements of the crops, and yet, from some defect in its physical characters, be nearly or altogether barren. In fact, it is impossible to examine a large number of analyses of soils without seeing that though in many instances they may give tolerably satisfactory information as to their relative values, yet we sometimes see two soils one fertile and the other barren although there is no appreciable difference in their chemical composition. An illustration of this is found in the following analyses of two soils both fertile, but in one of which red clover grows luxuriantly, in the other it invariably fails.
| Clover fails | Clover succeeds | |--------------|----------------| | Insoluble silicates | 83·90 | 81·34 | | Soluble silica | 0·08 | 0·02 | | Peroxide of iron | 4·45 | 6·68 | | Alumina | 2·40 | 3·00 | | Lime | 1·23 | 1·33 | | Magnesia | 0·45 | 0·25 | | Potash | 0·20 | 0·22 | | Soda | 0·07 | 0·09 | | Sulphuric acid | 0·05 | 0·08 | | Phosphoric acid | 0·38 | 0·07 | | Carbonic acid | 0·09 | 0·34 | | Chlorine | trace | trace | | Humic acid | 0·42 | 0·43 | | Humine | 0·10 | | Insoluble organic matters | 3·70 | 3·61 | | Water | 2·54 | 2·52 |
99·96 | 100·08
Nitrogen | 0·15 | 1·15
In default of any explanation deducible from the composition of the soil, we are induced to attribute the differences here observed to differences in their physical properties.
Now it appears that the mechanical constituents of the soil mentioned above possess certain properties, partly mechanical and partly chemical, which exert an important influence on its fertility. Sand and clay, the most important of the four, confer on the soil diametrically opposite properties; the former, when present in large quantity, producing what are designated as light, the latter stiff or heavy soils. Sand, being composed of hard indestructible grains of silicious matter, forms a soil of an open texture, through which water readily permeates; while clay, from its fine state of division, and peculiar adhesiveness or plasticity, gives a close-textured and retentive soil; and the proper intermixture of the two produces a light fertile soil, each tempering the peculiar properties of the other. Indeed, their mixture is manifestly essential, for sand alone contains none of the essential ingredients of the plant; and if present in large quantity, the openness of the soil is excessive, water flows through it with rapidity, manures are rapidly destroyed, and the accession of drought soon causes the plants which grow upon it to languish and die. Clay, on the other hand, is by itself equally objectionable; the closeness of its texture prevents the spreading of the roots of plants, and the access of carbonic acid, which, as we have already seen, is so important an agent in the changes occurring in the soil. In fact a pure clay, that is to say, a clay unmixed with sand, even though it may contain all the essential constituents of the plant is absolutely unfertile. Practically, of course, these extreme cases never occur; the heaviest clay soils are mixtures of true clay with sand, and the most sandy soils contain their proportion of clay; but frequently the preponderance of the one over the other is so great, as to produce soils greatly inferior to those in which the mixture is more uniform.
We have spoken of those substances merely as mechanically affecting the soil; but clay possesses also a very remarkable property, apparently of a chemical nature, although to what extent it is so is as yet unknown, and which gives it a high importance in the soil. It possesses a remarkable power of absorbing the soluble constituents of the soil, and preventing them, in part at least, from being washed out by the rains. This peculiar effect of clay has long been recognised by chemists; but its special importance in an agricultural point of view has been shown by Mr Thomson; and to the extended investigation of Way we owe the greater part of our definite information regarding it. It appears that all ordinary arable soils possess, in a more or less marked degree, the power of removing from their solution ammonia, potash, soda, and phosphoric acid, to a considerable extent, and lime and magnesia in smaller quantity. The amount of this absorption is easily seen by a simple experiment. It suffices to take a tall cylindrical vessel open at both ends, and filled with the soil to be operated upon, which is retained by a piece of rag tied over its lower end. A quantity of a dilute solution of ammonia being then poured upon the surface of the soil, and allowed to percolate, the first quantity which flows away is found to have entirely lost its peculiar smell and taste; and in a similar manner the removal of potash and soda may be illustrated. Mr Way has found, that not only is ammonia absorbed by the soil in the free state, but also when in combination with different acids. In the latter case the absorption is attended with a true chemical decomposition, for the fluid which flows from the soil contains the whole of the acid of the ammonia salt in combination with lime. Thus, if sulphate of ammonia be employed, we have sulphate of lime in the fluid, and if muriate of ammonia, we have muriate of lime escaping. Mr Way's experiments have shown, that an ordinary soil is capable of absorbing and bringing into an insoluble condition about 0·3 per cent. of ammonia, when either ammonia itself, its sulphate, or muriate, is employed. It thus appears, as far as absorption goes, to be immaterial whether the ammonia is free or combined. But it is different with potash, which is absorbed from the nitrate to the extent of about 0·6 per cent., and from a caustic solution of potash to double that amount. In these cases the acid of the substance employed appears to combine with lime, and the whole of it is obtained in the solution. From this it may be gathered, that lime is not readily absorbed from solutions of its salts; indeed, it would appear that the only salt of lime which comes under the absorbent power of the soil is the bicarbonate, from which lime is taken to the extent of 1·4 per cent. by the soil. The absorption of lime from this salt, and of phosphoric acid, which takes place to a considerable extent, probably occurs, however, quite independently of the clay present in the soil, and is occasioned by its lime, which forms an insoluble compound with phosphoric acid, and by removing half the carbonic acid of the bicarbonate of lime converts it also into an insoluble state.
Mr Way attributes the entire absorptive effects of the soil to the clay which they contain, but it may be questioned whether it is the exclusive agent at work. We have remarked, that the absorption of phosphoric acid and of lime from the bicarbonate is probably dependent on lime itself. But as regards alkalies and ammonia, there is another absorptive agent in the organic constituents of the soil. So powerful indeed is the affinity of these substances for ammonia, that chemists are at one as to the difficulty of obtaining the humic and other acids pure, owing to the obstinacy with which they retain it; and there cannot be a doubt that in many soils these substances are in this point of view of much importance. This is particularly the case in peat soils, which, though naturally barren, may be made to produce good crops by the application of sand or gravel; and as neither of these can cause any absorption of the valuable matters, we must attribute this effect to the organic matter. That peat does absorb ammonia may be shown by a simple experiment. If a quantity of dry peat be taken and ammonia poured on it, we find that its smell disappears; and this may be continued until upwards of 1·5 per cent. of dry ammonia has been absorbed, and this quantity is retained by the peat. There is another point worthy of inquiry at the present time, and which may prove of much importance. It is certain that the soil, when shaken up with ammonia, withdraws from its solution a quantity of that substance; but what is the effect of withdrawing the remaining solution, and agitating with more water? May it not happen that the ammonia at first absorbed may be again washed out of the soil. The analyses of the soils given in page 393 show at least, that the whole of the potash and soda is not in an insoluble condition in the soil; and though the quantity extracted by water is small, it is unequivocal; we know also, that the water which flows from the drains always contains a variety of alkaline and other salts in solution, manifestly derived from the soils through which it has percolated. In short, it must not be supposed that the substances absorbed are rendered absolutely insoluble; they become only relatively so; and it is probable that, under particular circumstances, a considerable proportion of them may be again removed. This is obviously the case in regard to ammonia, as absorbed by peat, for we find that the alkaline reaction is removed by that substance from a considerable quantity of dilute ammonia, although only a portion of it is retained when the soil has become dry. Here the presence of moisture appears to be of consequence, and no doubt other conditions, not at present understood, may have the effect of greatly modifying the phenomena.
The peculiarities hitherto alluded to are perhaps in some respects more chemical than mechanical, or at least partake to some extent of both; but the more strictly physical characters, such for instance as the relations of the soil to heat and moisture, &c., are not less important. It needs, indeed, only a moment's consideration to see how great must be the influence exerted by their power of absorbing heat and moisture. We know that in these respects soils differ greatly, and the possession of these properties in a high degree may cause two soils chemically identical to differ widely in productiveness. Thus, for instance, two soils may be identical in composition, but one may be highly hygroscopic, that is, may absorb moisture readily from the air, while the other may be very deficient in that property. Under ordinary circumstances no difference will be apparent between the produce of the two soils, but in a dry season the crop upon the former may be in a flourishing condition, while that on the latter may be languishing and enfeebled merely from its inability to absorb from the air, and supply to the plant the quantity of water required for its growth. In the same way, a soil which absorbs much heat from the sun's rays will surpass another which has not that property; and though in many cases this effect may be comparatively unimportant, it may make the difference between successful and unsuccessful cultivation in soils which lie in an unfavourable climate or exposure.
The investigation of the physical characters of soils has attracted little attention except on the part of Schübler, who published, nearly 30 years ago, a very elaborate series of researches on this subject, from which all our present information is derived. He determined, 1st, The specific gravity of the soils; 2d, The quantity of water which they are capable of imbibing; 3d, The rapidity with which they give off by evaporation the water they have imbibed; that is, their tendency to become dry; 4th, The extent to which they shrink in drying; 5th, Their hygroscopic power; 6th, The extent to which they are heated by the sun's rays; 7th, The rapidity with which a heated soil cools down, which indicates its power of retaining heat; 8th, Their tenacity, or the resistance they offer to the passage of agricultural implements; 9th, Their power of absorbing oxygen from the air. Each of these experiments was performed on several different soils, and on their mechanical constituents. Schübler's experiments are undoubtedly important, and though the methods employed are some of them not altogether beyond cavil, they have apparently been performed with great care. It is nevertheless desirable that they should be carefully repeated, for such facts ought not to rest on the authority of one experimenter, however skilful and conscientious, nor on a single series of soils, which may not give a fair representation of their general physical properties. In fact Schübler appears to imagine that having once determined the extent to which the sand, clay, and other mechanical constituents of the soil possess these properties, we are in a condition to predicate with regard to soils produced from their mixture in variable proportions, although this is by no means probable.
In examining these properties, Schübler selected for experiment, pure silicious sand, calcareous sand (carbonate of lime in coarse grains), finely powdered carbonate of lime, pure clay, humus, and powdered gypsum. He used also a heavy clay consisting of 11 per cent. of sand and 89 of pure clay, a somewhat stiff clay containing 24 per cent. of sand and 76 of clay, a light clay, with 40 per cent. of sand and 60 of pure clay, a garden soil consisting of 52-4 per cent. of clay, 36-5 of silicious sand, 1-8 of calcareous sand, 2 per cent. of finely divided carbonate of lime, and 7-2 of humus, and two arable soils, one from HofwyI, and one from a valley in the Jura, the former a somewhat stiff, the latter a light soil.
| Specific gravity | Water absorbed by 100 parts. | Of 100 parts water absorbed, there evaporates over hours at 66°. | Diminution during drying of 100 parts moist soil. | Quantity of hydrometric water absorbed by 77-565 grams of the soil spread on a surface of 144-45 square inches. | Power of retaining heat. Calc. sand, 100. | Tenacity of the soils, pure clay, 100. | Quantity of oxygen absorbed by 77-565 grams of the moist soil in 24 days, from 15 inches of atmospheric pressure, expressed in cubic inches. | |------------------|-------------------------------|-------------------------------------------------|-----------------------------------------------|-------------------------------------------------|---------------------------------|-----------------|-------------------------------------------------| | Silicious sand...| 2-753 | 25 | 68-4 | 0 | 0 | 0 | 95-6 | | Calcareous sand..| 2-892 | 29 | 75-9 | 0 | 0-154 | 0-231 | 0-231 | | Light clay.......| 2-701 | 40 | 52-0 | 6 | 1-617 | 2-002 | 2-156 | | Stiff clay.......| 2-632 | 50 | 45-7 | 8 | 1-925 | 2-310 | 2-618 | | Heavy clay.......| 2-603 | 61 | 34-9 | 11-4 | 2-310 | 2-772 | 3-080 | | Pure clay........| 2-591 | 70 | 31-3 | 18-3 | 2-849 | 3-234 | 3-696 | | Carbonate of lime.| 2-468 | 85 | 28-0 | 5 | 2-002 | 2-857 | 2-695 | | Humus............| 1-225 | 190 | 20-5 | 20-0 | 6-160 | 7-469 | 8-470 | | Gypsum...........| 2-358 | 27 | 71-7 | 0 | 0-077 | 0-077 | 0-077 | | Garden soil......| 2-332 | 96 | 24-5 | 14-9 | 2-635 | 3-465 | 3-850 | | Soil from HofwyI.| 2-401 | 52 | 32-0 | 12-0 | 1-232 | 1-771 | 1-771 | | Soil from Jura...| 2-526 | 47 | 40-1 | 9-5 | 1-978 | 1-463 | 1-540 |
The experiments detailed in the preceding table speak in a great measure for themselves, and scarcely require detailed comment. It must be remarked, however, that all the characters determined are not equally important. Those illustrating the relations of the soil to water are perhaps the most important. The superiority of a retentive over an open soil is sufficiently familiar in practice, and though this superiority is no doubt partly due to the former absorbing and retaining more completely the ammonia and other valuable constituents of the manures applied to it, it is also dependent to an equal if not greater extent upon the power it possesses of retaining moisture. A reference to the table makes it apparent that this power is presented under three different heads, which are certainly related to one another but are not identical. In the second column of the table we have the quantity of water absorbed by the soil, when thoroughly moistened as a sponge is, and it may be considered as representing the quantity of water which will be retained by these different soils when thoroughly saturated by long continued rains. The column immediately succeeding gives the quantity of that water which escapes by evaporation from the same soil after exposure for four hours to dry air at the temperature of 66°. The fifth, sixth, seventh, and eighth columns indicate the quantity of moisture absorbed, when the soil, previously artificially dried, is exposed to moist air for different periods. These characters are dependent principally, though not entirely, on the porosity of the soil. The last may also be in some measure due to the presence of deliquescent salts in the soil, but is partially occasioned by their peculiar structure. It is to be remarked that clay and humus are two of the most highly hygroscopic substances known, and it is peculiarly interesting to observe, that by a beneficent provision of nature, they also form a principal part of all fertile soils. The quantity of water imbibed by the soil is important to its fertility, in so far as it prevents it becoming rapidly dry after having been moistened by the rains. It is valuable also in another point of view, because if the soil be incapable of absorbing much water, it becomes saturated by a moderate fall of rain, and when a larger quantity falls, the excess of necessity percolates through the soil and carries off with it a certain quantity of the soluble salts. Important as this property is, however, it must not be possessed in too high a degree, but must permit the evaporation of the water retained with a certain degree of rapidity. Soils which do not admit of this taking place, become the cause of much inconvenience and injury in practice. By becoming thoroughly saturated during winter, they remain for a long time in a wet and unworkable condition, in consequence of which they cannot be prepared and sown until late in the season, and though chemically unexceptionable, such soils are always disadvantageous, and may in certain seasons prove absolutely valueless. The extent to which the imbibition of water takes place is extremely variable, and the rapidity of evaporation equally so, but apparently in the inverse ratio of the former; for we observe that silicious sand absorbs only one-fourth of its weight of water, and again gives off in the course of four hours four-fifths of that it had taken up, while humus, which absorbs nearly twice its weight, retains nine-tenths of that quantity after four hours exposure. Long-continued and slow evaporation of the water absorbed by a soil is injurious in another way, for it makes the soil "cold," a term of practical origin, but which very correctly expresses the peculiarity in question, which is due to the quantity of heat absorbed during evaporation, which prevents the soil acquiring a sufficiently high temperature from the sun's rays. The soils which have absorbed a large quantity of moisture shrink more or less in the process of drying, and form cracks, which often break the delicate fibres of the roots of the plants and cause considerable injury: the extent of this shrinking is given in the fourth column.
The relation of the soils to heat divides itself into two considerations: the amount of heat absorbed by the soil, and the degree in which it is retained. Of these the latter only is illustrated in the table. The former is dependent on so many special considerations that the results cannot be tabulated in a satisfactory manner. It is independent of the chemical nature of the soil, but varies to a great extent according to its colour and the angle of incidence of the sun's rays, and its state of moisture. It is, however, an important character, and has been found by Girardin to modify to a considerable extent the rapidity of ripening of the crops. He found in a particular year, that on the 25th of August 26 varieties of potatoes were ripe on a very dark-coloured sandy vegetable mould, 20 on an ordinary sandy soil, 19 on a loamy soil, and only 16 on a nearly white calcareous soil.
The tenacity of the soil is very variable, and indicates the great differences in the amount of power which must be expended in working them. According to Schübler, a soil whose tenacity does not exceed 10 is easily worked, but when it reaches 40 it becomes sufficiently difficult and heavy to work.
In looking at the tables, we see manifestly that there is one constituent of the soil to which a high importance must be attached in relation to its physical properties, and it is the more interesting to observe, as it is that to which we have attributed a minor chemical importance. It is humus, which will be observed to confer on the soil a high power of absorbing and retaining water, to diminish its tenacity and permit its being more easily worked, to add to its hygrometric power and property of absorbing oxygen from the air, and finally, from its dark colour to cause the more rapid absorption of heat from the sun's rays. It will be understood, that while humus does not directly supply food to the plant, it ministers indirectly in a most important manner to its well-being, and that to so great an extent that it must be considered an indispensable constituent of a fertile soil. But it is important to observe that it must not be present in too large a quantity, for an excess of it does away with all the good effects of a smaller supply, and produces soils notorious for their infertility.
