ELEMENTS

OF

AGRICULTURAL CHEMISTRY

BY

THOMAS ANDERSON, M.D.

F.R.S.E., F.C.S.

PROFESSOR OF CHEMISTRY IN THE UNIVERSITY OF GLASGOW, AND CHEMIST TO THE HIGHLAND AND AGRICULTURAL SOCIETY OF SCOTLAND.
EDINBURGH:
ADAM AND CHARLES BLACK.
1860.

ERRATUM.

Page 190, line 11, for "gallon" read "ton."

PRINTED BY R. AND R. CLARK, EDINBURGH.

Transcriber's note: Many of the tables needed to be split to fit space constraints.

PREFACE.

The object of the present work is to offer to the farmer a concise outline of the general principles of Agricultural Chemistry. It has no pretensions to be considered a complete treatise on the subject. On the contrary, its aim is strictly elementary, and with this view I have endeavoured, as far as possible, to avoid unnecessary technicalities so as to make it intelligible to those who are unacquainted with the details of chemical science, although I have not hesitated to discuss such points as appeared essential to the proper understanding of any particular subject.

The rapid progress of agricultural chemistry, and the numerous researches prosecuted under the auspices of agricultural societies and private experimenters in this and other countries, render it by no means an easy task to make a proper selection from the mass of facts which is being daily accumulated. In doing this, however, I have been guided by a pretty intimate knowledge of the wants of the farmer, which has induced me to enlarge on those departments of the subject which bear more immediately on the every-day practice of agriculture; and for this reason the composition and properties of soils, the nature of manures, and the principles by which their application ought to be governed, have been somewhat minutely treated.

In all cases numerical details have been given as fully as is consistent with the limits of the work; and it may be right to state that a considerable number of the analyses contained in it have been made in my own laboratory, and that even when I have preferred to quote the results of other chemists, they have not unfrequently been confirmed by my own experiments.

University of Glasgow,
1st November 1860.

CONTENTS.