Such are the important physical properties of the soil, and it is greatly to be desired that they should be more extensively examined. The great labour which this involves has, however, hitherto prevented its being done, and will, in all probability, render it impossible ever to do so except in a limited number of cases. Some of these characters are, however, of minor importance, and for ordinary purposes it might be sufficient to determine the specific gravity of the soil in the dry and moist state, the power of imbibing and retaining water, its hygrometric power, its tenacity and its colour. With these data we should be in a condition to draw probable conclusions regarding the others; for we find that the higher the specific gravity in the dry state, the greater is the power of the soil to retain heat, and the darker its colour the more readily does it absorb it. The greater its tenacity, the more difficult it is to work, and the greater difficulty will the roots of the young plant find in pushing their way through it. The greater the power of imbibing water, the more it shrinks in drying; and the more slowly the water evaporates, the colder is the soil produced. The hygrometric power is so important a character that Davy and other chemists have even believed it possible to make it the measure of the fertility of a soil; but though this may be true within certain limits, it must not be too broadly assumed, the results of recent experiments by no means confirming the opinion in its integrity, but indicating only some relation between the two.
The Subsoil.—The term soil is strictly confined to that portion of the surface turned over by the plough working at ordinary depth; which, as a general rule, may be taken at 10 inches. The portion immediately subjacent we call the subsoil, and it has considerable agricultural importance, and requires a short notice. In many instances, soil and subsoil are separated by a purely imaginary line, and no striking difference can be observed either in their chemical or physical characters. In such cases it has been the practice with some persons not to limit the term soil to the upper portion, but to apply it to the whole depth, however great it may be, which agrees in characters with the upper part, and only to call that subsoil which manifestly differs from it. This principle is perhaps theoretically the more correct, but great practical advantages are derived from limiting the name of soil to the depth actually worked in common agricultural operations. The subsoil is always analogous in its general characters to a soil, but it may be either identical with that which overlies it or not. Of the former, striking illustrations are seen in the wheat subsoils, the analyses of which have been already given. In the latter case we find that great differences may exist. Thus we may have a heavy clay lying on an open and porous sand, or on peat, and vice versa. Even where the characters of the subsoil appear the same as those of the soil, appreciable chemical differences are generally observed, especially in the quantity of organic matter, which is increased in the soil by the decay of plants which have grown upon it, and by the manure added. In general, then, all that we have said regarding the characters of soils both chemically and physically, will apply to the subsoils, except that, from the difficulty with which the air reaches the latter, some minor peculiarities are observed. The most important is the effect of the decay of vegetable matter, without access of air, which is attended by the reduction of the peroxide of iron to the state of protoxide, or more commonly by the production of sulphuret of iron, compounds which are extremely prejudicial to vegetation, and occasionally give rise to some difficulties when the subsoil is brought to the surface, as we shall afterwards have to notice.
The physical characters of the subsoil are often of much importance to the soil itself. As, for instance, where a light soil lies on a clay subsoil, in which case the value of the soil is much higher than if it reposed on an open or sandy subsoil. And in many similar modes is an important influence exerted; but these belong more strictly to the prac- Classification of Soils.—Numerous attempts have been made to form a classification of soils according to their characters and value, but they have not hitherto proved very successful; and the result of more recent chemical investigations has not been such as to encourage a farther attempt. We have not at present data sufficient for the purpose, nor if we had, would it be possible to arrange any soil in its class except after an elaborate chemical examination. The only classification at present possible must be founded on the general physical characters of the soil; and the ordinary mode followed in practice of dividing them into clays, loams, &c., &c., which we need not here particularise, fulfils all that can be done until we have more minute information regarding a large number of soils. Those of our readers who desire more full information on this point are referred to the works of Thaer, Schübler, and others, where the subject is minutely discussed.
THE MECHANICAL IMPROVEMENT OF THE SOIL.
In order that it may have the highest degree of fertility, a soil must possess the necessary physical properties and chemical composition in perfection. In comparatively few instances does this actually occur; for the greater proportion of soils, either from mechanical or chemical defects, are incapable of producing an abundant vegetation. These defects, however, admit of diminution, or even entire removal, by certain methods of treatment, the adaptation of which to particular cases is necessarily one of the most important branches of agricultural practice, as the elucidation of their mode of action is of its theory. The observations already made with regard to the characters of fertile soils, will have prepared the reader for the statement that these defects may be removed in two ways, either mechanically or chemically. The former method of improvement may at first sight appear to fall more strictly under the head of practical agriculture, of which the mechanical treatment of the soil forms so important a part, and that their improvement by chemical means should form the sole subject of our consideration here. But the line of demarcation between the mechanical and the chemical, which seems so marked, disappears on more minute observation, and we find that the mechanical methods of improvement are frequently dependent on chemical principles; and those which, at first sight, appear to be entirely chemical, are also in reality partly mechanical. It will be necessary for us, therefore, to consider shortly the mechanical methods of improving the soil.
Draining.—By far the most important method of mechanically improving the soil is by draining,—a practice the beneficial action of which is dependent on a great variety of circumstances. Its most obvious effect is probably that which it produces on the temperature of the soil. We have already remarked that the germination of a seed is dependent on the soil in which it is sown acquiring a certain temperature, and the rapidity of the after-growth of the plant is, in part at least, dependent on the same circumstance. The necessary temperature is speedily attained by the heating action of the sun's rays, when the soil is dry; but when it is moist, the heat is expended in evaporating the moisture with which it is saturated; and it is only after this has been effected that it acquires a sufficiently high temperature to produce the rapid growth of the seeds committed to it. But when the soil is drained, the superfluous moisture is drawn off, and it is ready to take advantage of the heating effect of the sun's rays in early spring; and thus the period of germination, and by consequence also that of ripening, is advanced. The extent to which this takes place is necessarily variable, but it is generally considerable; and in some districts of Scotland the extensive introduction of draining has made the harvest on the average of years from 10 to 14 days earlier than it was before. It is unnecessary to insist on the importance of such a change, which in upland districts may make cultivation successful when it was previously almost impossible. The removal of moisture by drainage affects the physical characters of the soil in another manner: it makes it lighter, more friable, and more easily worked; and this change is occasioned by the downward flow of the water carrying with it to the lower part of the soil the finer argillaceous particles, leaving the coarser and sandy matters above, and in this way a marked improvement is produced on heavy clays. The abundant escape of water from the drains acts chemically by removing any noxious matters the soil may contain, and by diminishing the amount of soluble saline matters, which sometimes produce injurious effects. It thus prevents the saline incrustation, which is frequently seen in dry seasons on soils which are naturally wet, and which is produced by the water, which rises to the surface by capillary attraction, depositing, as it evaporates, the soluble substances it contained, and leaving a hard crust which prevents the access of air to the interior of the soil. The access of air to the soil, which is one of the most important elements of its fertility, is promoted in a high degree by draining, as, by removing the water which stagnates in the lower part of the soil, it permits the air to reach it. It provides also for the frequent change of the air which permeates the soil; for every shower that falls expels from it a quantity of the air it contains, and as the moisture flows off by the drains, a new supply of air enters to take its place, and thus the important changes which the atmospheric oxygen produces on the soil, are promoted in a high degree. The air which thus enters acts on the organic matters of the soil, and produces the carbonic acid, which we have already seen is so intimately connected with many of its chemical changes. In the absence of atmospheric air, the organic matters undergo different decompositions, they pass into states in which they are slowly acted on, and are incapable of supplying a sufficient quantity of carbonic acid to the soil. They also act upon the peroxide of iron, contained in all soils, reduce it to the state of protoxide, or, with the simultaneous reduction of the sulphuric acid, they produce sulphuret of iron, forms of combination which are well known to be most injurious to vegetation.
The removal of water from the lower part of the soil, and the admission of air, which is the consequence of draining, submits that part of it to the same changes which take place in its upper portion, and has the effect of practically deepening the soil to the extent to which it is thus laid dry. The roots of the plants growing on the soil, which stop as soon as they reach the moist part, now descend to a lower level, and derive from that part of it supplies of nourishment formerly unavailable. The deepening of the soil has further the effect of making the plants which grow upon it less liable to be burned up in seasons of drought, a somewhat unexpected result of making a soil drier, but which manifestly depends on its permitting the roots to penetrate to a greater depth, and so to get beyond the surface portion, which is rapidly dried up, and to which they were formerly confined.
It is thus obvious that the drainage of the soil modifies its properties both mechanically and chemically. It exerts also various other actions in particular cases which we cannot here stop to particularise. It ameliorates the climate of districts in which it is extensively carried out, and even affects the health of the population in a favourable manner. The sum of its effects must necessarily differ greatly in different soils, and in different districts; but a competent Agricultural authority has estimated, that, on the average, land which has been drained produces a quarter more grain per acre than that which is undrained. But this can scarcely be said to exhaust the benefits derived from it; for draining is merely the precursor of further improvement. It is only after it has been carried out that the farmer derives the full benefit of the manures which he applies. He gains also by the increased facility of working the soil, and by the rapidity with which it dries after continued rain, which enables him to get on at their proper season with agricultural operations, which would otherwise have to be postponed for a considerable time.
We can scarcely be expected here to say much regarding the mode in which draining ought to be carried out, but we may remark that much inconvenience and loss has occasionally been produced by too close adhesion to particular systems. No rules can be laid down as to the depth or distance between the drains which can be universally applicable, but the intelligent drainer will seek to modify his practice according to the circumstances of the case. As a general rule, the drains ought to be as deep as possible, but in numerous instances it may be more advantageous to curtail their depth and increase their number. If, for instance, a thick impervious pan resting on a clay were found at the depth of three feet below the surface, it would serve no good purpose to make the drains deeper; but if the pan were thin, and the subjacent layer readily permeable by water, it might be advantageous to go down to the depth of four feet, trusting to the possible action of the air which would thus be admitted, gradually to disintegrate the pan, and increase the depth of soil above it. It is a common opinion that if we reach, at a moderate depth, a tenacious and little permeable clay it is no use making the drains deeper than this; but this is an opinion which should be adopted with caution, both because no clay is absolutely impermeable, even the most tenacious admitting the passage of water, and because the clay may have been brought down by water from the upper part of the soil, and may have stopped there merely for want of some deeper escape for the water, and which drains at a lower level might supply. It may even happen that it might be necessary to vary the depth of the drains in different parts of the same field, and the judicious drainer may sometimes save a considerable sum by a careful observation of the peculiarities of the different parts of the ground to be drained.
Subsoil and Deep Ploughing.—It frequently happens, when a soil is drained, that the subsoil is so stiff as to permit the passage of water imperfectly, and to prevent the tender roots of the plant from penetrating it, and reaching the new supplies of nourishment which are laid open to it. In such cases the benefits of subsoil ploughing and deep ploughing are conspicuous. The mode of action of these two methods of treatment is similar but not identical. The subsoil plough merely stirs and opens the subsoil, and permits the more ready passage of water and the access of air and of the roots of plants, the former to promote the necessary decompositions, the latter to avail themselves of the valuable matters set free. Deep ploughing again produces more extensive changes; it brings up new soil to the surface, mixes it with the original soil, and thus not only brings up new supplies of valuable matters to it, but frequently changes its chemical and mechanical characters, rendering a heavy soil lighter by the admixture of a light subsoil and vice versa. Both are operations which are useless unless they are combined with draining, for it must manifestly serve no good purpose to attempt to open up a soil unless the water which lies in it be previously removed. In fact, subsoiling is useless unless the subsoil has been made thoroughly dry, and it has been found by experience that no good effects are obtained if it be attempted immediately after draining, but that a sufficient time must elapse, in order to permit the escape of the accumulated moisture, which often takes place very slowly. Without this precaution, the subsoil, after being opened by the plough, soon sinks together, and the good effects anticipated are not realised. The necessity for allowing some time to elapse between draining and further operations is still more apparent in deep ploughing, when the soil is actually brought to the surface. In that case it requires to be left for a longer period after draining, in order that the air may produce the necessary changes on the subsoil; for if it be brought up after having been for a long time saturated with moisture, and containing its iron in the state of protoxide, and the organic matter in a state in which it is not readily acted upon by the air, the immediate effect of the operation is frequently injurious in place of being advantageous. One of the best methods of treating a soil in this way is to make the operation a gradual one, and by deepening an inch or two every year gradually to mix the soil and subsoil; as in this way from a small quantity being brought up at a time no injurious effects are produced. Deep ploughing may be said to act in two ways, firstly, by again bringing to the surface the manures which have a tendency to sink to the lower part of the soil, and, secondly, by bringing up a soil which has not been exhausted by previous cropping, in fact a virgin soil.
The success which attends the operation of subsoiling or deep ploughing must manifestly be greatly dependent on the character of the subsoil, and good effects can only be obtained when its chemical composition is such as to supply in increased quantity the essential constituents of the plant; and it is no doubt owing to this that the opinions entertained by practical men, each of whom speaks from the results of his own experience, are so varied. The effects produced by deep ploughing on the estates of the Marquis of Tweeddale, are familiarly known to most Scottish agriculturists, and they are at once explained by the analyses of the soil and subsoil here given, which show that the latter, though poor in some important constituents, contains more than twice as much potash as the soil.
| Component | Soil | Subsoil | |----------------------------|--------|---------| | Insoluble silicates | 57-623 | 82-72 | | Soluble silica | 0-393 | 0-12 | | Alumina and oxide of iron | 4-129 | 8-60 | | Lime | 0-341 | 0-18 | | Magnesia | 0-290 | 0-24 | | Sulphuric acid | 0-027 | 0-03 | | Phosphoric acid | 0-240 | trace | | Potash | 0-052 | 0-12 | | Soda | 0-050 | 0-04 | | Water | 1-956 | 3-26 | | Organic matter | 5-220 | 4-02 |
In addition to the difference in the amount of potash, something is probably due to the difference in the quantity of alumina and oxide of iron in the subsoil, which on this account must probably be more tenacious than the soil itself, which appears to be rather light. In many other instances, the use of the subsoil plough has occasioned much disappointment, and has led to its being deprecated by many practical men; but of late years its use having become better understood its merits are more generally admitted. We believe that, in all cases in which the soil is deep, more or less marked good effects must be produced by its use, but of course there must be cases in which, from the defective com-
---
1 Mr Dudgeon of Spylaw. Highland Society's Transactions, vol. xii. p. 505. position of the subsoil or other causes, it must fail. It may sometimes be possible *a priori* to detect these cases, but in a large majority of them we suspect our knowledge is too limited to enable us to do so.
**Improving the Soil by Burning.**—It has long been familiarly known, that a decided improvement has been produced on some soils by burning. Its advantages have chiefly been observed on two sorts, heavy clays, and peat soils; and on these varieties it has been practised to a great extent. The action of heat on the heavy clays appears to be of a two-fold character, depending partly on the change effected in its physical properties, and partly on a chemical decomposition produced by the heat. The operation of burning is effected by mixing the clay with brushwood and vegetable refuse, and allowing it to smoulder for some time. It is an operation of some nicety, and its success depends on the temperature being kept as low as possible. It has been further found that its success is by no means equal in all clays, but that there are some which are rather injured than benefited by it. The cause of its beneficial action appears to depend on a change which takes place in the state in which the potash exists in the soil, as a consequence of which it becomes more soluble than it was before. It has been found, that after heating, a dilute acid will extract from clays improved by burning a much larger quantity of potash than it did before; and as we know that the substances not extractable by acids are in a state in which they are unavailable to the plant, or at least can only be slowly obtained by it, we can easily understand how such a soil should be improved by burning, and also how some clays which contain little or no potash should not be affected by the process. The necessity for preserving the temperature of the burning mass of clay as low as possible is also rendered obvious; for it has been found by direct experiment, that at high temperatures another change occurs, whereby the potash, which at lower temperatures becomes soluble, passes again into an insoluble state.
A part of the beneficial effects is no doubt also due to the change produced in the physical characters of the clay by burning, which makes it lighter and more friable, and by mixture with the unburnt clay ameliorates the whole. This improvement in the physical characters of the clay also requires that it shall be burnt with as low a heat as possible; for if it rises too high, the clay coheres into hard masses which cannot again be reduced to powder, and the success of the operation of burning may always be judged of by the readiness with which it falls into a uniform friable powder.
The improvement of peat by burning has been practised to some extent in Scotland, though less frequently of late years than formerly; but it is still the principal method of reclaiming peat soils in many countries, and particularly in Finland, where large breadths of land have been brought into profitable cultivation by means of it. The *modus operandi* of burning peat is very simple; it acts by diminishing the superabundant quantity of humus or other organic matters, which, in the previous section we have seen to be so injurious to the fertility of the soil. It may act also in the same way as it does on clay, by making part of the inorganic constituents more really soluble, although it is not probable that its effect in this way can be very marked. Its chief action is certainly by destroying the organic matters, and by thus improving the physical character of the peat, and causing it to absorb and retain a smaller quantity of water than it naturally does. For this reason it is that it proves successful only on thin peat bogs, for if they be deep the inorganic matters soon sink into the lower part, and the surface relapses into its old state of infertility. It is probably for this reason that the practice has been so much abandoned in Scotland, the more especially as other and more economical modes of treating peat soils have come into use.
**Mixing of Soils.**—The mixing of soils is a very obvious method of improving those which are defective, and nothing but its expense limits its utility. It has been applied to the improvement of heavy soils and of peats, the former being mixed with sand or marl so as to diminish its tenacity; the latter with clay or gravel to add to its inorganic matters, and in both instances it has proved successful. The admixture of peat with open sandy soils, and with heavy clays has also been tried, although to a small extent, and we are not aware of the results obtained. It is very probable that the practice of mixing soils might be judiciously extended if we had sufficiently accurate information regarding the chemical composition of those mixed. It must be manifest, indeed, that the admixture of a highly fertile soil, even in moderate quantity, with another of inferior quality, must of necessity be attended with some effect. We believe, indeed, that the rich trap soils of some parts of Scotland have been mixed with those of inferior quality, but the extent of the benefit derived from the practice has not been made public.
**Manures.—Their Chemical Composition and Mode of Action.**
It is obvious from the statements we have made in a previous section, that even fertile soils contain many of the essential constituents of plants in limited and some of them in very small quantity; and the necessary consequence is, that the growth of successive generations of plants would soon exhaust the whole supply of these substances which they are capable of affording. In a state of nature, the plant which grows upon a soil dies there, or annually sheds its leaves, and returns to it the substances it had drawn into its system, in a state in which they are ready to afford nourishment to the next year's vegetation. But under the artificial circumstances of cultivation, when the crops produced are more or less completely removed from the soil, exhaustion would sooner or later take place if the substances removed from it were not again returned in the form of manure.
The action of a manure, however, is more complicated than this statement would lead us to suppose: for it is not confined to the mere addition to the soil of the substances required to sustain its fertility, but they exert an influence upon the soil itself, promote the changes which are continually in progress within it, and cause the liberation of a larger quantity of its valuable constituents than the atmosphere alone could do. It is clear that different manures may affect these objects in different ways and to different extents. Some may confine their influence to the mere addition of the necessary elements of the plants, others may exert a powerful influence on the changes occurring in the soil. Some again may supply all the constituents of the plants, others only one or two. The former of these is a very obscure and ill-understood branch of the action of manures; the latter is better known, and is an important element in our estimation of their value, those manures necessarily having the highest value which contain the greatest number of substances required by the plant, and those which afford only one or more having their value regulated by the difficulty which the plant has in obtaining the constituents these manures contain in abundance. There has thus arisen the distinction between general and special manures, a distinction which is important both in theory and practice, and which ought never to be lost sight of.
**General Manures.**—General manures, then, are those which are capable of supplying to the plant *all* or nearly all its constituents, and this is necessarily done in the most effectual manner by farm-yard manure, to which theory and practice concur in giving the highest rank.
**Farm-Yard Manure.**—Farm-yard manure is a mixture of the dung and urine of domestic animals with the straw which has been used as litter, and its composition and value will of course depend on that of these substances, which we must first consider. The dung of animals consists of that part of their food which passes through the intestinal canal without undergoing assimilation; the urine contains that portion which has been assimilated and is again excreted in consequence of the changes which are proceeding in the tissues of the animal. Their composition is naturally very different, and must be separately considered.
Urine.—Urine consists of a variety of earthy and alkaline salts, and of certain organic substances, generally rich in nitrogen, dissolved in a large quantity of water. The composition in the different domestic animals has been examined by different chemists; we quote the analyses of Fromberg, as giving the most complete view of the subject.
| Horse | Swine | Ox | Goat | Sheep | |-------|-------|----|------|-------| | Extractive matter soluble in water | 2-132 | 0-142 | 2-248 | 0-100 | 0-340 | | Extractive matter soluble in spirit | 2-550 | 0-387 | 1-421 | 0-454 | 3-330 | | Salts soluble in water | 2-340 | 0-909 | 2-442 | 0-850 | 1-957 | | Salts insoluble in water | 1-680 | 0-088 | 0-155 | 0-060 | 0-032 | | Urea | 1-244 | 0-273 | 1-976 | 0-678 | 1-262 | | Hippuric acid | 1-260 | ... | 0-550 | 0-125 | ... | | Mucous | 0-005 | 0-005 | 0-007 | 0-006 | 0-025 | | Water | 88-589 | 98-196 | 91-201 | 98-007 | 92-897 |
100-000 100-000 100-000 100-000 99-863
Composition of the ash of these Urines.
| Horse | Swine | Ox | Goat | Sheep | |-------|-------|----|------|-------| | Carbonate of lime | 12-50 | ... | 1-07 | trace | 0-82 | | Carbonate of magnesia | 9-46 | ... | 6-93 | 7-3 | 0-46 | | Carbonate of potash | 46-00 | 12-10 | 77-28 | trace | ... | | Carbonate of soda | 10-33 | ... | 53-0 | 42-25 | ... | | Sulphate of potash | ... | ... | 13-30 | ... | 2-98 | | Sulphate of soda | 13-04 | 7-00 | ... | 25-0 | 7-72 | | Phosphate of soda | 19-00 | ... | ... | ... | ... | | Phosphate of lime | ... | 8-80 | ... | ... | 0-70 | | Chloride of sodium | 6-94 | 53-10 | 0-30 | 14-7 | 32-01 | | Chloride of potassium | ... | trace | ... | ... | 12-00 | | Silica | 0-55 | ... | 0-35 | ... | 1-06 | | Oxide of iron and loss | 1-09 | ... | 0-77 | ... | ... |
100-00 100-00 100-00 100-00 100-00
Human urine has been accurately examined by Berzelius. His analysis gives the following numbers:
| Natural | Dry Residue | |---------|-------------| | Urea | 3-010 | 44-70 | | Lactic acid, lactate of ammonia, and extractive matters | 1-714 | 25-58 | | Uric acid | 0-100 | 1-49 | | Mucous | 0-032 | 0-48 | | Sulphate of potash | 0-371 | 5-54 | | Sulphate of soda | 0-316 | 4-72 | | Phosphate of soda | 0-294 | 4-39 | | Biphosphate of ammonia | 0-165 | 2-46 | | Chloride of sodium | 0-445 | 6-64 | | Muriate of ammonia | 0-150 | 2-46 | | Phosphates of magnesia and lime | 0-100 | 1-49 | | Silica | 0-003 | 0-05 | | Water | 93-300 | ... |
100-000 100-00
Among the special organic constituents of the urine are three substances of much importance, as they contain a large quantity of nitrogen, which they yield in the form of ammonia, as a consequence of certain changes to which they are liable. The composition of these substances is as follows:
| Carbon | Urea Acid | Uric Acid | Hippuric Acid | |--------|-----------|-----------|--------------| | 20-00 | 36-0 | 60-7 | | | Hydrogen | 6-60 | 2-4 | 5-0 | | Nitrogen | 46-70 | 33-4 | 8-0 | | Oxygen | 26-70 | 28-2 | 26-3 |
100-00 100-00 100-00
It is evident from these analyses that the urines of different animals must differ greatly in value, those of the ox, swine, and goat, containing a very much smaller quantity of solids than the others. They differ also in regard to their saline ingredients; and while salts of potash and soda form the principal part of the ash of the urine of the ox, sheep, goat, and horse, and phosphoric acid and phosphates are entirely absent, that of the pig contains a considerable quantity of the latter substances, and in this respect more nearly resembles the urine of man. Human urine is also much richer in urea and nitrogenous constituents generally, and has a higher value than any of the others.
Dung.—The solid excrement of animals is equally variable in composition. That of the domestic animals which had the ordinary winter food was found to have the following composition:
| Horse | Cow | Sheep | Swine | |-------|-----|-------|-------| | Per-cent of water in the fresh excrement | 77-25 | 82-45 | 50-47 | 77-13 | | Ash in the dry excrement | 13-36 | 15-23 | 13-49 | 37-17 |
100 parts of ash contained—
| Horse | Cow | Sheep | Swine | |-------|-----|-------|-------| | Silica | 62-40 | 62-54 | 50-11 | 13-19 | | Potash | 11-30 | 2-91 | 3-32 | 3-60 | | Soda | 1-98 | 0-98 | 3-28 | 3-44 | | Chloride of sodium | 0-03 | 0-23 | 0-14 | 0-89 | | Phosphate of iron | 2-73 | 8-63 | 3-98 | 10-55 | | Lime | 4-63 | 57-1 | 18-15 | 2-63 | | Magnesia | 3-84 | 11-47 | 5-45 | 2-24 | | Phosphoric acid | 8-93 | 4-75 | 7-52 | 0-41 | | Sulphuric acid | 1-83 | 1-77 | 2-69 | 0-90 | | Carbonic acid | trace | trace | trace | 0-60 | | Oxide of manganese | 2-13 | ... | ... | 61-37 | | Sand | ... | ... | ... | ... |
99-80 99-29 99-64 99-82
Human faeces contain about 73 per cent. of water, and leave about 1 per cent. of ash, of which the composition is—
| Potash | 6-10 | | Soda | 5-07 | | Lime | 26-46 | | Magnesia | 10-54 | | Oxide of iron | 2-50 | | Phosphoric acid | 36-03 | | Sulphuric acid | 3-13 | | Carbonic acid | 5-07 | | Chloride of sodium | 4-33 |
99-23
It is to be observed that the urine and dung of animals differs conspicuously in the composition of the ash, the urine being characterised by the abundance of alkaline salts, the latter containing only a small proportion of these, but being rich in earthy matters, and especially in phosphoric acid. The difference in the quantity of nitrogen they contain is also very marked, and is distinctly shown by the following analyses by Boussingault, which give the quantity of carbon, hydrogen, nitrogen, and oxygen in the dung and urine of the horse and the cow. It thus appears that the urine of the horse, in its natural state, contains three times as much nitrogen as its dung, and that of the cow twice as much; and the difference, especially in the horse, becomes still more conspicuous when they are dry.
Taking the facts just mentioned into account, it is obvious that the quality of farm-yard manure must depend, 1. On the kind of animal from which it is produced; 2. On the quantity of straw which has been used as litter; 3. On the nature of the food with which the animals have been supplied; and, 4. On the care which has been taken to prevent the escape of the urine, or of the ammonia produced by its decomposition. The extent to which these different circumstances modify the quality of farm-yard manure has not been very fully determined by analyses; nor is it probable that its composition varies very greatly, indeed, the analyses which we possess, although they are not very numerous, show a great degree of similarity in the farm-yard manure of different places, when prepared in the same manner. The results obtained by different analyses are contained in the following table, which, in addition to the ordinary farm-yard manure, gives also the composition of that produced by feeding cattle in boxes and of stable dung. The authority is given with each analysis.
The value of farm-yard manure must manifestly depend on the quantity of all the constituents of the plant contained in it. If, however, we examine minutely into the action of the individual elements, it becomes obvious that two present a much higher value than the others, and may, in most cases, be taken as the measure of the value of any manure. These substances are nitrogen and phosphoric acid. Now, it is to be observed, that though both exist in the soil, they are present in but small quantity, even in those of the highest fertility. But this is not all, for the condition in which they are met with is far from being that in which they are readily available to the plant. The phosphoric acid of our soils is not only in an insoluble state, but in one in which it is not readily reached or dissolved by the roots of the plant, and consequently can only become available as it is liberated by the continuous and slow decomposition constantly occurring in the soil. As regards nitrogen, we have already very distinctly referred to the fact that it can only be absorbed in the state of ammonia or of nitric acid, and most abundantly and readily in the former condition. But the soil contains its nitrogen in the form of vegetable debris, which do not contain that element in the form of ammonia, but only yield it as the result of decompositions, which, like all those occurring in the soil, proceed with extreme slowness. The importance of having the phosphoric acid in a soluble state, and the nitrogen in the form of ammonia, is conspicuously seen from the marked effects obtained from the use of superphosphate of lime (in which a large quantity of the phosphoric acid is soluble) and the salts of ammonia, to which we shall afterwards have to advert. By reference to the analyses of the urine and dung of our domestic animals in the previous page, we see that they do not contain ammonia as such, although they are rich in substances capable of yielding it, and that soluble phosphates are found only in the urine of man and the pig. The value of farm-yard manure is to be estimated by the abundance of these elements; but for all ordinary purposes, in considering the matter, we may put the phosphoric acid out of the question, and confine our attention to the nitrogen.
In the production of farm-yard manure of the highest quality, the object of the farmer must be, first, to produce a manure containing the largest possible amount of nitrogen; and secondly, to convert that nitrogen more or less completely into ammonia. In regard to the first of these points, it will be at once seen from what we have said of the comparative composition of the dung and urine, that the more effectually the latter is collected and its escape from the dung preserved, the higher will be the value of the manure; and it is for this reason that chemists have so anxiously impressed upon the agricultural public the great importance of preserving, and, if possible, retaining in the mass the fluid that drains from the dung heap, which must necessarily con- tain a considerable quantity of the nitrogenous constituents of the urine. This may be managed to a considerable extent, by arranging the manure heap in such a manner that the fluid which drains from it can be again pumped up over the solid matter, so that the latter may be saturated by it. A still more effectual plan is to mix the dung with some substance which may absorb the urine. For this purpose it is desirable that the absorbent substance shall be one which has an affinity for ammonia, so that it may not only retain the urine mechanically, but, by combining chemically with the ammonia produced by its decomposition, may prevent the escape of that substance into the air, which, from its volatility, is of course very liable to do. Many substances, such as gypsum, sulphate of iron, chloride of manganese, sulphate of magnesia, and sulphuric acid, have been proposed for this purpose, and have occasionally been used, though not extensively. They all answer the purpose of fixing the ammonia, that is, of preventing its escaping into the air more or less effectually, but they do not add sufficiently to the porosity of the manure heap to enable it to absorb the fluid, which is a matter of some importance. For this purpose clay, or the vegetable refuse of the farm, may be employed. But by far the best substance, when it can be got, is dry peat, which not only absorbs the fluid, but will serve to fix the ammonia without the addition of any other substance. We have already referred to the absorbent power of peat in the section on soils, but we may mention here that accurate experiment has shown that a particular peat will absorb about 2 per cent.\(^1\) of ammonia, and when dry will still retain from 1 to 1\(\frac{1}{2}\) per cent., or nearly three times as much as would be yielded by the whole nitrogen of an equal weight of farm-yard manure. Peat charcoal has been recommended for the same purpose, but careful experiment has shown that it does not absorb ammonia, although it removes putrid odour; and though it may be usefully employed when it is wished to deodorise the manure heap, it must not be trusted to as an absorbent of ammonia.
In addition to these methods of managing farm-yard manure, we have of late years had the introduction of box-feeding, one of the great advantages of which is said to be the production of a manure of superior quality to that obtained in the old way. In box-feeding none of the dung or urine is removed from under the animals, but is trampled down by their feet, and new quantities of litter being constantly added, the whole is consolidated into a compact mass, by which the urine is entirely retained. That, under these circumstances, the manure should be of high quality is certainly consistent with theory, and the analysis of box manure given above confirms the opinion. It is, however, a solitary analysis, and until confirmed by more extended analyses too much dependence must not be placed upon it. The value of box manure must mainly depend upon the solid retaining the whole of the liquid manure; but this is exactly the point on which practical men differ, the keen supporters of box-feeding asserting that it does, while others find that a certain quantity escapes. Should this prove to be frequently the case, the advantages of box-feeding, as a means of producing good manure, will be less than its supporters imagine.
Whether box manure is really superior to that prepared by the ordinary method is very questionable, but there is no doubt that it surpasses a large proportion of that actually produced. It is more than probable, however, that the more careful management of the manure heap may produce equally good effects. It is manifest that the same number of cattle, fed in the same way, and on the same food, and supplied with the same quantity of litter, must always excrete the same quantities of valuable matters, and the only question to be solved is, whether these valuable matters are more effectually preserved in the one way than the other. It will be readily seen that this cannot be done by the analysis of the manure alone, but we must conjoin with it a determination of the total weight of manure produced; for though, weight for weight, box manure may be better than ordinary farm-yard manure, the total quantity obtained from a given number of cattle may be so much greater that the deficiency in quality may be compensated for. At the present time our knowledge is too limited to admit of a definite opinion on this subject, but it is highly deserving of the combined investigation of the farmer and the chemist.
The value of the manure produced is also dependent on the nature of the food supplied to the cattle, and the period of the fattening process at which it is collected. When lean beasts are put up to fatten they at first exhaust the food much more completely then they do when they are nearly fattened, and the manure produced is very inferior at first, and goes on gradually improving in quality as the animal becomes fat. That the quality of the food affects the value of the manure is an opinion which has long been entertained by practical men, and it is no doubt correct, though not to the extent which some persons believe. It is held by some farmers, that while cattle are fed with oil-cake, the increased value of the manure is equal to from one-half to one-third that of the oil-cake. This is certainly an exaggeration, but there cannot be a doubt that some increase of value must take place; for when oil-cake is employed, the quantity of nutritive matters consumed by the animal is larger than when it is fed on turnip alone; but it is not possible at present to estimate the difference, for want of experiments especially directed to this point. From analyses made some time since, we have come to the conclusion that, weight for weight, the dung and urine of cattle fed with oil-cake are richer in valuable matters than those fed on turnips alone; but the experiments were not sufficiently extensive to enable us to draw a definite conclusion, and no determination of the total quantity excreted could be made.
Supposing the conditions which produce the manure containing the largest quantity of nitrogen to have been fulfilled, we have now to consider those which affect its evolution in the form of ammonia. This change is effected by fermentation. When a quantity of manure is left to itself for some time it is found to become hot, and gradually to diminish in bulk, and if it be now turned over it is found to evolve the smell of ammonia more or less distinctly. This ammonia is produced, in the first instance, from the urine, the nitrogenous constituents of which are rapidly decomposed, and the fermentation thus set up in the mass of manure extends to the solid dung, and finally to the straw of the litter, and gradually proceeds until a large quantity of ammonia is produced. The same change occurs in the manure if mixed with the soil, but in that case it is much slower, and experience has shown that much greater effects are produced if the manure has been fermented previous to being used. Science at once explains the necessity for the process, and shows that by its means the nitrogen is converted into a state in which the crop it is applied to can rapidly absorb it, and that the practice of applying well-fermented dung to the quickly growing crops, and fresh dung to those which come slowly to maturity, is consistent with theory. But it points out also that the method of doing this
---
\(^1\) Report on the economic uses of peat. Highland Society's Transactions, N.S., vol. iv, p. 549. by fermentation is, in so far, defective, that it cannot be effected without some loss of ammonia; however carefully it may be managed; and though up to the present time the fermentation of a manure has been deemed essential to its success, it may be questioned whether, now that we have other sources of ammonia, it might not be more economical to apply farm-yard manure in an unfermented state, so as to avoid the loss of ammonia which takes place during the process, and to supply the quantity required for the early growth of the plant by the use of some of the salts of ammonia or other ammoniacal manures. This is a speculative opinion, which must be submitted to experiment; and its value must depend on the amount of loss which takes place by fermentation in the manure heap, regarding which we have at present no information.