Page
Introduction [1]
CHAPTER I.
The Organic Constituents of Plants.
Carbon ... Carbonic Acid ... Hydrogen ... Nitrogen ... Nitric Acid ... Ammonia ... Oxygen ... Sources whence obtained ... The Atmosphere ... The Soil ... Source of the Inorganic Constituents of Plants ... Manner in which the Constituents of Plants are absorbed [8]
CHAPTER II.
The Proximate Constituents of Plants.
The Saccharine and Amylaceous Constituents ... Cellulose ... Incrusting Matter ... Starch ... Lichen Starch ... Inuline ... Gum ... Dextrine ... Sugar ... Mucilage ... Pectine and Pectic Acid ... Oily or Fatty Matters ... Margaric, Stearic, and Oleic Acids ... Wax ... Nitrogenous or Albuminous Constituents of Plants and Animals ... Albumen ... Fibrine ... Casein ... Diastase [40]
CHAPTER III.
The Changes which take place in the Food of Plants during their Growth.
Changes occurring during Germination ... Changes during the After-Growth of the Plant ... Decomposition of Carbonic Acid ... Decomposition of Water ... Decomposition of Ammonia ... Decomposition of Nitric Acid [54]
CHAPTER IV.
The Inorganic Constituents of Plants.
The Amount of Inorganic Matters in Different Plants ... The Relative Proportions of Ash in the Different Parts of Plants ... Influence of the Nature of the Soil on the Proportion of Mineral Matters in the Plant ... The Composition of the Ashes of Plants ... Classification of Different Plants [63]
CHAPTER V.
The Soil—Its Chemical and Physical Characters.
The Origin of Soils ... Composition of Crystalline and Sedimentary Rocks ... their Disintegration ... Chemical Composition of the Soil ... Fertile and Barren Soils ... Mechanical Texture of Soils ... Absorbent Action of Soils ... their Physical Characters ... Relation to Heat and Moisture ... The Subsoil ... Classification of Soils [83]
CHAPTER VI.
The Improvement of the Soil by Mechanical Processes.
Draining ... Its Advantageous Effects ... Subsoil and Deep Ploughing ... Improving the Soil by Paring and Burning ... Warping ... Mixing of Soils ... Chalking [137]
CHAPTER VII.
The General Principles of Manuring.
Fundamental Principles upon which Manures are applied ... Special and General Manures ... Importance of this distinction ... Views regarding the Theory of Manures ... Remarks on Special Manures ... Action of Manures on the Chemical and Physical Properties of a Soil ... Remarks on the Application of Manures[152]
CHAPTER VIII.
The Composition and Properties of Farm-yard and Liquid Manures.
Farm-yard Manure ... Urine ... Composition of ... Dung ... Composition of ... Farm-yard Manure ... Composition of ... Management of Dung-Heaps ... Box-feeding ... Fermentation and application of Manure ... Liquid Manure ... Composition and application of ... Sewage Manure ... Its composition and application [166]
CHAPTER IX.
The Composition and Properties of Vegetable Manures.
Rape-Dust, Mustard, Cotton and Castor Cake ... Composition of various Oil-Cakes ... Malt-Dust, Bran, Chaff, etc. ... Straw and Saw-dust ... Manuring with Fresh Vegetable Matter ... Green Manuring ... Sea-Weed ... Composition of various Sea-Weeds ... Leaves ... Peat [195]
CHAPTER X.
The Composition and Properties of Animal Manures.
Guano, different varieties of ... Average composition of ... Division into Ammoniacal and Phosphatic ... Characters of ... Adulteration of ... Application of ... Pigeons' Dung ... Urate and Sulphated Urine ... Night-Soil and Poudrette ... Hair, Skin, Horn, Wool, etc. ... Blood ... Fish ... "Fish-Guano"—Bones [204]
CHAPTER XI.
The Composition and Properties of Mineral Manures.
Mineral Manures ... Sulphate and Muriate of Ammonia ... Sulphomuriate of Ammonia ... Ammoniacal Liquor ... Nitrates of Potash and Soda ... Muriate and Sulphate of Potash ... Chloride of Sodium, or Common Salt ... Carbonates of Potash and Soda ... Silicates of Potash and Soda ... Sulphate of Magnesia ... Phosphate of Lime ... Bone-ash ... Coprolites ... Apatite ... Sombrero Guano ... Superphosphates and Dissolved Bones ... Biphosphate of Lime or Soluble Phosphates ... Phospho-Peruvian Guano ... Lime ... Chalk ... Marl ... Application and Action of Lime on Soils ... Sulphate of Lime or Gypsum [226]
CHAPTER XII.
The Valuation of Manures.
The Principle on which Manures are valued ... Its application to different simple and complex Manures ... Method of Calculation ... General Remarks [255]
CHAPTER XIII.
The Rotation of Crops.
Its necessity explained ... Quantity of Mineral Matters in the produce of an Acre of Different Crops ... The Theory of Rotation[266]
CHAPTER XIV.
The Feeding of Farm Stock.
The Principles of Feeding ... The Composition of different Animals in different stages of Fattening ... The Composition of the Food of Animals ... Milk ... The Principal Varieties of Cattle Food ... General Observations on Feeding [276]

AGRICULTURAL CHEMISTRY.

INTRODUCTION.