Liquid Manure.—This term is applied to the urine of the animals fed on the farm, and to the drainings from the manure-heap, which, in place of being returned to it, are in some instances allowed to drain away, and are collected in tanks, from which they are distributed according to the old plan by a watering-cart, or according to the method recently introduced in Ayrshire, by pipes laid under-ground in the fields, and through which the manure is either pumped by steam-power, or, where the necessary inclination can be obtained, is distributed by gravitation. That liquid manure must necessarily be valuable, is an inference which may be at once drawn from the analyses of the urine of different animals already given, and which, when it is collected apart from the solid matters, may be taken as representing the composition of liquid manure. It will be at once seen from these analyses, that liquid manure so obtained must be extremely rich in ammonia, but deficient in phosphates; and as the nitrogenous matters of urine pass with great rapidity into the form of ammonia, it must act quickly, and produce its best effects on those crops which admit of being rapidly forced on to maturity. The deficiency of phosphates in the urine of the common domestic animals is an objection to the use of that fluid alone. But where the drainings of the manure-heap are employed, this difficulty is done away with, for they generally contain a certain quantity of phosphates, either in solution, or more probably in suspension in the fluid. The following analyses by Professor Johnston give the composition, No. 1 of the drainings of the manure-heaps when exposed to rain; No. 2 of the drainings when moistened with cows' urine pumped over it. The numbers give the quantities in grams contained in a gallon of the fluid:
| | No. 1 | No. 2 | |----------------|-------|-------| | Ammonia | 96 | 21-5 | | Organic matter | 200-8 | 77-6 | | Ash | 268-8 | 518-4 |
Total solids in a gallon ..... 479-2 617-5
The ash contained,— - Alkaline salts ..... 207-8 420-4 - Phosphates ..... 25-1 44-5 - Carbonate of lime ..... 18-2 31-1 - Carbonate of magnesia, and loss ..... 4-3 3-4 - Silica and alumina ..... 13-4 19-0
268-8 518-4
The method of liquid manuring employed by Mr Kennedy at Myremill, the results of which have excited so much interest, is different from liquid manuring in its strict sense, for not only are the drainings of the manure-heap employed, but the whole solid excrements are mixed with water in a tank, and rape-dust and other substances occasionally added, and distributed through the pipes.
No system of manuring has produced more striking effects than liquid manuring; and especially on grass-lands, the rapidity of its action is such as to produce an extremely abundant vegetation, much greater, indeed, than could be produced by the application of the solid manure. The luxuriance of the growth of grass by its means is, however, apt to lead us to overrate its general effects; and it is by no means so certain that it can be advantageously applied to the general operations of the farm. Its effect on the cereals is certainly much less marked than that on ryegrass and root-crops, and we have not yet seen the effects of its continued use. Experience and theory concur in holding that a supply of solid matters capable of evolving carbonic acid, is essential to the fertility of the soil, so that, by acting on the mineral matters, it may cause their continuous decomposition. The objection to liquid manure is, that it does not supply these substances in sufficient quantity. This difficulty is no doubt got rid of, to some extent, in the Ayrshire plan, which, after all, is rather a new method of applying the solid manure of the farm than strictly liquid manuring; but still, as no litter is employed, the quantity of organic matters capable of evolving ammonia is greatly less than it is by the old method; and we must consider the Ayrshire mode of liquid manuring as a great experiment of which all will await the results with interest. Of course the pecuniary question is also of much importance in this method; but that is a matter which we only indicate here, as it is treated of in full in the article on Practical Agriculture.
Vegetable Manures.—Many vegetable substances are employed as manures, and their value is variable, and must be estimated in the same manner as that of farm-yard manure, the quantity of nitrogen and phosphoric acid greatly exceeding in importance that of the other constituents. Although like farm-yard manure they may be made to undergo fermentation so as to convert their nitrogen into ammonia, they are generally, indeed almost invariably, applied in the unfermented state, seldom alone, and most commonly combined with farm-yard manure.
Rape-Dust, Castor-Cake, Poppy-Cake, &c.—Rape-dust has long been employed as a manure, and the success which has attended its use has led to the introduction of the refuse cake from some other oil seeds, such as that of the castor-oil seed, which cannot be employed for feeding. Like the seeds of all plants, these substances are rich in nitrogen, and their ash, containing of course all the constituents of the plant, supplies the necessary inorganic elements. The following are analyses of these substances, which, in addition to the amount of nitrogen and phosphates, show also that of water and oil, to which we shall have occasion afterwards to refer in relation to the feeding value of some of them.
| | Rape-dust | Poppy-cake | Castor-cake | |----------------|-----------|------------|------------| | Water | 10-68 | 11-63 | 11-19 | | Oil | 11-10 | 5-95 | 9-08 | | Albuminous compounds | 29-53 | 31-16 | 25-16 | | Ash | 7-79 | 12-98 | 5-64 | | Other constituents | 40-90 | 38-18 | 48-93 |
100-00 100-00 100-00
Nitrogen ..... 4-88 4-94 3-95 3-20 Silica ..... 1-18 3-36 1-32 1-96 Phosphates ..... 3-87 6-93 2-19 2-81 Phosphoric acid in combination with alkaline | 0-39 3-27 0-15 0-64
A general similarity will be observed in the composition of all these substances; they are all rich in nitrogen, and contain as much of that element as is found in about ten times their weight of farm-yard manure, and a somewhat similar proportion exists in the amount of phosphates and probably of their other constituents. They have all been employed with success, but the most accurate experiments have been made with rape-dust, which has been longer and more extensively used than any of the others. It has been employed alone for turnips, or mixed with farm-yard manure, and also as a top-dressing to cereals. The most marked advantage is derived from it when applied in the latter way on land which has been much exhausted, on which its effects are often very striking. Several circumstances are essential to the production of its full effects. It requires moisture, and hence it often proves a failure in very dry seasons, and on dry soils. It must not be applied in too large a quantity, experience having shown that after a certain point has been reached, an increase in the quantity not only does not increase but positively diminishes the crop. The most advantageous application is found to be from five to seven cwt. per acre. The observations in regard to the use of rape-dust probably apply with equal force to the other substances of the same class; but their application being more recent and more limited, the results of their use have not been made public.
**Malt-Dust, Bran, Cluff, &c.**—All these substances have been applied as manures, and their value is principally dependent on the quantity of nitrogen they contain, which in malt-dust amounts to 4-5 per cent. of nitrogen, and in bran to about 3-2. They must therefore be made to rank with rape-dust in point of value.
**Straw** has been occasionally employed as a manure, and sometimes even as a top-dressing for grass land. It is generally admitted, however, that its application in the dry state, and especially as a top-dressing, is a practice not to be recommended, as it decomposes too slowly in the soil; and it is always desirable to ferment it in the manure heap, so as to facilitate the production of ammonia from its nitrogen. Still circumstances may occur in which it becomes necessary to employ it in the dry state, and it will generally prove most valuable on heavy soils, which it serves to keep open, and so promotes the access of air, and enables it to act on the soil. On light sandy soils it generally proves less advantageous, as its tendency of course is to increase the openness of the soil, and render it less able to retain the essential constituents of the plant. The manurial value of straw alone is low, as it contains only about 0-2 per cent. of nitrogen, or about half as much as farm-yard manure.
**Saw-Dust** has little value as a manure, as it undergoes decomposition with extreme slowness. It is a good mechanical addition to heavy soils, and diminishes their tenacity; and though its manurial effects are small, it sooner or later undergoes decomposition, and yields what valuable matters it contains. It is a useful absorbent of liquid manure, and may be advantageously added to farm-yard manure for that purpose.
**Manuring with Fresh Vegetable Matter.—Green Manuring.**—The term green manuring is applied to the ploughing in of green vegetable matter which has been grown on the soil for that purpose. The success which attends it, especially on soils poor in organic matter, is very marked. Its utility is manifestly dependent on its affording to the soil a supply of matter which by its decomposition may yield carbonic acid to act on the soil, as well as nitrogen and inorganic matters. The action is not, however, confined to this, for it serves also as a means of bringing up from the lower parts of the soil the valuable matters it contains, and of mixing them again with the surface part. Many of the plants found most useful for green manuring send down their roots to a considerable depth; and when they are ploughed in, all the substances which they have brought up are of course deposited in the upper few inches of the soil. Plants when ploughed in in the fresh state also decompose rapidly, and are therefore able immediately to improve the subsequent crop; and as this decomposition takes place in the soil without the loss of ammonia and other valuable matters, which infallibly occurs when they are fermented on the dung-heap, it will be obvious that in no other mode can equally good results be obtained by the use of these plants.
Many plants have been employed as green manure, and different opinions have been expressed as to their relative values. In the selection of any one for the purpose, that should of course be taken which grows most rapidly, and produces within a given time the largest quantity of valuable matters. No general rule can be given for the selection, as the plant which fulfils those conditions best will differ in different soils and climates. The plants most commonly employed in this country are spurry, white mustard, and turnips. Rye, clover, buck-wheat, white lupins, rape, borage, and some others, have been largely employed abroad. Some of these are obviously unfitted for the climate of the British Islands; and the others, although they have been tried occasionally, do not appear to have been very extensively employed. The turnip is sown broadcast at the end of harvest, and ploughed in after two months. White mustard and spurry are employed in the same way as a preparation for winter wheat, and with the best results. The latter is sometimes sown as a spring crop in March, ploughed in in May, and another crop sown which is ploughed in in June, and immediately followed by a third. The effect of this treatment is such that the worst sands may be made to bear a remunerative crop of rye.
**Sea-Weed.**—Sea-weeds are very extensively employed on the coasts of Scotland and England, in quantities varying from 10 to 20 tons per acre as a manure. Their action is necessarily similar to that of green manure ploughed in, as they contain all the ordinary constituents of land plants. Their nitrogen usually amounts to about 2-1 per cent. of the dry substances, and they are much richer in ash than ordinary land plants. The dry fucus saccharinus contains 28 per cent., and fucus vesiculosus about 20 per cent. The following are analyses of the ash of three species of sea-weeds from the Firth of Forth.
| Laminaria digitata | Fucus serratus | Fucus nodosus | |-------------------|---------------|--------------| | Potash | 31·812 | 30·870 | 14·320 | | Chloride of potassium | 19·764 | 6·148 | 29·885 | | Iodide of potassium | 1·385 | | | | Chloride of sodium | 23·966 | 25·859 | 15·557 | | Lime | 5·351 | 7·927 | 7·647 | | Magnesia | 3·454 | 6·368 | 5·636 | | Peroxide of iron | 1·333 | 0·230 | 0·135 | | Sulphuric acid | 9·598 | 17·870 | 24·812 | | Phosphoric acid | 3·287 | 2·480 | 0·848 | | Silica | 0·050 | 2·248 | 1·160 |
The great value of sea-weed is dependent on the rapidity with which it decomposes. In fact, when spread on the land, it is seen to soften and disappear in a very short time. It is therefore a rapid manure, and its effects are almost entirely confined to the crop to which it is applied. It may be used as a top-dressing to grass land; but it is most beneficial when ploughed in green, or when made into a compost with lime and earth. On the western coast of Scotland and in the Hebrides sea-weed is the chief manure. It gives excellent crops of potatoes, but they are said to be of inferior quality, unless marl or shell sand is employed at the same time.
The observations which have been made regarding the manurial value of these substances, immediately lead to the inference that all vegetable matters possess a certain value, and that they ought to be carefully collected and preserved. In fact, the careful farmer adds every thing of the sort to his manure heap, where, by undergoing fermentation along with the manure, their nitrogen becomes immediately available to the plant; while during the fermentation the seeds of weeds are destroyed, and the risk of the land being rendered dirty by their springing up when the manure is used, is prevented.
**Animal Manures.**—Animal substances generally contain a much larger quantity of nitrogen than vegetables, and as they undergo decomposition and yield it in the form of ammonia more rapidly, their value is much higher.
**Flesh** is an important manure. That of horses is prepared and sold to some extent. The dead animal after being skinned is cut up and boiled in large cauldrons until the flesh separates from the bones. The latter are removed, and the flesh dried upon a flat stove. The flesh as sold has the following composition:
| Component | Percentage | |--------------------|------------| | Water | 12·17 | | Organic matter | 78·44 | | Phosphate of lime, &c. | 3·82 | | Alkaline salts | 3·64 | | Sand | 1·93 | | **Total** | 100·00 |
Nitrogen: 9·22
Ammonia to which the nitrogen is equivalent: 11·20
Another sort of "flesh manure" has been recently imported from South America. It is a mixture of the flesh and smaller bones of cattle which have been slaughtered for their tallow, and remains in the vats in which the separation of the tallow is effected by steaming. Owing to the variable proportion of bones and flesh, and to the mixture of sand, which takes place, owing to the careless way in which it has been preserved, it varies somewhat in composition. Its composition is,
| Component | Percentage | |--------------------|------------| | Water | 9·05 | | Fat | 11·13 | | Animal matter | 39·52 | | Phosphate of lime | 28·74 | | Carbonate of lime | 3·81 | | Alkaline salts | 0·57 | | Sand | 7·18 | | **Total** | 100·00 |
Nitrogen: 5·56
Ammonia to which the nitrogen is equivalent: 6·67
Another sample contained 5·77 of nitrogen, 17·16 of phosphate of lime, and 18·78 of sand.
Considerable difference must necessarily exist in the effects of these two manures, owing to the difference of their composition. The first must owe its value entirely to nitrogen, the quantity of phosphate of lime and alkaline salts being too small to exert any influence of importance. In the South American manure, however, the quantity of bones raise that of phosphate of lime in the first instance to above a fourth, and in the second to nearly a fifth, of the whole weight, and must therefore cause it to act, to a great extent, in the same manner as bones, to the manurial value of which we shall presently refer.
**Fish** have been employed in considerable quantity as a manure. That most extensively employed in this country is the sprat, which is occasionally caught in enormous quantities on the Norfolk coast, and used as an application for turnips. They are sold at 8d. per bushel, and their composition is,
| Component | Percentage | |--------------------|------------| | Water | 64·6 | | Organic matter | 33·3 | | Ash | 2·1 | | **Total** | 100·00 |
Nitrogen: 1·90
Phosphoric acid: 0·91
The refuse of herring and other fish curing establishments, whales' blubber, and similar fish refuse are all useful as manure, and are employed whenever they can be obtained. They are not usually employed alone, but are more advantageously made into composts with their own weight of soil, and allowed to ferment thoroughly before being applied.
**Blood** is a most valuable manure, but it is not much employed in this country, at least in the neighbourhood of large towns, as there is a demand for it for other purposes, and it can rarely be obtained by the farmer in large quantity. In its natural state it contains about 3 per cent. of nitrogen, and after being dried up the residue contains about 15 per cent. It is best used in the form of a compost with peat or mould, and this forms an excellent manure for turnips, and is also advantageously applied as a top-dressing to wheat.
**Hair, Skin, and Horn.**—The refuse of manufactories in which these substances are employed, are frequently used as manures. They are all highly nitrogenous substances, and owe their entire value to the nitrogen they contain, their inorganic constituents being in too small quantity to be of any importance, wool and hair having only 2 per cent., and horn 0·7 per cent. of ash. In the pure and dry state, and after subtraction of the ash, their composition is,
| Component | Skins | Hairs | Hair | Wool | Horn | |--------------------|-------|-------|------|------|------| | Carbon | 50·99 | 50·65 | 50·65| 51·99| 51·99| | Hydrogen | 7·07 | 6·36 | 7·03 | 6·72 | 6·72 | | Nitrogen | 18·72 | 17·14 | 17·71| 17·28| 17·28| | Oxygen | 23·22 | 20·85 | 24·61| 24·01| 24·01| | Sulphur | | | 5·00 | | |
The refuse actually obtained is always moist and often mixed with foreign matters, and is consequently inferior to this. Refuse horse hair generally contains 11 or 12 per cent. of nitrogen. Wool contains very different quantities according to the kind of refuse. Woollen rags contain 12·7 per cent. of nitrogen; woollen cuttings about 14; and what is called shoddy only 5·5 per cent. Horn shavings are extremely variable in their amount of nitrogen. When pure they sometimes contain as much as 12·5 per cent., but a great deal of the horn shavings from comb manufactories, &c., contains much sand and bone dust, by which their percentage of nitrogen is greatly diminished, and it sometimes does not exceed 5 or 6 per cent.
All these substances are highly valuable as manures, but as they undergo decomposition more slowly than flesh or blood they are more applicable to slow growing crops, and to those which require a strong soil. Woollen rags have been largely employed as a manure for hops, and are believed to surpass every other substance for that crop. As a manure applicable to the ordinary purposes of the farm they have scarcely met with that attention which they deserve, probably because their first action is slow and the farmer is more accustomed to look to immediate than to future results; but they possess the important qualification of adding permanently to the fertility of the soil.
**Urate and Sulphated Urine.**—We have already discussed the urine of animals, in reference to farm-yard manure. But human urine, the composition of which was then stated, is of much higher value than that of the lower animals, and many attempts have been made to preserve and convert it into a dry manure. Urate is prepared by adding gypsum to urine, and collecting and drying the precipitate produced. It contains a considerable quantity of the phosphoric acid of the urine, but very little of its ammonia; and as the principal value of urine depends on the latter, it is necessarily a very inefficient method of turning it to account. A better method has been proposed by Dr Stenhouse, who adds lime-water to the urine, and collects the precipitate, which, when dried in the air, contains 1·91 per cent. of nitrogen, and about 41 per cent. of phosphates. This method is subject to the same objection as that by which urate is made, namely, that the greater part of the ammonia is not precipitated. This might probably be got over to some extent by the addition of sulphate of magnesia, or, still better, of chloride of magnesium, which would throw down the phosphate of magnesia and ammonia. By much the best mode of employing urine is in the form of sulphated urine, which is made by adding to urine a sufficient quantity of sulphuric acid to neutralise its ammonia, and evaporating to dryness. In this form all the valuable constituents are retained, and excellent results are obtained from it. Its effects, though mainly attributable to its ammonia, are also in part dependent on the phosphates and alkaline salts which it contains; and it is therefore capable of supplying to the plant a larger number of its constituents than the animal matters already mentioned.
Night-Soil and Poudrette.—The value of night-soil as a manure is well known. It depends, of course, partly on the urine, and partly on the feces of which it is formed. Its disagreeable odour has prevented its general use, and various methods have been contrived both for deodorising and converting it into a solid and portable form. The same difficulties which beset the conversion of urine into the solid form are found here, and in most of the methods employed the loss of ammonia is great. It is sometimes mixed with lime or gypsum, and dried with heat, and sometimes with animal charcoal or peat charcoal. By none of these methods, however, is it obtained of high quality; and a good method of making it portable at small expense is still a desideratum. It usually contains about 2 per cent. of nitrogen, and 6 of phosphoric acid.
Guano is the solid excrement of carnivorous sea-birds, which is accumulated in immense quantities on the coasts of South America and other tropical countries. It has been used as a manure in Peru from time immemorial, but the accounts given by the older travellers of its marvellous effects were considered to be fabulous, until Humboldt, from personal observation, confirmed all their statements. It was first imported into this country in 1840, in which year a few barrels of it were brought home; and from that time its importation rapidly increased. Soon after large deposits of it were found in Ichaboe; and it has since been brought from many other localities.
The value of guano differs greatly according to the locality from which it is obtained. That from the rainless districts of Peru contains the ingredients of the dung comparatively little changed, a considerable proportion of the uric acid and ammonia of the urine existing in some instances in its natural state, and a small quantity only having undergone decomposition. But that from other districts has suffered a more or less complete decomposition according to the moisture of the climate, which reduces the quantity of organic matters and ammonia, until, in some varieties, they are so small as to be of little importance. The following are minute analyses of three specimens of Peruvian guano, showing all the different constituents it contains, and the amount of difference which may exist:
| Component | L | H | III | |----------------------------|---|---|-----| | Urate of ammonia | 10·70 | 9·9 | 3·24 | | Oxalate of ammonia | 12·38 | 10·6 | 13·35 | | Oxalate of lime | 5·44 | 7·0 | 16·36 | | Phosphate of ammonia | 19·25 | 6·0 | 6·45 | | Phosphate of magnesia and ammonia | 2·6 | 4·20 | | Sulphate of potash | 4·50 | 5·5 | 4·23 | | Sulphate of soda | 1·45 | 3·8 | 1·12 | | Sulphate of ammonia | 3·36 | ... | ... | | Mariate of ammonia | 4·81 | 4·2 | 6·50 | | Phosphate of soda | ... | ... | 5·29 | | Chloride of sodium | ... | ... | 0·10 | | Phosphate of lime | 15·55 | 14·3 | 9·94 | | Carbonate of lime | 1·89 | ... | ... | | Sand and alumina | 1·29 | 4·7 | 5·80 | | Water | 9·14 | ... | ... | | Undetermined humus-like organic matters | 10·00 | 32·3 | 23·42 |
These analyses illustrate two points—first, that in different samples the decomposition may have advanced to different extents; for we observe that the quantity of uric acid, or rather of urate of ammonia, is greatly less in the last analysis than in the other two, and much smaller than in the fresh dung, which contains from 50 to 90 per cent. of uric acid; and secondly, that guano is rich in all the constituents of the plant, but especially in ammonia, the best form in which nitrogen can be supplied in uric acid, which, by decomposition, yields ammonia, and in phosphoric acid. But such analyses are too elaborate for ordinary purposes; and a less complete analysis is usually made, in which the total quantity of ammonia, with that which exists ready formed, and will be yielded by the uric acid, the quantity of water, the loss by ignition (that is, the total quantity of organic matter and ammoniacal salts), the water, sand, and alkaline salts, are determined. The subjoined tables give the average composition of different sorts of guano determined in this way. They are mostly averages deduced from a very large number of analyses, excepting those of recent Ichaboe, and of old Bolivian. In the second table are given the analyses of a number of other sorts of guano, but some of them only from a single analysis, so that they probably do not accurately represent the average, although they may give some idea of the composition of each sort. Two of the sorts—old Ichaboe and old Bolivian—are not now imported, the supplies being exhausted.
Table showing the Average Composition of different varieties of Guano.
| Component | Angamos | Persian | Ichaboe | Bolivian | Latham Island | Saldanha Bay | Australian | Patagonian | Chilian | |----------------------------|---------|---------|---------|----------|---------------|--------------|------------|------------|---------| | Water | 12·36 | 13·73 | 24·21 | 15·89 | 12·55 | 16·44 | 14·15 | 24·96 | 21·03 | | Organic matter & ammoniacal salts | 59·92 | 53·16 | 39·30 | 32·49 | 35·89 | 12·28 | 26·14 | 10·96 | 14·93 | | Phosphates | 17·01 | 23·48 | 30·00 | 19·63 | 27·63 | 56·09 | 23·13 | 54·47 | 56·40 | | Sulphate of lime | ... | ... | ... | ... | 9·65 | 2·82 | ... | 4·55 | 1·30 | | Carbonate of lime | ... | ... | ... | ... | 12·87 | 2·20 | ... | 8·82 | 3·06 | | Alkaline salts | 7·20 | 7·97 | 4·19 | 8·62 | 15·29 | 11·33 | 5·97 | 4·06 | 6·10 | | Sand | 3·51 | 1·66 | 2·30 | 6·72 | 8·64 | 2·51 | 8·09 | 0·51 | 1·54 | | Ammonia | 21·10 | 17·50 | 8·50 | 10·42 | 8·99 | 2·57 | 3·26 | 1·26 | 1·62 | | Phosphoric acid in alkaline salts | 1·20 | 2·50 | ... | ... | 3·11 | ... | ... | ... | 3·00 |
100·00 100·00 100·00 100·00 100·00 100·00 100·00 100·00 100·00 100·00 100·00 100·00 Table showing the Composition of some of the less common varieties of Guano.
| | Sea Bear Bay | Holmes's Bird Island | Ascension Island | Possession Island | Algoa Bay | New Island | Bird's Island | |----------------------|--------------|---------------------|------------------|-------------------|-----------|------------|--------------| | Water | 30-82 | 25-00 | 15-97 | 10-92 | 30-35 | 28-78 | 16-52 | | Organic matter and ammoniacal salts | 31-78 | 32-10 | 23-15 | 15-42 | 6-35 | 13-78 | 14-84 | | Phosphates | 24-33 | 27-36 | 32-54 | 46-41 | 21-24 | 22-46 | 25-21 | | Sulphate of lime | 3-84 | ... | ... | 7-46 | 36-42 | ... | 40-47 | | Carbonate of lime | 0-58 | ... | ... | ... | ... | 13-78 | ... | | Alkaline salts | 7-38 | 8-82 | 15-92 | 6-15 | 3-32 | 12-62 | 1-16 | | Sand | 1-27 | 6-72 | 12-42 | 13-64 | 1-62 | 11-58 | 1-80 |
| | 100-00 | 100-00 | 100-00 | 100-00 | 100-00 | 100-00 | 100-00 | | Ammonia | 10-45 | 7-75 | 6-06 | 1-34 | 0-54 | 0-84 | 1-26 | | Phosphoric acid in alkaline salts | ... | ... | ... | ... | ... | ... | ... |
As with farm-yard manure, the value of guano is estimated by the quantity of nitrogen and phosphates which it is capable of yielding to the crop. As the nitrogen, however, exists in great part as ammonia, and the remainder in a state in which it readily passes into that substance, it is customary in the analysis to state the quantity of ammonia and not that of nitrogen, but it is easy to calculate the amount of the latter by bearing in mind that 17 parts of ammonia correspond to 14 of nitrogen.
By examining the tables given above, it is obvious that guanos may be divided into two classes, the one characterised by the abundance of ammonia, the other by that of phosphates; and which, for convenience sake, may be called ammoniacal and phosphatic guano. Peruvian and Angamos are characteristic of the former, and Saldanha Bay and Bolivian of the latter class. Of course the value of these varieties is very different; and as guano is an expensive manure, it is of much importance that some ready means of estimating the value of different samples should be known to the farmer. The principles that guide the chemist in making such an estimate are very simple. As the value of a guano depends on the quantity of ammonia and phosphates it contains, and as these are commercial articles, which have a definite value in the market, all that is necessary is to ascertain that value, and to calculate from it that of the guano. Now when sulphate of ammonia is bought, we find by calculation from its price, that the dry ammonia contained in it costs very nearly 6d. per pound, and in bones the cost of phosphate of lime is about 3½d. per pound.
In addition to this, we have also a certain quantity of phosphoric acid in the alkaline salts, which is equal in round numbers to double its weight of phosphate of lime. But this phosphoric acid being in a soluble state is worth more than twice as much as phosphate of lime. It is somewhat difficult to estimate its exact value, but we shall probably not be far wrong in assuming it at 3½d. per lb. If we calculate the value of Peruvian guano upon this principle we obtain the following results per ton:
- 17½ per cent. of ammonia equal to 392 lb. per ton, value at 6d. per lb., L.9 16 0 - 23-48 per cent. of phosphates equal to 526 lb. at 3½d. per lb., 1 13 0 - 2½ per cent. of phosphoric acid equal to 56 lb. per ton at 3½d. per lb., 14 0
Total value per ton, L.12 3 0
But the price of Peruvian guano is from L.9, 10s. to L.10 per ton; and this being the case, either it must be bought for its ammonia alone or else we buy ammonia in guano at a cheaper rate that we do in any other form. The latter is probably the most correct view of the case; and if so we find that the price paid for ammonia in Peruvian guano is rather less than 3½d. per lb. Calculated on this principle, the value of a ton of average Peruvian guano is L.9, 13s. 8d., which is almost exactly the price at which it is sold. By calculating in this way, then, we can at once estimate the value of any sample of guano. A very simple method of effecting the calculation is the following:—If a pound of ammonia be worth 5½d., a ton will be worth L.45; in the same way we find that a ton of phosphates is worth L.7, and of phosphoric acid in the alkaline salts L.28. If then we multiply the per-cent-age of ammonia by 45, that of phosphates by 7, and of phosphoric acid by 28, the sum of the products will give the price in pounds sterling of 100 tons of guano, and then dividing by 100 and doubling the first decimal, we have the price of a ton within a shilling or two. Applying this method of calculation to the Government Bolivian guano we find its value to be as follows:
- Phosphates, 56-00 × 7 = 392-6 - Phosphoric acid, 3-11 × 28 = 87-0 - Ammonia, 2-57 × 45 = 115-6
Value of 100 tons, 593-2
The value of a ton will therefore be about L.5, 18s. But the usual selling price of Bolivian guano is about L.8 per ton, so that it actually sells at a much higher price than its value calculated in this way would warrant. By a similar calculation we find the value of Saldanha Bay would be L.4, 13s., its selling price at present being L.6, 10s.
This method of calculation, therefore, while it may be used with advantage for the comparison of different ammoniacal guanos, is not applicable to the phosphatic class, as it gives a lower value than that at which they are sold. It may be urged that these guanos are sold at a higher price than the value of their constituents warrants, but the question is not limited to this point. The two classes of guanos are bought for different purposes, the ammoniacal guanos for the sake of their ammonia principally, the phosphatic for their phosphates, and these prices would not be given for the latter sorts of guano unless the farmer found his advantage by it. We shall see afterwards that though Peruvian guano is generally the best, there are certain soils on which the phosphatic guanos nearly or altogether equal it; and on these soils, of course, Bolivian guano at L.8 is actually cheaper than Peruvian at L.9, 10s.; but this is only the case in particular instances, and taken as a whole it may be said that Peruvian, notwithstanding its high price, is the cheapest of all guanos.
In purchasing guano particular precautions are required on the part of the farmer, in order to avoid the risk of obtaining an adulterated article. He ought to attend to the following points in regard to Peruvian guano.
1st. The guano should be light coloured. If it is dark, the chances are that it has been damaged by sea-water.
2d. It should be dry, and when a handful is well squeezed together it should cohere very slightly.
3d. It should not have too powerful an ammoniacal odour.
4th. It should contain lumps which, when broken, appear of a paler colour than the powdery part of the sample.
5th. When rubbed between the fingers it should not be gritty.
These characters must not, however, be too implicitly relied on, for they are all imitated with wonderful ingenuity by the skilful adulterator, and they are applicable only to Peruvian guano; the others being so variable that no general rules can be given for determining whether they are genuine. With them as well as with Peruvian guano, the only safe mode of detecting adulteration is by analysis; and it is desirable even where there is no chance of adulteration having been practised, to determine in this way the value of the guano, as different cargos of the same sort differ materially in this respect. In the table above we have given the average composition of the different guanos, but in order to show how much individual cargos may differ from the mean, we give here analyses of samples of the highest and lowest quality of the genuine guanos of most importance.
| Guanos | Peruvian | Bolivian | |--------|----------|----------| | Highest | Lowest | Highest | Lowest | Highest | Lowest | | Water | 12-60 | 7-09 | 10-37 | 21-49 | 11-53 | 16-20 | | Organic matter and ammoniacal salts | 65-62 | 50-83 | 55-73 | 46-26 | 11-17 | 12-86 | | Phosphates | 10-83 | 8-70 | 25-20 | 18-93 | 62-99 | 52-55 | | Alkaline salts | 7-50 | 16-30 | 7-50 | 10-64 | 9-83 | 13-83 | | Sand | 3-45 | 17-68 | 1-20 | 2-68 | 4-38 | 4-16 |
Calculating, in the manner before given, the value of these two samples of Peruvian guano, we find the highest to be worth L10, 6s. per ton, the lowest no more than L7, 18s. Something would have to be added for the phosphoric acid in the alkaline salts which have not been determined in these analyses, but this would not materially alter their relative values.
The adulteration of guano is carried on to a very large extent, and we are certainly within bounds in asserting that one-half of all the guano sold in this country is adulterated. The chief adulterations are a sort of yellow loam, very similar in appearance to guano, sand, gypsum, common salt, and apparently also ground coprolites. The extent to which it is adulterated may be estimated from the following analyses taken at random from those of a large number of guanos, all of which were sold as first-class Peruvian.
| Water | 12-85 | 15-19 | 12-06 | 27-86 | 6-32 | | Organic matter and ammoniacal salts | 26-64 | 44-31 | 31-44 | 30-41 | 27-42 | | Phosphates | 15-54 | 20-95 | 22-08 | 22-17 | 33-61 | | Sulphate of lime | 6-07 | 9-40 | 12-81 | 7-92 | 22-50 | | Sand | 38-70 | 10-15 | 7-83 | 1-64 | 10-15 |
In all these cases a very large depreciation in value has taken place; the first of them, by calculation, being worth only L5, 5s. per ton. Large quantities of similarly adulterated guanos are annually sold at the price of the genuine article. The adulteration is principally carried on in London, but it is believed that it is also practised, though not so largely, in other places. In most instances these guanos are sold without analysis, and with the assurance on the part of the seller that they are genuine guano, which some farmers seem to consider all that is required. Others are sold with analysis, and sometimes it occurs that large quantities of adulterated and inferior guanos are sold by the analysis of a genuine sample, but of course this is the practice of the fraudulent dealer only. In order to insure obtaining a genuine guano, none should ever be purchased without an analysis, a comparison of which with the average composition of good guano, enables the buyer to ascertain its quality, and when the supply is obtained another sample should be selected of which an analysis should be obtained, in order to ascertain that the stock corresponds with the analysis by which it was sold—a very necessary precaution. It may be objected that this involves expense, but surely the cost of an analysis is a very trifling matter when a farmer buys perhaps L100 or L200 worth of a manure which adulteration may reduce to half its value.
The value and use of guano are now so well understood, that it will scarcely be necessary to enlarge on the mode of its application. Although owing its chief value to ammonia and phosphates, it contains also all the other ingredients of the plant, and everything required in a manure except the large quantity of organic matters capable of producing carbonic acid, on the importance of which to the soil we have already enlarged. It is capable of entirely replacing farm-yard manure, and excellent crops of turnips and potatoes have been raised by it alone, and at less cost than by farm-yard manure. But though this can be done, it is a practice not to be recommended, for the quantity of valuable matters in an ordinary application of guano is much smaller than in farm-yard manure, and not sufficient permanently to sustain the fertility of the soil. Five cwt. of Peruvian guano, which is a fair application to an acre, contains about 97 lb. of ammonia and 138 of phosphates, but 30 tons of farm-yard manure contain about 280 of ammonia and 360 of phosphates, and consequently the effects of the guano must be much more rapidly exhausted than those of the farm-yard manure. In fact guano is a rapidly acting manure, and its effects are principally observed on the crop to which it is applied. It is not of course to be denied that a certain effect will be experienced on the subsequent crops, but it must of necessity be small. The inference from these facts is, that, though guano may at an emergency be used as an entire substitute for farm-yard manure, the practice is one to be generally avoided. But the rapidity of action of guano makes it a most important auxiliary to farm-yard manure, and it is an auxiliary that the greatest benefit has been derived from it. Experience has shown that one-half the farm-yard manure may be replaced by guano with the production of a larger crop than by the former alone in its full quantity. The proportion of guano usually employed is from three to five cwt., and it is said that a much larger quantity produces prejudicial effects on the subsequent crop, although it is not very easy to see on what it depends.