That the phenomena of vegetation are dependent on certain chemical changes occurring in the plant, by which the various elements of its food are elaborated and converted into vegetable matter, was very early recognised by chemists; and long before the correct principles of that science were established, Van Helmont maintained that plants derived their nourishment from water, while Sir Kenelm Digby, Hook, Bradley, and others, attributed an equally exclusive influence to air, and enlarged on the practical importance of the conclusions to be deduced from their views. These opinions, which were little better than hypotheses, and founded on very imperfect chemical data, are mentioned by Jethro Tull, the father of modern agriculture, only to deny their accuracy; and he contended that the plants absorb and digest the finer particles of the earth, and attributed the success of the particular system of husbandry he advocated to the comminution of the soil, by which a larger number of its particles are rendered sufficiently small to permit their ready absorption by the roots. Popular opinion at that time was in favour of the mechanical rather than the chemical explanation of agricultural facts, and Tull's work had the effect of confirming this opinion, and turning attention away from the application of chemistry to agriculture. Indeed, no good results could have followed its study at that time, for chemistry, especially in those departments bearing more immediately on agriculture, was much too imperfect, and it was only towards the close of the last century, when Lavoisier established its true principles, that it became possible to pursue it with any prospect of success.

Very soon after Lavoisier's system was made known, Lord Dundonald published his "Treatise on the Intimate Connexion between Chemistry and Agriculture," in which the important bearings of the recent chemical discoveries on the practice of agriculture were brought prominently under the notice of the farmer, and almost at the same time De Saussure commenced those remarkable researches, which extended over a long series of years, and laid the foundation of almost all our accurate knowledge of the chemistry of vegetation. Saussure traced with singular care and accuracy the whole phenomena of the life of plants, and indicated the mode in which the facts he established might be taken advantage of in improving the cultivation of the soil. But neither his researches, nor Lord Dundonald's more direct appeal to the farmer, excited the attention they deserved, or produced any immediate effect on the progress of agriculture. It was not till the year 1812 that the interest of practical men was fairly awakened by a course of lectures given by Sir Humphrey Davy, at the instance of Sir John Sinclair, who was at that time president of the Board of Agriculture. In these lectures, written with all the clearness and precision which characterised their author's style, the results of De Saussure's experiments were for the first time presented to the farmer in a form in which they could be easily understood by him, the conclusions to which they led were distinctly indicated, and a number of useful practical suggestions made, many of which have been adopted into every-day practice, and become so thoroughly incorporated with it, that their scientific origin has been altogether forgotten. A lively interest was excited by the publication of Davy's work, but it soon died out, and the subject lay in almost complete abeyance for a considerable number of years. Nor could any other result be well expected, for at that time agriculture was not ripe for chemistry, nor chemistry ripe for agriculture. The necessities of a rapidly increasing population had not yet begun to compel the farmer to use every means adapted to increase the amount of production to its utmost limit; and though the fundamental principles of chemistry had been established, its details, especially in that department which treats of the constituents of plants and animals, were very imperfectly known. It is not surprising, therefore, that matters should have remained almost unchanged for 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.

Saussure's researches formed the main groundwork of Liebig's treatise, as they had before done for Davy's; but the progress of science had supplied many new facts which confirmed the opinions of the older chemists in most respects, and enabled Liebig to generalise with greater confidence, and illustrate more fully the principles upon which chemistry ought to be applied to agriculture. Few works have ever produced a more profound impression. Written in a clear and forcible style, dealing with scientific truths in a bold and original manner, and producing a strong impression, as well by its earnestness as by the importance of its conclusions, it was 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 these 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 lauded them as the commencement of a new era, but who had absurdly expected an instantaneous effect, changed their opinion, and contemned, as strongly as they had before supported, the application of chemistry to agriculture.

That this effect should have been produced is not unnatural; for practical men, having at that time little or no knowledge of chemistry, were necessarily unable to estimate its true position in relation to agriculture, and forgetting that this department of science was still in its early youth, and burthened with all the faults and errors of youth, they treated it as if it were already perfect in all its parts. Neither could they distinguish between the fully demonstrated scientific truths, and the uncertain, though probable conclusions deduced from them; and when the latter, as occasionally happened, proved to be at variance with practice, it is not surprising: that this 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 hypothetical inference, which, however probable, is, after all, merely a suggestion requiring to be substantiated by experiment. This omission, which the scientific reader can supply for himself, becomes a source of serious misapprehension in a work addressed to persons unacquainted with science, who adopt indiscriminately both the facts and the hypotheses of the author. And this is no doubt the cause of the vary different estimation in which the work of the Giessen Professor was held by scientific and practical men.