Guano has also been most advantageously employed as a top-dressing to grass land and to young corn.
In selecting the variety to be employed, several circumstances must be attended to. It will be found as a general rule that on strong soils, under good cultivation, the best effects are obtained from the ammoniacal guanos, but on light soils these guanos are less applicable, as the soluble ammoniacal compounds they contain are rapidly washed out, and much of their effects lost. On such soils the phosphatic guanos come up to, or even surpass, the others. No definite rules can be given for determining the soils on which these different varieties are most applicable, but each individual must determine by experiment that which best suits his own farm; and the inquiry is of much importance to him, as, of course, if the phosphatic guanos will answer as well as the ammoniacal, there is a large saving in the cost of the manure. A very excellent practice is to employ a mixture of equal parts of the two sorts of guano.
Pigeons' Dung.—The dung of all birds, which more or less closely resembles guano, may be employed with much advantage as a manure, but that of the pigeon and the common fowl are the only ones which can be got in quantity. Pigeons' dung, according to Boussingault, contains 8-3 per cent. of nitrogen, equivalent to 10-0 of ammonia. Its value, therefore, will be more than half that of guano, but it varies greatly, and a sample imported from Egypt into this country, and analysed by Professor Johnston, contained only 5-4 per cent. of ammonia. Hens' dung has not been accurately analysed, but its value must be about the same as pigeons'.
Bones.—Bones appear to have been employed agriculturally to a considerable extent in the last century as a dressing to old exhausted pasture lands in Cheshire; but it is only during the present century that they have been employed on arable land. The bones employed as a manure are always moist, and vary to some extent in quality. The following is an analysis of a very excellent sample, consisting, we believe, mostly of the bones of the horse:
| Component | Percentage | |--------------------|------------| | Water | 6-20 | | Organic matter | 39-13 | | Phosphate of lime | 48-95 | | Lime | 2-57 | | Magnesia | 0-30 | | Sulphuric acid | 3-15 | | Silica | 0-30 | | Ammonia which the organic matter is capable of yielding | 4-80 |
In general, bones may be said to contain about half their weight of phosphate of lime, and 10 or 12 per cent. of water. But besides these, bones are met with in other forms in commerce, in which their organic matter has been extracted either by boiling or burning. The latter is especially common in the form of the spent animal charcoal of the sugar refiners, which usually contains from 70 to 80 per cent. of phosphate of lime, but of course does not yield ammonia, the organic matters having been entirely destroyed by heat.
From the analysis given above, it is obvious that the manurial value of bones is dependent partly on their phosphates and partly on the ammonia they yield. It has been common to attribute their entire effects to the former, but this is manifestly erroneous. It is true that in some instances this may be the case, but there is no doubt that the ammonia must generally be of importance, and ought to be taken into account in estimating their value. When bones are applied for the sake of their phosphates alone, burnt bones or the spent animal charcoal of the sugar-refiners are to be preferred.
At the first introduction of bones they were applied in large fragments, and in quantities of from 20 to 30 cwt. per acre. As their use became more general they were gradually employed in smaller pieces, until at last they were reduced to dust, and it was found that, in a fine state of division, a few hundredweights produced as great an effect as the larger quantity of the unground bones. Even the most complete grinding which can be attained, however, leaves the bones in a much less minute state of division than guano, and they necessarily act more slowly than it does, the more especially as they contain no ready-formed ammonia. They may be still further reduced by fermentation, which acts by decomposing the organic matter, and causing the production of ammonia. Or by solution in sulphuric acid, which converts the insoluble phosphate of lime into a soluble state.
Dissolved Bones.—The method of dissolving bones in sulphuric acid has proved a very important boon to agriculture. It depends upon the fact that there exists a phosphate of lime containing half as much lime as that which is found naturally in the bones, and which is artificially produced by the sulphuric acid, which withdraws one-half of the lime, forming with it sulphate of lime, while the whole of the phosphoric acid of the bones remains in combination with the other half, and in a soluble form. By employing a sufficiently large quantity of sulphuric acid, the whole quantity of phosphoric acid in the bones may be thus brought into a soluble state, but in actual practice it is found preferable to leave part of it in both states; as where it is entirely soluble, its effect is too great during the early part of the season, and deficient at its end. In order to dissolve bones, we employ from one-half to one-fourth of their weight of strong sulphuric acid; but one-third will be found to be the quantity most generally applicable. The bones are put into a vessel of wood, stone, or lead (iron is to be avoided, as it is rapidly corroded by the acid), and mixed with one-third their weight of water, which may with some advantage be used hot. One-third their weight of sulphuric acid is then added, and mixed as uniformly as possible with the bones. Considerable effervescence takes place, and the mass becomes extremely hot. At the end of two or three days it is turned over with the spade, and after standing for some days longer, generally becomes pretty dry. Should it still be too moist to be sown, it must be again turned over, and mixed with some dry substance to absorb the moisture. For this purpose all substances containing lime or its carbonate must be carefully avoided, as they bring back the phosphates into the insoluble state, and undo what the sulphuric acid has done. Dry loam, peat, decaying leaves, or similar substances may be used. An excellent plan is to sift the bones before dissolving, and, applying the acid only to the coarser part, to mix in the finer dust which has passed through the sieve, to dry up the mass. Considerable trouble attends the manufacture of superphosphate on the small scale, and large manufactories have sprung up in which it is manufactured for sale, and is sold at about L7 per ton. This probably exceeds the cost at which superphosphate can be prepared at the farm, but the saving of trouble induces many persons to purchase it. Since the introduction of coprolites, that substance has come into general use among manufacturers as a substitute for bones, owing to its cheapness and the large quantity of phosphates it contains. But as it is devoid of nitrogenous matters, and is consequently incapable of yielding ammonia, a certain quantity of bones is always employed along with it, or sulphate of ammonia, or some other ammoniacal salt is added to the mixture. The following are analyses of different samples of commercial superphosphate of fair quality. The first and second are made entirely from bones, the third apparently from coprolites alone. We have added also a single analysis of an inferior sort.
| Component | Percentage | |--------------------|------------| | Water | 10-50 | | Organic matter | 26-47 | | Phosphates | 34-29 | | Sulphate of lime | 12-14 | | Sulphuric acid | 14-40 | | Alkaline salts | 0-72 | | Sand | 1-48 | | Ammonia | 3-17 | | Soluble phosphates | 22-97 |
These analyses give the total quantity of phosphates in the body of the analysis, and below the proportion of these phosphates which is rendered soluble by acids, as well as the quantity of ammonia contained in the organic matter. The two first of them are rather above the average quality, and it may be stated generally that the farmer must expect to find in a good superphosphate, about 30 per cent. of phosphates, of which from 12 to 15 ought to be in the soluble state; and about 15 per cent. of ammonia if the superphosphate is made from bones, a smaller quantity if from coprolites. Many superphosphates, however, are sold of greatly inferior quality, and containing little or no soluble phosphates; and these are generally made from coprolites and with a deficient quantity of sulphuric acid. Of this the last analysis is an example, but it is no uncommon thing to see samples which prove to contain no soluble phosphates, but even some per cent. of carbonate of lime, which is incompatible with their existence. The farmer should lay it down as a rule, never to purchase a superphosphate in the analysis of which carbonate of lime or of magnesia occurs.
The manurial value of bones depends principally, but not entirely, on their phosphates, and it is for these almost exclusively that they are employed. They were first made use of in Cheshire as an application to old pasture lands, on which their effects were truly marvellous. That here, at least, the phosphates alone were the cause of their beneficial effects is very clear. These lands had been from time immemorial pastured by milch cows; and in the milk and cheese removed, a large quantity of phosphoric acid is carried off, and so the pasture at length became deteriorated. The bones supplied this element, and hence their good effects. The principal use of bones at the present time is in the culture of the turnip, and it is on that crop that dissolved bones have proved so beneficial. They exert a remarkable influence in forcing on the plant through the early period of its growth, and so bringing it out of the stage during which it is most liable to suffer from the attacks of insects and other risks; and are best applied in conjunction with farm-yard manure or with guano; although the turnip can be raised with them alone. Dissolved bones are a rapid manure, and the greater part of their influence is exerted on the first crop. But undissolved bones are very permanent, and their effects have been observed many years subsequent to a liberal application.
Many other substances have been used as manures, but those to which we have referred are of much greater importance than any others. It is at once obvious from the remarks already made, that manures vary greatly in quality, and it is most desirable that some means should be contrived for estimating their comparative values. Considerable difficulties stand in the way of doing this effectually. Boussingault, who has paid much attention to this subject, is of opinion that the value of a manure may be determined solely from the quantity of nitrogen it contains, irrespective of its other constituents, and in his Rural Economy he has given a table constructed on this principle, of which the following is an abridgment.
| Substance | Nitrogen Equivalent | |---------------------------|--------------------| | Farm-yard manure | 0·41 | | Dung from inn-yard | 0·79 | | Wheat straw | 0·24 | | Rye straw | 0·17 | | Oat straw | 0·28 | | Pea straw | 1·79 | | Potato tops | 0·37 | | Withered beet-root leaves | 0·5 | | Carrot leaves | 0·85 | | Oak leaves | 1·18 | | Fucus digitatus | 0·86 |
In this table the first column gives the quantity of nitrogen in 100 parts of the moist manure. The second gives the equivalent, that is, the quantity of any manure which may be substituted for another, farm-yard manure being taken at 100. Thus, to give an example of its use, suppose the farmer wished to employ a certain quantity of dried blood in place of farm-yard manure, he finds in the table that for every 100 lb. of the latter, he requires only 3·2 lb. of the former. It is evident, however, that this principle cannot be accurately carried out with all manures. In the comparison of farm-yard manure with straw and other analogous substances it probably approximates very closely to the truth; but with bones, guano, and other substances, much of the value of which is certainly dependent on their phosphates, it must manifestly give incorrect results, and with the phosphatic guanos especially we should obtain values much less than practical experience has shown them to possess. Moreover, no account is taken of the state in which the nitrogen exists, although we have already seen that this is far from unimportant. Still some value attaches to such a table, and except with those manures which contain phosphates or other substances in larger quantity, it may often prove useful.
**Mineral Manures.**
All the substances to which we have hitherto alluded are capable of adding to the soil all, or the greater part of, the essential constituents of the plant. Even bones supply not merely ammonia and phosphates, but contain quantities of alkaline salts and other matters, which, though small, are not to be neglected. But many substances are also employed which contain only a single constituent, and their use is found to be followed by very remarkable results. We shall mention these substances in succession, commencing with those which yield nitrogen.
**Sulphate and Muriate of Ammonia.**—These and other salts of ammonia have been tried experimentally as manures, and it has been ascertained that they may all be used with equal success; and as the sulphate is by much cheaper, it is that which probably will always be employed to the exclusion of every other. It contains, when pure, 25·7 per cent. ammonia. That which is now manufactured for agricultural purposes is of very excellent quality, and when genuine, contains almost exactly the proper quantity of ammonia. Its purity may always be roughly estimated, by putting a small quantity on a shovel and heating it over a fire, when it ought to volatilise completely, or leave only a trifling residue. Some care, however, is necessary in applying this test, as in the hands of inexperienced persons it is sometimes fallacious. The salts of ammonia may be applied in the same way as guano; but they are most advantageously employed as a top-dressing, and principally to grass lands. In this way very remarkable effects are produced, and within a week after the application, the difference between the dressed and undressed portions of a field is already conspicuous. Experience has shown that success is best insured when the salt is applied during or immediately before rain, so that it may be at once incorporated with the soil; as when used in dry weather little or no benefit is derived from it. It seems also to exert a peculiarly beneficial effect upon clover; and hence it ought to be employed only on clover-hay, as where ryegrass or other grasses form the whole of the crop we have better manures.
Ammoniacal Liquor of the Gas Works, and of the Ivory-Black Manufacturers.—Both of these are excellent forms in which to apply ammonia, when they can be obtained. The ammoniacal liquor of the gas-works is very variable in quality, but contains generally from 4 to 8 ounces of dry ammonia per gallon, which corresponds in round numbers to from 1 to 2 lb. of sulphate of ammonia. It is best applied with the watering-cart, but must be diluted before use with three or four times its bulk of water, as if concentrated it burns up the grass. It is also well to use it during wet weather. The ammoniacal liquor of the ivory-black works contains above 12 per cent. of ammonia, or about four or five times as much as gas liquor. It has been used in some parts of England, made into a compost, and applied to the turnip and other crops, and, it is said, with good effect. Bone oil, which distils over along with it, has also been used in the form of a compost; it contains a large quantity of ammonia and of nitrogen in other forms of combination; the total quantity of nitrogen it contains being 9½ per cent., which is equivalent to 10½ of ammonia. Only part of this nitrogen is actually in the state of ammonia; and some circumstances connected with the chemical relations of the other nitrogenous compounds in this substance render it probable that they may pass very slowly into ammonia, and may therefore be of inferior value; but the substance is worth a trial, as it is very cheap. It must be carefully composted with peat, and turned over several times before being used.
Nitrates of Potash and Soda.—Nitrate of potash has been frequently employed as a manure, but its place is now entirely taken by nitrate of soda, which, from its superior cheapness, will always be preferred. Like the ammoniacal salts, it is a source of nitrogen, of which it yields about 16 per cent., and is therefore richer in that element than Peruvian guano. It is employed as a top-dressing to grass lands and to young corn, and with the most striking effects, even when the quantity employed has been extremely small. In a recent experiment, Mr Pusey found 42 lb. per acre to increase the produce of barley by 7 bushels per acre, and very favourable results have been obtained by other experimenters. The beneficial effects of nitrate of soda appear to be almost entirely confined to the grasses and cereals. At least experience here has shown that it produces little or no effect on clover; and one farmer has stated, that having recently adopted the practice of sowing clover with a very small proportion of ryegrass only, he has been led to abandon the use of nitrate of soda, which he formerly employed abundantly, when ryegrass formed a principal part of his crop. The action of nitrate of soda is very remarkable, not only in this respect, but also because a given quantity of nitrogen in it appears to produce a greater effect than the same quantity in sulphate of ammonia or guano. At the same time, this statement must be taken as very general, for our experiments are still too few to permit us to state it as a definite fact. Nitrate of soda is best conjoined with common salt, which checks its tendency to make the grain crops run to straw, and prevents their lodging, which, when it is employed alone, they are very apt to do. With hay this precaution is less necessary, and it is better to conjoin the nitrate with an equal quantity of sulphate of ammonia, the combination of the two giving better results than either separately.
Salts of Potash and Soda.—The substances just mentioned may be considered to owe all their value to their nitric acid, but other salts of the alkalies have been employed as manures, although, with the exception of common salt, to a limited extent. Sulphate of soda has been tried on clover and grass, but mixed with nitrate of soda, and with good effect, although we cannot tell how much may have been due to the nitrate.
Chloride of Sodium, or Common Salt, has at different times been employed as a manure, but its effects are so variable and uncertain, that its use, in place of increasing, has of late years rather diminished, it having frequently been found that on soils in all respects similar, or even on the same soil, in different years, it will sometimes prove advantageous, at others positively injurious. It appears, however, to be a valuable addition to other manures, especially to guano and nitrate of soda, as it prevents the tendency which crops manured with these substances have to lodge. The mode in which this effect is produced is obscure; and, so far as we know, no explanation has yet been given of it. It is supposed to cause the plant to absorb more silica from the soil; but this is a speculative explanation of its action, and has not been supported by definite experiment. Although little effect has been observed from salt, it deserves a more accurate investigation, as notwithstanding the extent to which it has been employed, we are singularly deficient in definite experiments with it.
Silicates of Potash and Soda have been employed with the view of supplying silica to the plant, but the results have been far from satisfactory. Good effects have been observed from the application of silicate of soda to the potato; but our experience of it is much too limited to enable us to form any estimate of its general value.
Carbonates of Potash and Soda have only been tried experimentally, and that to a small extent. The remarks we have made in the section of the ashes of plants regarding the subordinate value of soda, will enable the reader to see that greater effects are to be anticipated from the former than from the latter of these salts. They may, however, exert a chemical action in the soil, altogether independent of their absorption by the plant, but its nature and amount are still to determine.
Sulphate of Magnesia can be obtained at a low cost, and has been used as a manure in some instances with very marked success. It has been chiefly applied as a top-dressing to clover hay, but it seems probable that it might prove of use to the cereals, the ash of which is peculiarly rich in magnesia.
Many other saline substances have been tried as manures; but in most instances to too limited an extent to permit any definite conclusions as to their value. The experiments have also been too frequently performed without those precautions necessary to exclude fallacy, so that the results already arrived at must not be accepted as establishing facts, but rather as indications of the direction in which further experiments would be valuable. There is little doubt that many of these substances might be usefully employed, if the conditions necessary for their successful application were eliminated; and no subject is at present more deserving of elucidation by careful and well-devised field experiments. Various mixtures of saline manures have been employed, and frequently with good effects. The most marked, however, have been from those of vegetable origin. Thus, wood ashes and peat ashes have been employed with more or less success, and their utility is clearly attributable to their affording a supply of all the inorganic constituents of the plant. Wood ashes, when they can be obtained, are a most valuable manure; coal and peat ashes appear to be inferior as a general rule; but in Belgium and Holland the use of peat ashes is common, and the effects are said to be excellent. They have at different times been imported into this country, but do not appear to have established a reputation as a manure.