Liebig's treatise was followed, in the year 1844, by the publication of Boussingault's Economic Rurale, a work winch excited at the time infinitely less interest than Liebig's, although it is really quite as important a contribution to scientific agriculture. It is distinguished by entering more fully into the special details of the application of chemistry to agriculture, and contains the results of the author's numerous searches both in the laboratory and the field. Boussingault possesses the qualification, at present somewhat rare, of combining a thorough 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, are the largest and most valuable contribution to the exact data of scientific agriculture which has yet been made public.

The year 1844 was also distinguished by the foundation of the Agricultural Chemistry Association of Scotland, an event of no small importance in the history of scientific agriculture. That association 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, and has formed the model of similar establishments in London, Dublin, and Belfast, as well as in Germany; and it is peculiarly creditable to the intelligence and energy of the practical farmers of Scotland, that with them commenced a movement, which has already found imitators in so many quarters, and conferred such great benefits on agriculture. Within the last ten or twelve years, and mainly owing to the establishment of agricultural laboratories, 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 this subject has considerably increased, though still greatly less than its exigencies require.

Notwithstanding all that has recently been done, it must not be forgotten that we have scarcely advanced beyond the threshold, and that it is only by numerous and frequently repeated experiments that it is possible to arrive at satisfactory results. Agricultural inquiries are liable to peculiar fallacies due to the perturbing influence of climate, season, and many other causes, the individual effects of which can only be eliminated with difficulty, and much error has been introduced, by hastily generalising from single experiments, in place of awaiting the results of repeated trials. Hence it is that the progress of scientific agriculture must necessarily be slow and gradual, and is not likely to be marked by any great or startling discoveries. Now that the relations of science to practice are better understood, the extravagant expectations at one time entertained have been abandoned, and, as a necessary consequence, the interest in agricultural chemistry has again increased, and the conviction daily gains ground that no one who wishes to farm with success, can afford to be without some knowledge of the scientific principles of his art.

CHAPTER I.

THE ORGANIC CONSTITUENTS OF PLANTS.

When the water naturally existing in plants is expelled by exposure to the air or a gentle heat, the residual dry matter is found to be composed of a considerable number of different substances, which have been divided into two great classes, called the organic and the inorganic, or mineral constituents of plants. The former are readily combustible, and on the application of heat, catch fire, and are entirely consumed, leaving the inorganic matters in the form of a white residuum or ash. All plants contain both classes of substances; and though their relative proportions vary within very wide limits, the former always greatly exceed the latter, which in many cases form only a very minute proportion of the whole weight of the plant. Owing to the great preponderance of the organic or combustible matters, it was at one time believed that the inorganic substances formed no part of the true structure of plants, and consisted only of a small portion of the mineral matters of the soil, which had been absorbed along with their organic food; but this opinion, which probably was never universally entertained, is now entirely abandoned, and it is no longer doubted that both classes of substances are equally essential to their existence.

Although they form so large a proportion of the plant, its organic constituents are composed of no more than four elements, viz.:—

Carbon.
Hydrogen.
Nitrogen.
Oxygen.

The inorganic constituents are much more numerous, not less than thirteen substances, which appear to be essential, having been observed. These are—

Potash.
Soda.
Lime.
Magnesia.
Peroxide of Iron.
Silicic Acid.
Phosphoric Acid.
Sulphuric Acid.
Chlorine.

And more rarely

Manganese.
Iodine.
Bromine.
Fluorine.

Several other substances, among which may be mentioned alumina and copper, have also been enumerated; but there is every reason to believe that they are not essential, and the cases in which they have been found are quite exceptional.

It is to be especially noticed that none of these substances occur in plants in the free or uncombined state, but always in the form of compounds of greater or less complexity, and extremely varied both in their properties and composition.

It would be out of place, in a work like the present, to enter into complete details of the properties of the elements of which plants are composed, which belongs strictly to pure chemistry, but it is necessary to premise a few observations regarding the organic elements, and their more important compounds.

Carbon.—When a piece of wood is heated in a close vessel, it is charred, and converted into charcoal. This charcoal is the most familiar form of carbon, but it is not absolutely pure, as it necessarily contains the ash of the wood from which it was made. In its purest form it occurs in the diamond, which is believed to be produced by the decomposition of vegetable matters, and it is there crystallized and remarkably transparent; but when produced by artificial processes, carbon is always black, more or less porous, and soils the fingers. It is insoluble in water, burns readily, and is converted into carbonic acid. Carbon is the largest constituent of plants, and forms, in round numbers, about 50 per cent of their weight when dry.

Carbonic Acid.—This, the most important compound of carbon and oxygen, is best obtained by pouring a strong acid upon chalk or limestone, when it escapes with effervescence. It is a colourless gas, extinguishing flame, incapable of supporting respiration, much heavier than atmospheric air, and slightly soluble in water, which takes up its own volume of the gas. It is produced abundantly when vegetable matters are burnt, as also during respiration, fermentation, and many other processes. It is likewise formed daring the decay of animal and vegetable matters, and is consequently evolved from dung and compost heaps.

Hydrogen occurs in nature only in combination. Its principal compound is water, from which it is separated by the simultaneous action of an acid, such as sulphuric acid and a metal, in the form of a transparent gas, lighter than any other substance. It is very combustible, burns with a pale blue flame, and is converted into water. It is found in all plants, although in comparatively small quantity, for, when dry, they rarely contain more than four or five per cent. Its most important compound is water, of which it forms one-ninth, the other eight-ninths consisting of oxygen.

Nitrogen exists abundantly in the atmosphere, of which it forms nearly four-fifths, or, more exactly, 79 per cent. It is there mixed, but not combined with oxygen; and when the latter gas is removed, by introducing into a bottle of air some substance for which the former has an affinity, the nitrogen is left in a state of purity. It is a transparent gas, which is incombustible and extinguishes flame. It is a singularly inert substance, and is incapable of directly entering into union with any other element except oxygen, and with that it combines with the greatest difficulty, and only by the action of the electric spark—a peculiarity which has very important bearings on many points we shall afterwards have to discuss. Nitrogen is found in plants to the extent of from 1 to 4 per cent.

Nitric Acid.—This, the most important compound of nitrogen and oxygen, can be produced by sending a current of electric sparks through a mixture of its constituents, but in this way it can be obtained only in extremely small quantity. It is much more abundantly produced when organic matters are decomposed with free access of air, in which case the greater proportion of their nitrogen combines with the atmospheric oxygen. This process, which is known by the name of nitrification, is greatly promoted by the presence of lime or some other substance, with which the nitric acid may combine in proportion as it is formed. It takes place, to a great extent, in the soil in India and other hot climates; and our chief supplies of saltpetre, or nitrate of potash, are derived from the soil in these countries, where it has been formed in this manner. The same change occurs, though to a much smaller extent, in the soil in temperate climates.

Ammonia is a compound of nitrogen and hydrogen, but it cannot be formed by the direct union of these gases. It is a product of the decomposition of organic substances containing nitrogen, and is produced when they are distilled at a high temperature, or allowed to putrefy out of contact of the air. In its pure state it is a transparent and colourless gas, having a peculiar pungent smell, and highly soluble in water. It is an alkali resembling potash and soda, and, like these substances, unites with the acids and forms salts, of which the sulphate and muriate are the most familiar. In these salts it is fixed, and does not escape from them unless they be mixed with lime, or some other substance possessing a more powerful affinity for the acid with which it is united.

Oxygen is one of the most widely distributed of all the elements, and, owing to 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 almost all the other chemical elements. 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. When diluted with nitrogen, it supports the respiration of animals; but in the pure state it proves fatal after the lapse of an hour or two. It is found in plants, in quantities varying from 30 to 36 per cent.