It has been held by some chemists, and particularly by Liebig, that, provided we apply to the soil the mineral constituents of the plant, without adding either nitrogen or organic matters, we fulfil all the conditions necessary to the growth of the plant. This opinion has certainly not been confirmed by experiment in this country; the presence of ammonia, or at all events of nitrogen, in some form or other, having always been found necessary, and the application of the mineral matters, even when their proportions have been regulated by reference to the composition of the ash of the plant to which they have been applied, has proved a failure. Notwithstanding this, Liebig still holds to this view, which he has found supported by experiments of his own. It is extremely difficult to reconcile these discordant statements and facts; but we suspect strongly that something must depend on climate and soil; we know at least as regards one manure, superphosphate, that climate has its effect. In England dissolved coprolites have been successfully employed as a manure for the turnip, but in Scotland they have proved by no means so successful. An addition of ammonia is necessary in the moister and colder climate of Scotland; and this is so well known to some manufacturers that they avoid sending to our market any superphosphates made from coprolite alone, but take care to add a sufficient quantity of nitrogenous matters to satisfy the wants of the climate. It is not impossible that the different requirements of the climates may be the real cause of these differences of opinion and that Liebig may be right for the climate of Giessen, as our experimenters are for this country. Boussingault, who first insisted on the importance of ammonia or nitrogen in manures, has had his opinion fully confirmed by the valuable researches of Lawes, to which we must refer our readers for a very full discussion of the whole subject.
Lime.—Lime is by far the most important of the mineral manures, and is an almost indispensable agent in all agricultural improvement. It has been employed in the form of chalk, limestone, marl, shell-sand, and as quick and slaked lime. To the composition of limestones we have already referred when treating of the origin of soils, and have pointed out that they are divisible into two classes—one consisting of nearly pure carbonate of lime, the other of a mixture of carbonate of lime and magnesia. It will be unnecessary, therefore, to refer further to this subject. Chalk is a nearly pure carbonate of lime; marl is a pulverulent deposit of carbonate of lime, sometimes nearly pure, at others mixed with a variable proportion of clay and sand; and shell-sand is the debris of shells which has been cast up on the seashore, and which contain a greater or less admixture of sand.
Pure carbonate of lime contains exactly 56 per cent. of lime, and a good limestone ought to contain from 90 to 95 per cent. of the carbonate, equivalent to 50-10 per cent. of lime. It may therefore be said generally, that a good limestone should contain about half its weight of lime. When limestone is exposed to heat, its carbonic acid is driven off, and the lime is left in the quick state; and the quick lime, by exposure to the air, absorbs moisture from it and slakes; and if it be exposed for a longer time, it also absorbs carbonic acid, and passes back, more or less completely according to the length of time it is exposed, into the state of carbonate. While lime may be applied in the state of carbonate, either as chalk, marl, or pounded limestone, and with a certain amount of advantage, much greater effects are obtained from the use of the lime itself in the quick or slaked state. These advantages are dependent partly on the mechanical effect of the burning and slaking, which enable us to reduce the lime to a much more minute state of division, and consequently to incorporate it more uniformly and thoroughly with the soil, and partly on the more powerful chemical action of the quick or caustic lime, by which a greater effect is produced upon the soil. Other minor advantages are also secured, such as the production of a certain quantity of sulphate of lime, &c., which, though comparatively trifling, may, under particular circumstances and in some soils, be of considerable importance.
The action of lime is of a complicated character. Like all the inorganic constituents of plants, it may of course serve as food for those growing in the soil to which it is added. But this is manifestly a very subordinate part of its action.—1st, Because no soil exists which does not contain lime in sufficient quantity to supply that element to the plants. 2d, Because its effects are not restricted to those soils in which it exists naturally in small quantity; and, 3d, Because it is found that a small application, such as would suffice for the wants of the crops, is not sufficient to produce its best effects. In fact, by far the most important action of lime is that which it exerts on the chemical and mechanical properties of the soil; and it is this which necessitates its application in very large quantities.
The proportion of lime applied varies very greatly in different places. As much as ten tons per acre have frequently been applied, and in some instances much more. Of late years, however, we believe that these very large applications have become less common, because it is found that better effects are produced by a smaller quantity more frequently repeated. Its quantity depends greatly on the nature of the soil; on heavy clays, especially if undrained, very large applications are required; on light soils much smaller; even the depth of a soil must be considered, and a smaller quantity will suffice when it is shallow. The geological origin of the soil is also not without its influence; for we find that its beneficial effect is peculiarly seen on granite, porphyry, and gneiss soils, both because these are naturally deficient in lime, and because they undergo very slowly those decompositions which liberate their active constituents.
The greater part of the action of lime is indeed dependent on its exerting a chemical decomposition on the soil; and it acts equally on both the great divisions of its constituents, the inorganic and the organic. On the former, it acts by decomposing the silicates, which form the main part of the soil, and by liberating the alkalies they contain, it causes a larger supply of these substances to become available to the plant. On the organic constituents its effects are principally expended in promoting the decomposition which converts their nitrogen into ammonia; and thus a supply of food, which might remain for a long period locked up, is set free in a state in which the plant can at once absorb it. But these chemical decompositions are attended by a corresponding change in the mechanical characters of the soil. Heavy clays are observed to become lighter and more open in their texture; and those which are too rich in organic matter have it rapidly reduced in quantity, and the excessive lightness which it occasions diminished.
The effects of an application of lime are not generally observed immediately, but become apparent in the course of one or two years, when it has had time to exert its chemical influence on the soil; but from that time its effects are seen gradually to diminish and finally to cease entirely. The period within which this occurs necessarily varies with the amount of the application and the nature of the soil, but it may be said generally that lime will last from ten to fifteen years. The cessation of its effects is due to several circumstances, partly of course to the absorption of lime by the plants, partly to its being washed out of the soil by the rains, and partly to its tendency to sink to a lower level in the soil, a tendency which most practical men have had opportunities of observing. In the latter case, deep-ploughing often produces a marked effect, and sometimes makes it possible to postpone for a year or two the reapplication of lime. All these circumstances have their influence in bringing to an end its action, but the most important is, that after a time it has exhausted its decomposing effect on the soil, having destroyed all the organic matter, or liberated all the insoluble mineral substances which the quantity added is competent to do, and so the soil passes back to its old state. It does even more, for unless active measures are taken to sustain the fertility of the soil by other means, it is found that its fertility is apt to become less than it was before the use of lime. And that it should be so is manifest, if we consider that the lime added has liberated a quantity of inorganic matter, which, in the natural state of the soil, would have become slowly available to the plant, and that it must have acted chiefly in those very portions which, from having already undergone a partial decomposition, were ready to pass into a state fitted for absorption, and thus as it were, must have anticipated the supplies of future years. This effect has been frequently observed by farmers, and is indeed so common, that it has passed into a proverbial saying, that "lime enriches the fathers and impoverishes the sons." But this is true only when the soil is stinted of other manures, for when it is liberally treated the exhausting effect of lime is not observed, and it must be laid down as a practical rule that the use of lime necessitates a liberal treatment of the soil in all other respects. But when lime has been once employed, it becomes almost necessary to resort to it again; and generally so soon as its effects are exhausted a new quantity is applied, not so large as that which is used when the soil is first limed, but still considerable. When this is done very frequently, however, bad effects ensue; the soil gets into a particular state in which it is so open that the grain crops become uncertain, and such land is said, in practical language, to be overlimed.
The explanation commonly assumed by those unacquainted with chemistry is, that the land has become too full of lime; but a moment's consideration of the very small fraction of the soil, which even the largest application of lime forms, will serve to show that this cannot be the cause. And analyses of overlimed soils have proved that the lime does not exceed the ordinary quantity found in fertile soils. The explanation of the phenomenon probably is, that the rapid decomposition of organic matter by the lime, and its escape as carbonic acid has so opened the pores of the soil as to give it the peculiar appearance so well known in practice. The cure for overliming is found to be the employment of such means as consolidate the soil, such as eating off with sheep, rolling, or laying down to permanent pasture.
The immediate effect of lime on the vegetation of the land to which it is applied is very striking. It immediately destroys all sorts of moss, makes a tender herbage spring up, and eradicates a number of weeds. It improves the quantity and quality of most crops, and causes them to arrive more rapidly at maturity. The extent to which it produces these effects is due to the form in which it is applied. In general they are produced more distinctly and more rapidly when it is applied in the quick state, more slowly if it be in the mild state, that is to say, quick lime which has been exposed for a long time to the air, and still more slowly as marl or chalk. The particular circumstances under which these different forms of lime are best employed is a very extensive subject, and would lead us beyond our limits; and for further information we must refer the reader to Professor Johnston's treatise on the use of lime in agriculture.
**Sulphate of Lime, or Gypsum.**—Gypsum has been applied in large quantity as a manure, and is found to exert a very remarkable influence upon clover, and leguminous crops generally. It is used in quantities varying from 2 cwt. per acre up to a very large quantity, and almost invariably with good results, in some instances even with the production of double crops. Much speculation has taken place as to the cause of this action which is so specific in its character, and from Sir Humphry Davy down to the present time, many chemists and agriculturists have considered the matter. Sir Humphry Davy attributed its action to its supplying sulphur to those plants which, according to him, contain a larger quantity of that element than other plants. That opinion has been since entertained by others, but it can scarcely be considered as well founded, for the more accurate experiments recently made do not point to any conspicuous differences between the quantities of sulphur contained in these and other plants. It is, moreover, to gypsum alone that these effects are due, and if it were merely as a source of sulphur that it was employed, there are other salts which could be equally, perhaps more advantageously, used; such, for instance, as sulphate of soda. It is more probable that the action of sulphate of lime may depend on its value as an absorbent of ammonia, and to its taking the atmospheric ammonia, and supplying it to the plants. Great difficulties unquestionably surround this explanation, and though supported by some persons, much may be said against it; as, for instance, why should its effect be so very marked on particular plants. In fact, while we have experiments which prove in the most unquestionable manner the utility of gypsum, we require others made with the express object of elucidating the cause of its action.
**Phosphate of Lime.**—In treating of bones we have alluded sufficiently to the value of phosphate of lime, but when conjoined with animal matters, ammonia, and other valuable substances. We have now simply to refer to the existence of certain varieties of mineral phosphates, some of which have been used, and others proposed, as manures. The sparite of Estremadura was some years since proposed as a manure, and a commission was sent by our Government to inquire into the extent of the supplies, and the possibility of its being imported, but it was found to be limited in extent and too inaccessible to be of much importance, and we believe no attempt has been made to import it. The same mineral is met with in New Jersey and other districts of America, and has been sent to this country, but its price was too high to admit of its being employed. Phosphate of lime also occurs in England, principally in Suffolk, in the form of what are called coprolites, although it is doubtful whether they really deserve that name, which was originally given by geologists to very different substances. The coprolites are now collected in very large quantities, and some thousand tons must be annually employed. They are extremely hard, and require very powerful machinery to reduce them to powder, and hence their price is considerable, we believe about L3 per ton. From this hardness they are also less easily attacked by the plant, and are consequently best employed dissolved in sulphuric acid. Coprolites have the following composition:— They are therefore rich in phosphates, containing, in fact, more of these substances than guano; but as they are hard, and not easily dissolved by the plant, an inferior value must be attributed to them. Much coprolite of inferior quality, and containing a larger proportion of carbonate of lime, is sold; and the purchaser must ascertain by analysis that the article he buys is good.
THE ROTATION OF CROPS.
It is a necessary consequence of the facts detailed in the previous sections, that a crop growing on any land must necessarily exhaust it more or less; that is, must remove from it a certain quantity of the elements which confer fertility upon it. That this is the case has been long admitted in practice, and it has also been established that the exhausting effects of different species of plants are very different; that while some rapidly impoverish the soil, others may be cultivated for a number of years without material injury, and others even apparently improve it. Thus, it is a notorious fact that white crops exhaust, while grass improves the soil; but the improvement in the latter case is really dependent on the fact, that when the land is laid down in pasture, nothing is removed from it, the cattle which feed on its produce returning again all that they had removed; so that, when we take into account the fact that the plants derive a part, and in some instances a very large part, of their nutriment from the air, the fertility of the soil must manifestly be improved, or at all events supported in its previous state.
When, however, the plant, or some of its parts, is removed from the soil, there must be a reduction in the amount of its fertility dependent on the quantity of its valuable constituents which each plant contains; and thus it occurs that when a plant has grown on any soil, and has removed from it a large quantity of nutritive matters, that it becomes incapable of producing an equally large crop of the same species; and if the attempt is made to grow it in successive years, the land becomes incapable of producing it at all, and is then said to be thoroughly exhausted. But if the exhausted land be allowed to lie for some time without a crop, it is found, more or less rapidly according to circumstances, to regain its fertility, and to produce again the same substance in remunerative crops. The observation of this fact led to the introduction of naked fallows, which, up to a comparatively recent period, were an essential feature in agriculture. But after a time it was observed that the land which had been exhausted by successive crops of one species was not absolutely barren, but was still capable of producing a luxuriant growth of other plants. Thus peas, beans, clover, or potatoes, might be cultivated with success on land which would no longer sustain a crop of grain, and these plants came into use in place of the naked fallow under the name of fallow crops. On this was founded the rotation of crops; for it was clear that a judicious interchange of the plants sown might enable the soil to regain its fertility for one crop at the time when it was producing another; and when exhausted for the second, it might be again ready to bear crops of the first.
The necessity for a rotation of crops has been explained in several ways. The oldest is that of Decandolle, who founded his theory on the fact that the plants excrete certain substances from their roots. He found that when a plant was grown in water, a substance was excreted from the roots; and he believed that this extremititious substance was thrown out because it was injurious to the plant, and that, remaining in the soil, it acted as a poison to those of the same species, and so prevented the growth of another crop. But this excretion, though poisonous to the plants from which it was excreted, he believed to be nutritive to those of another species which thus grew luxuriantly where the others failed. Nothing can be more simple than this explanation, and it was readily embraced at the time it was propounded and considered fully satisfactory. But when more minutely examined, it becomes apparent that the facts on which it is founded are of a very uncertain character. Decandolle's observations regarding the radical excretions of plants have not been confirmed by subsequent observers. On the contrary, they have found that though some plants, when growing in water, do excrete a particular substance in small quantity, that nothing of the sort appears when they are grown in a silicious sand. And hence the inference is, that the peculiar excretion of plants growing in water is rather the result of disease than a natural product. But even admitting the existence of these matters, it would be impossible to accept the explanation founded upon them, because we know that, on individual soils, the repeated growth of particular crops is perfectly possible, as, for instance, on the virgin soils of America, from which many successive crops of wheat have been taken; and in these cases the alleged excretion must have taken place without producing any deleterious effect on the crop. Besides, it is in the last degree improbable that these excretions, consisting of soluble organic matters, should remain in the soil without undergoing decomposition, as all similar substances do; and even if they did, we cannot, with our present knowledge of the food of plants, admit the possibility of the direct absorption of any organic substance whatever. We believe, indeed, that the idea of radical excretions, as an explanation of the rotation of crops, must be considered as being entirely abandoned.
We now seek for its explanation in the different quantities of valuable matters which different plants remove from the soil, and more especially to their mineral constituents. To the great differences which exist in the composition of the ash of different plants we have referred in the section on that subject; and we have pointed out that a distinction has been made between lime, potash, and silica plants. This distinction has its origin in the explanation of the rotation of crops, to which we now refer. In fact, it is believed that if, to take a particular instance, a plant which requires a large quantity of potash be grown on a soil, it will, in a greater or less time, exhaust all, or nearly all, the potash which that soil contains in an available form, and will consequently cease to produce a luxuriant crop of it. But if we replace it by another plant which requires only a small quantity of potash and a large quantity of lime, it will flourish, because it finds what is necessary to its growth. In the meantime, the changes which are proceeding in the soil, are liberating new quantities of the inorganic matters from those forms of combination in which they are not immediately available, and when after a time the plant which requires potash is again sown on the soil, it finds a sufficient quantity to serve its purpose. We have already, in treating of the ashes of plants, pointed out the extent of the differences which exist; but these will be made more obvious by the annexed table, giving the quantity of the different mineral matters contained in the produce of an imperial acre of the different crops. We have omitted the oxide of iron and manganese as unimportant, and have added the quantity of nitrogen, which is of considerable interest, though of course not directly important as regards rotation. ### Table showing the number of Pounds of Mineral Matters and Nitrogen removed from an Acre of Land by average Crops of different Grains, &c.
| Crop | Silica | Potash | Soda | Lime | Magnesia | Chloride of Chloride of Phosphoric Acid | Sulphuric Acid | Nitrogen | |---------------|--------|--------|------|------|----------|----------------------------------------|----------------|----------| | Wheat, grain | 1·2 | 10·5 | 1·0 | 1·2 | 4·1 | ... | 15·3 | 0·1 | | straw | 96·8 | 27·2 | 3·7 | 11·1 | 3·0 | ... | 4·0 | 4·6 | | Barley, grain | 17·5 | 11·7 | 0·8 | 1·0 | 4·1 | 2·3 | 0·5 | 16·0 | | straw | 129·7 | 6·0 | 1·6 | 18·4 | 2·7 | ... | 3·4 | 4·9 | | Oat, grain | 23·0 | 8·0 | 1·2 | 3·8 | 3·8 | 2·6 | 17·2 | 3·3 | | straw | 83·5 | 18·7 | 20·0 | 11·1 | 7·2 | ... | 3·8 | 3·0 | | Beans, grain | 0·4 | 23·1 | 0·4 | 4·8 | 3·7 | 0·2 | 10·1 | 2·5 | | straw | 5·9 | 32·1 | 7·0 | 32·1 | 7·4 | 1·3 | 13·8 | 11·1 | | Pease, grain | 0·6 | 19·3 | 0·4 | 2·9 | 3·1 | 0·6 | 0·4 | 16·8 | | straw | 0·7 | 30·7 | 4·5 | 67·5 | 11·8 | ... | 6·4 | 8·1 | | Turnip, bulb | 4·0 | 112·0 | 28·0 | 32·0 | 8·0 | ... | 26·0 | 32·0 | | tops | ... | 50·0 | 1·0 | 70·0 | 2·0 | 11·0 | 14·0 | 11·0 | | Potato, tuber | 2·0 | 87·0 | 7·0 | 2·0 | 7·0 | ... | 9·0 | 18·0 | | top | 60·0 | 4·0 | 2·0 | 58·0 | 9·0 | ... | 4·0 | 12·0 | | Meadow hay | 68·0 | 48·0 | 5·0 | 39·0 | 16·0 | ... | 6·0 | 12·0 | | Ryegrass | 147·0 | 31·0 | 10·0 | 25·0 | 6·0 | ... | 6·0 | 16·0 | | Red clover | 10·0 | 44·0 | 5·0 | 103·0| 32·0 | ... | 9·0 | 18·0 | | Flax, straw | 90·7 | 14·1 | 14·1 | 17·8 | 11·4 | ... | 3·3 | 15·4 | | seed | 0·6 | 13·4 | 0·6 | 3·4 | 5·2 | ... | 0·1 | 15·2 |
From an inspection of this table, we at once perceive the difference of effects which different crops must produce on the soil. Thus, a wheat crop (grain and straw) removes from the soil 98 lbs. of silica; and as we know that that substance exists in small quantity in an available state in most soils, we understand how a succession of wheat or other grain crops (some containing even a larger quantity of silica than wheat) should fail to flourish, unless a sufficient quantity of that element be annually set free to supply the loss; while a crop of turnips removing only 4 lbs. of silica may be produced, and at the same time permit the accumulation of a quantity of soluble silica ready for another crop of grain. The turnip again, which carries off no less than 112 lbs. of potash, soon exhausts the soil of that element; and when a grain crop, removing only from 17 to 37 lbs. according to circumstances, replaces it, we have the conditions necessary for the restoration of that which the turnip had removed. So with the other elements we find that the turnip removes 40 lbs. of sulphuric acid, wheat only 4·7; and clover requires above 135 lbs. of lime and magnesia, and wheat only 19 of the two. And thus the small quantity of individual substances removed by one plant, compensates for the large quantity withdrawn by another; and by a judicious interchange we have the soil always in a condition to supply a sufficient quantity of the elements necessary for any crop which grows on it.