It is worthy of observation, that of the four organic elements, carbon only is fixed, and the other three are gases; and likewise, when any two of them unite, their compound is either a gaseous or a volatile substance. The charring of organic substances, which is one of their most characteristic properties, and constantly made use of by chemists as a distinctive reaction, is due to this peculiarity; for when they are heated, a simpler arrangement of their particles takes place, the hydrogen, nitrogen, and oxygen unite among themselves, and carry off a small quantity of carbon, while the remainder is left behind in the form of charcoal, and is only consumed when access of the external air is permitted.

Now, in order that a plant may grow, its four organic constituents must be 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, when placed in similar circumstances, shows no disposition to increase, but rapidly languishes and dies. The obvious inference from these facts is, that these substances cannot be absorbed when in the elementary state, but that it is only after they have entered into certain forms of combination that they acquire the property of being readily taken up, and assimilated by the organs of the plant.

It was at one time believed that many different compounds of these elements might be absorbed and elaborated, but later and more accurate experiments have reduced the number to four—namely, carbonic acid, water, ammonia, and nitric acid. The first supplies carbon, the second hydrogen, the two last nitrogen, while all of them, with the 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 each.

The Atmosphere as a source of the Organic Constituents of Plants.—Atmospheric air consists of a mixture of nitrogen and oxygen gases, watery vapour, carbonic acid, ammonia, and nitric acid. The two first are the largest constituents, and the others, though equally essential, are present in small, and some of them in extremely minute quantity. When deprived of moisture and its minor constituents, 100 volumes of air are found to contain 21 of oxygen and 79 of nitrogen. Although these gases are not chemically combined in the air, but only mechanically mixed, their proportion is exceedingly uniform, for analyses completely corresponding with these numbers 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 so counterbalanced by others acting in an opposite direction, as to retain a uniform proportion between the main constituents of the atmosphere, and to prevent the undue accumulation of one or other of them at any one point.

No such uniformity exists in the proportion of the minor constituents. The variation in the quantity of watery vapour is a familiar fact, the difference between a dry and moist atmosphere being known to the most careless observer, and the proportions of the other constituents are also liable to considerable variations.

Carbonic Acid.—The proportion of carbonic acid in the air has been investigated by Saussure. From his experiments, made at the village of Chambeisy, near Geneva, it appears that the quantity is not constant, but varies from 3·15 to 5·75 volumes in 10,000; the mean being 4·15. These variations are dependent on different circumstances. It was found that the carbonic acid was always more abundant during the night than during the day—the mean quantity in the former case being 4·32, in the latter 3·38. The largest quantity found during the night was 5·74, during the day 5·4. Heavy and continued rain diminishes the quantity of carbonic acid, by dissolving and carrying it down into the soil. Saussure found that in the month of July 1827, during the time when nine millimetres of rain fell, the average 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. A moist state of the soil, which is favourable to the absorption of carbonic acid, also diminishes the quantity contained in the air, while, on the other hand, continued frosts, by retaining the atmosphere and soil in a dry state, have 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; 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, though small, are quite distinct, and have been confirmed by subsequent experimenters.

Ammonia.—The presence of ammonia in the atmosphere appears to have been first observed by Saussure, who found that when the sulphate of alumina is exposed to the air, it is gradually converted into the double sulphate of alumina and ammonia. Liebig more recently showed that ammonia can always be detected in rain and snow water, and it could not be doubted that it had been absorbed from the atmosphere. Experiments have since been made by different observers with the view of determining the quantity of atmospheric ammonia, and their results are contained in the subjoined table, which gives the quantity found in a million parts of air.