Viewed in this light, we see that there are several important practical deductions to be drawn from these observations regarding the principles of rotation. We observe that the quantities of mineral matters withdrawn by the plants of the same class are generally similar, and thus we infer that we ought as much as possible to cause crops of the most opposite class to alternate with one another, and to repeat each plant as seldom as possible, so that even when we are obliged to return to the same class we should, if circumstances permit, employ a different member of it. Thus, for instance, in place of immediately repeating wheat, when we wish another grain crop, it would theoretically be preferable to employ oats or barley, and to replace the turnip by mangold-wurzel or some other root. It is obvious, however, that this system cannot be carried out in practice to its full extent; for the superior value of individual crops causes their repetition more frequently than that of those which make a less return. But experience has so far concurred with theory, that it has taught the farmer the advantage of long rotations; and we have had the successive introduction of the three, four, five, and six course shift, and even of longer periods in some instances.
In all this the farmer only imitates the practice of nature; for it has been long observed that when one generation of plants dies out, it is immediately replaced by another. In the forests of Sweden we have a remarkable illustration of this, for when a pine forest is felled and the land left to itself, there spring up not pines but birch trees; and every one is familiar with the fact that when a gap occurs in a thorn-hedge it is useless to attempt to fill it up by putting in a young thorn, but that some other plant must be used.
Such is the theory of rotation; but is it absolutely necessary that it should be rigidly adhered to? We think not. Because in the art of agriculture we place the plants in artificial circumstances, and instead of allowing them to depend entirely on the soil we supply them with a quantity of manure containing all the elements of the plant, and if it be used in sufficiently large quantity we may grow year after year the same crop. And accordingly the order of rotation which is theoretically the best may be, and every day is, violated in practice. This must necessarily be done at the expense of a certain quantity of the valuable matters of the manure added, and is so far a practice which ought theoretically to be avoided. But in actual practice the matter is to be decided on other grounds. The object then is, not to produce the largest crops, but those which make the largest money return, and thus it may be practically economical to grow a crop of high commercial value more frequently than is theoretically advantageous. The farmer must therefore seek to do away as far as possible with the disadvantages which such a course entails, and this he will endeavour to do by a liberal treatment of the soil, and by as careful a management of the other crops of this nature as possible.
But while the farmer may do this to some extent, he must bear in mind that the frequent repetition of some crops cannot be practised with impunity, for they are liable to AGRICULTURAL CHEMISTRY.
The Food of Animals.—In examining the conditions which must be attended to in the food of animals, we may with advantage take an example from that which nature has provided for the sustenance of their young. The milk may, in fact, be considered as a typical food, and necessarily the best fitted to fulfill the purposes for which it is intended. Now, we find it, exclusive of its inorganic constituents, to contain three different classes of nutritious matters. 1st, Nitrogenous or albuminous substances; 2d, Fatty matters; 3d, Sugar; the first adapted to the production of flesh, the second to the formation of the fat of the body, and the third going partly to supply the respiratory process, and partly to be converted into fat. Now, to sustain the animal in its usual state, a sufficient quantity of those substances must be given to supply the waste of the tissues, and the process of respiration, along with a quantity which is never absorbed, but passes through the alimentary canal. In round numbers this latter quantity amounts to not less than half the whole nutritive matters. The quantity of albuminous substances absorbed and used to supply the waste of the tissues varies very greatly according to circumstances shortly to be mentioned, but in an experiment of Boussingault's on a cow, amounted to about 18 ounces. The consumption of sugar, starch, and similar substances, to maintain respiration is much larger; for it has been ascertained that in the course of 24 hours an ox will convert into carbonic acid by respiration, from 4 to 5 lb. of carbon, to supply which from 10 to 12 lb. of sugar or starch are required. It is necessary, therefore, that the food to be supplied should contain these substances, and a sufficiency of them for the purpose; and what we have already said of the composition of vegetables shows that all do contain these substances, though their quantity is very variable. In comparing the value of different sorts of food, it is necessary to consider the quantity of nitrogenous and other matters which they contain. But we find by experience, that all these substances are not of equal importance, some being supplied in sufficient quantity for all purposes, others being found in many sorts of food in comparatively small quantity. It appears, indeed, that the nitrogenous constituents are by much the most important, and for many purposes the value of the food may be estimated almost entirely from them. But these substances are the flesh-forming element of the food only; the production of fat is dependent partly on the fatty matter, and partly on the starch and sugar of the plant. At one time it was believed that the presence of fatty matters in the food was essential to the production of fat in the animal; but careful experiments have entirely refuted this opinion, and have shown that the fat may be produced from sugar or starch alone, and hence some chemists and physiologists have even gone so far as to hold that the fat of the food does not go to form the fat of the animal. We apprehend, however, that this is an extreme view of the case, and it can scarcely be doubted that the fat must be of importance, though it may not be absolutely essential. Now, in comparing different sorts of food, we come to the conclusion, that those are most valuable which contain the largest quantity of albuminous substances, oil and saccharine matters; but there are few or no foods which do not contain these latter substances in sufficient quantity to supply the wants of any animals which feed upon them. We may therefore take the quantities of albuminous matters and oil as the measure of the nutritive value of any sort of food. That this is borne out by practice, we may see at once by selecting any two sorts of food: let us take the turnip and linseed-cake. Now the former of these is very poor in nutritive matters, and hence requires to be supplied in large quantity to the animal; the latter is rich both in albuminous and oily matters, and only a small quantity of it is required. The practical farmer, when he gives cattle linseed-cake, generally considers that he may replace 100 lb. weight of turnips by 5 lb. of cake; and that with this apparently trifling quantity the cattle fatten better than they did with the large quantity of turnip. Now, when we inquire into the relative quantities of nitrogenous and oily matters contained in these two substances, we have at once an explanation of the observed fact. Analysis shows us, that the quantities of albuminous matters and of oil contained in 100 parts of those two substances are as follows:
| Substance | Oil-Cake | Turnip | |-----------------|----------|--------| | Albuminous matter | 27.69 | 1.27 | | Oil | 12.79 | 0.20 |
If we calculate from these results, we find that 100 lb. of turnips contain the same quantity of albuminous matter as 4.5 lb. of oil-cake, and no more oil than is supplied by 1.5 lb. of oil-cake, so that the 5 lb. of oil-cake contain a larger quantity of nutritive matter than the large quantity of turnip for which they are a substitute.
The consideration of these facts is of much importance in the economic feeding of animals, for it is manifest that very great differences must exist in the nutritive value of different sorts of food; and now that the farmer finds it desirable to use other substances than those produced on the farm, it is of importance that he should possess some means of estimating the relative value of different sorts of food. We give here, as an assistance in doing so, a table showing the percentage of albuminous and oily matters contained in 100 parts of different crops.
| Crop | Albuminous Matters | Oil | |------------------------------|--------------------|-----| | Poppy cake | 31.46 | 5.75| | Rape cake | 29.53 | 11.10| | Crambolina cake | 28.79 | 9.50| | Common Scotch tares | 28.57 | 1.30| | Hopetown tares | 28.32 | 1.49| | Linseed cake | 27.69 | 12.79| | Field beans | 27.05 | 1.58| | Winter tares (foreign) | 26.73 | 1.58| | Spring tares (foreign) | 26.54 | 1.26| | Cotton seed cake | 25.16 | 9.03| | Beans (65 lb. per bushel) | 24.70 | 1.59| | Linseed | 24.44 | 34.00| | Lentils (foreign) | 24.57 | 1.51| | Lentils (Scotch growth) | 24.25 | 1.79| | Gray peas | 24.25 | 3.30| | Foreign beans | 23.49 | 1.51| | Kidney beans | 20.06 | 1.22| | Maple peas | 19.43 | 1.72| | Clover hay, second crop | 13.52 | ... | | Sunflower seed | 12.70 | 29.98| | Oats | 10.16 | 6.12 | | Buckwheat | 9.84 | 2.69 | | Guinea corn | 9.27 | 3.46 | | Wheat | 9.01 | 1.99 | | Common Scotch bean straw | 8.25 | ... | | Barley | 7.74 | 1.88 | | Hay (new) | 6.16 | ... | | Winter bean straw | 5.71 | ... | | Hay (old) | 4.00 | ... | | Crimson clover | 3.30 | ... | | Yellow clover | 3.26 | ... | | Lucerne | 3.11 | ... | | Cow grass | 2.75 | ... | | Red clover | 2.59 | ... | | Chevalier barley straw | 1.90 | ... | | Early Angus oat straw | 1.50 | ... | | Red wheat straw | 1.50 | ... | | White wheat straw | 1.37 | ... | | Turnip | 1.27 | 0.20 |
The blank spaces in the second column occur where the oil is in too small quantity to admit of accurate determination.
A simple inspection of this table gives a great deal of information; but by the use of the rule of three it is easy to calculate the quantity of one substance which corresponds in albuminous or in oily matters to another. It is to be observed, however, that the substances containing the former in large quantity, do not necessarily, or even frequently contain a proportionate amount of the other, so that, practically, in making the comparison, we must rely upon one only, and we commonly select the quantity of albuminous matters as the most important. It will be understood, that while these two constituents are the most important as regards the estimation of the value of any food, they are not the only substances essential to its nutritive value. On the contrary, the non-nitrogenous, or, as they are sometimes called, the respiratory elements, because they supply the carbon consumed in the process of respiration, and each individual inorganic substance is essential; but as these substances are met with in abundance in all sorts of food, they are unimportant in the estimation of their relative value.
The values as deduced from these numbers must be considered as an approximation only; but they are very close approximations when the substances are of analogous characters; as, for instance, in the case of different sorts of grain. They are, however, liable to modification, by a number of different circumstances: thus, for example, rape-cake has, according to the table, a higher value than linseed-cake, but it possesses a peculiar bitter flavour, which makes it unpalatable to the cattle; and it also frequently produces scouring, so that it may not always give as good effects as might be anticipated from it. Something also is to be attributed to the general nature of the substance, and to the condition in which its constituents exist; and it would appear that the presence of a large quantity of woody fibre reduces the value of food, by enveloping its nitrogenous and other matters, and preventing their absorption during their passage through the intestines of the animal; and it is probably on this account that straw, though richer in nitrogenous matters, has a much lower nutritive value than the turnip.
The nutritive effects obtained from the food, are also increased by mixing together different sorts. Indeed this is found to be an exceedingly important point; for as we have seen that the milk which nature supplies as the appropriate food of the young animal, contains a mixture of all the different classes of nutritive elements, so it appears the best effects are produced by an imitation of this also. But few of the substances which we employ contain all the necessary elements of the food in proper proportion; we find one deficient in fatty matters, another in albuminous, and so on, and in order to produce a food of the most suitable kind, we require to mix together several substances; and when this is judiciously done so as to insure a proper relation between the individual nutritive elements, a higher effect is obtained than could have been got by the use of these substances separately. Thus the farmer who is feeding with bean-meal in considerable quantity, will generally find that a better effect is obtained by replacing a part of it with some of the more oleaginous seeds or cakes. There is, however, another circumstance independent of chemical composition which modifies the nutritive value of the different sorts of food. It is necessary in order to its proper digestion, that the food shall have a certain bulk, for without it the peristaltic motions of the intestines are not properly performed, digestion is incomplete, and a quantity of nutritive matter is wasted. For this reason the highly nutritive foods must always be conjoined with those which occupy a large bulk in the stomach. But, on the other hand, the bulk of the food must not be excessive, for then the stomach is overloaded, digestion and absorption of the food are checked, and the health of the animal becomes impaired. The circumstances which diminish the waste of the Food.
We have already remarked that there are three great purposes to which the food swallowed is appropriated; the increase of weight in the animal—the object the feeder has in view and desires to promote—the supplying the waste of the tissues, and the process of respiration, both of which are sources of waste of food, and which it must necessarily be his aim to diminish as much as possible. The circumstances which must be attended to in order to do this are sufficiently well understood. It has been clearly established that the natural heat of the animal is sustained by the consumption of a certain quantity of its food in the respiratory process, during which it undergoes exactly the same change as those which occur during combustion. In fact, a certain quantity of the food is no more than so much fuel intended to sustain the heat of the body. We observe, however, that the temperature of the body is always the same, whatever be that of the surrounding air. Now it is obvious that if the temperature of the animal is to remain the same in winter as in summer, a larger quantity of fuel (i.e., food) must be consumed for this purpose, just as a room requires more fire to keep it warm in winter than in summer, and hence it naturally follows that if we keep the animal in a warm locality we economise the fuel. In order to do this, then the housing of the cattle is a matter of importance, and here practice has arrived at conclusions strictly concordant with science. The old feeders kept their cattle in large open courts, where they were exposed to every vicissitude of the weather. But as intelligence advanced, we find them substituting, first, what are called hammels and then stalls, in which the animals are kept, during the whole time of fattening, at an equable temperature. The effect of this is necessarily to introduce a considerable economy of the food required to sustain the animal heat; but it also effects a saving in another way, for it diminishes the waste of the tissues. It has been ascertained in the most conclusive manner, that this waste is dependent on the amount of muscular exertion. Thus if we sit still for an hour a certain amount of waste in our tissues takes place, but if we run or engage in any violent muscular exertion, we increase this waste, and consequently require a larger quantity of food to supply it. The confining the animals in stalls has the effect of diminishing the amount of muscular action, and introduces an important economy in the food. Even the making the houses dark, and thus preventing the attention of the animal being disturbed by various objects, has its effect in promoting this economy.
An extension of the same principle has led to the use of the food artificially heated, but it is doubtful whether the advantages derived from it are commensurate to the increased expenses of the process; at least opinions differ among the best informed practical men on this subject.
The rapidity of fattening is dependent on many other agricultural circumstances. One of the most important is the breed, universal experience has shown that the short-horn manifests in this respect a marked superiority. This and many similar facts are, however, less chemical than physiological, and could not be considered here without entering upon many matters not connected with our subject.
CONCLUDING REMARKS.
We have thus endeavoured to give our readers as full an account of the present state of agricultural chemistry as our limits permit. In a science so new, and embracing so many minute facts, the task is not without difficulty. It has been our object, however, as far as possible to avoid details, and to give rather such principles as have been established, and to illustrate them by what appeared to be the most satisfactory and best observed facts. In many branches of the subject these are but few, and the conclusions founded on them must necessarily be uncertain, and in some instances may possibly be proved erroneous by further observations. That a department of science cultivated for so short a period, and requiring for its proper pursuit the co-operation of two classes of men, the farmer and the chemist, who have hitherto had so little in common, should be imperfect, is not to be wondered at. On the contrary, we are of opinion that the progress made within the last few years, considering all the disadvantages under which it has been placed, from the rash and unweighed theories with which it has been overloaded, the excessive and imprudent zeal of its supporters, the opposition of another class, and the equally fatal lukewarmness of a third, a great deal has been done. Facts of much practical value have been elicited, and an immense stimulus has been given to careful observation and inquiry into principles on the part of the farmer. That this is already beginning to bear its fruit is unquestionable, and it is impossible to look at the opinions and practice of modern farmers of the best class, without observing how much they are influenced by science. That much, however, still remains to be done, is only too obvious from many of the statements we have made, and even in what we consider familiar matters, the chemist is frequently stopped by the want of field experiments sufficiently definite to support or refute his positions. Indeed there are few departments of scientific agriculture that would not be benefited by experiments of a more minute and careful nature than those which, in a less advanced state of the art, and for purely practical purposes, were sufficient. We doubt much, however, whether this can be carried out in detail until a regular professional education in the principles as well as the practice of agriculture is provided for the young farmer, a want which is every day becoming more felt, and the fulfilment of which cannot long be postponed.