Of these results, the earlier ones of Kemp, Pierre, and Graeger are undoubtedly erroneous, as they were made without those precautions which subsequent experience has shown to be necessary. Even those of the other observers must be taken as giving only a very general idea of the quantity of ammonia in the air, for a proportion so minute as one fifty-millionth cannot be accurately determined even by the most delicate experiments. For this reason, more recent experimenters have endeavoured to arrive at conclusions bearing more immediately upon agricultural questions, by determining the quantity of ammonia brought down by the rain. The first observations on this subject were made by Barral in 1851, and they have been repeated during the years 1855 and 1856 by Mr. Way. In 1853, Boussingault also made numerous experiments on the quantity of ammonia in the rain falling at different places, as well as in dew and the moisture of fogs. He found in the imperial gallon—

It thus appears that in Paris the quantity of ammonia in rain-water is just six times as great as it is in the country, a result, no doubt, due to the ammonia evolved during the combustion of fuel, and to animal exhalations, and to the same cause, the large quantity contained in the moisture of fogs in Paris may also be attributed. Barral and Way have made determinations of the quantity of ammonia carried down by the rain in each month of the year, the former using for this purpose the water collected in the rain-gauges of the Paris Observatory, and representing, therefore, a town atmosphere; the latter, that from a large rain-gauge at Rothamsted, at a distance from any town. According to Barral the ammonia annually deposited on an acre of land amounts to 12·28 lbs., a quantity considerably exceeding that obtained by Way, whose experiments being made at a distance from towns, must be considered as representing more accurately the normal condition of the air. His results for the years 1855 and 1856 are given below, along with the quantities of nitric acid found at the same time.

Nitric Acid.—The presence of nitric acid in the air appears to have been first observed by Priestley at the end of the last century, but Liebig, in 1825, showed that it was always to be found after thunder-storms, although he failed to detect it at other times. In 1851 Barral proved that it is invariably present in rain-water, and stated the quantity annually carried down to an acre of land at no less than 41·29 lbs. But at the time his experiments were made, the methods of determining very minute quantities of nitric acid were exceedingly defective, and Way, by the adoption of an improved process, has shown that the quantity is very much smaller than Barral supposed, and really falls short of three pounds. His results for ammonia, as well as nitric acid, are given in the subjoined table.

No attempts have been made to determine the proportion of nitric acid in air, but its quantity is undoubtedly excessively minute, and materially smaller than that of ammonia. At least this conclusion seems to be a fair inference from Way's researches, as well as the recent experiments of Boussingault on the proportion of nitric acid contained in rain, dew, and fog, made in a manner exactly similar to those on the ammonia, already quoted. According to his experiments an imperial gallon contains—

Although it thus appears that Barral's results have been only partially confirmed, enough has been ascertained to show that the quantity of ammonia and nitric acid in the air is sufficient to produce a material influence in the growth of plants. The large amount of these substances contained in the dew is also particularly worthy of notice, and may serve to some extent to explain its remarkably invigorating effect on vegetation.

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.

Sulphuretted Hydrogen and Phosphuretted 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.

The preceding statements lead to the important conclusion, that the atmosphere is capable of affording an abundant supply of all the organic elements of plants, because it not only contains nitrogen and oxygen in the free state, but also in those forms of combination in which they are most readily absorbed, as well as a large quantity of carbonic acid, from which their carbon may be derived. At first sight it may indeed appear that the quantity of the latter compound, and still more that of ammonia, is so trifling as to be of little practical importance. But a very simple calculation serves to show that, though relatively small, they are absolutely large, for the carbonic acid contained in the whole atmosphere amounts in round numbers to

2,400,000,000,000 tons,

and the ammonia, assuming it not to exceed one part in fifty millions, must weigh

74,000,000 tons,

quantities amply sufficient to afford an abundant supply of these elements to the whole vegetation of our globe.

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 is small, and the combustible large, these proportions are reversed in the soil, which consists chiefly of inorganic or mineral matters, mixed with a quantity of combustible or organic substances, rarely exceeding 8 or 10 per cent, and often falling considerably short of this quantity.

The organic matter exists in the form of a substance called humus, 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 applied by chemists to a rather numerous group of substances, very closely allied in their properties, several of which 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 extracted by means of caustic potash; and the third, those 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 acids, while the soil still retains what has been called insoluble humus.

The acids above named do not differ greatly in chemical characters, but they have been subdivided into the humic, geic, and crenic groups, which present some differences in properties and composition. They are compounds of carbon, hydrogen, and oxygen, and are characterised by so powerful an affinity for ammonia 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 formed during their decay by a succession of changes, which may be easily traced by observing the course of events when a piece of wood or any other vegetable substance is exposed for a length of time to air and moisture. It is then found gradually to disintegrate with the evolution of carbonic acid, acquiring first a brown and finally a black colour. At one particular stage of the process it is converted into one or other of two substances, called humin and ulmin, both insoluble in alkalies, and apparently identical with the insoluble humus of the soil; but when the decomposition is more advanced 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 as the result of an oxidation occurring at certain periods of the decay.

The roots and other vegetable debris remaining in the soil undergo a similar series of changes, and form the humus, which is found only in the surface soil, that is to say, in the portion which is now or has at some previous period been occupied by plants, and the quantity of humus contained in any soil is mainly dependent on the activity of vegetation on it. Numerous analyses of humus compounds extracted from the soil 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, due manifestly to the fact that their production is the result of a gradual decomposition, which renders it impossible to extract from the soil one pure substance, but only a variable mixture of several, so similar to one another in properties, that their separation is very difficult, if not impossible. For this reason great discrepancies exist in the statements made regarding them by different observers, but this is a matter of comparatively small importance, as their exact composition has no very direct bearing on agricultural questions, and it will suffice to give the names and chemical formulæ of those which have been analysed and described,—

It is only necessary to observe further, that these formulæ indicate a close connection with woody fibre, and the continuous diminution of the hydrogen and increase of oxygen shows that they must have been produced by a gradually advancing decay.

The earlier chemists and vegetable physiologists attributed to the humus of the soil a much more important function than it is now believed to possess.

It was formerly considered to be the exclusive, or at least 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. But though this view has still some supporters, among whom Mulder is the most distinguished, it is now generally admitted that humus is not a direct source of the organic constituents of plants, and is not absorbed as such by their roots, although 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, for humus, as 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 in the same form, in order to supply the generation which succeeds them. But this is very far from being 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 very fully, has made an approximative calculation of the total quantity of humus on the earth's surface, and of the 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.

The particular phenomena of vegetation also afford abundant evidence that humus cannot be the only source of carbon. Thus Boussingault has shown that on the average of years, the crops cultivated on an acre of land remove from it about one ton more organic matter than they receive in the manure applied to them, although there is no corresponding diminution in the quantity of humus contained in the soil. An instance which leads still more unequivocally to the same conclusion is given by Humboldt. He states that an acre of land, planted with bananas, yields annually about 152,000 pounds weight of fruit, containing about 32,000 pounds, or almost exactly 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 obvious from these and many other analogous facts that humus cannot be the only or even a considerable source of the carbon of plants, although it is still contended by some chemists that it may be absorbed to a small extent. But even this is at variance with many well-known facts. For if humus were absorbed, it might be expected that vegetation would be most luxuriant on soils containing abundance of that substance, especially if it existed in a soluble and readily absorbable form; but so far from this being the case, nothing is more certain than that peat, in which these conditions are fulfilled, is positively injurious to most plants. On the other hand, our daily experience affords innumerable examples 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, weighing 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 Prince Salm Horstmar, on oats and rape sown in a soil deprived of organic matter by ignition, in which they grew readily, and arrived at complete maturity. One oat straw attained 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 Polstorf, these observers having found that it was possible to obtain a two-hundred-fold produce of barley in an entirely artificial soil, provided care was taken to give it the physical characters of a fertile soil. They prepared a mixture of six parts of sand, two of chalk, one of white bole, and one of wood charcoal; to which was added a small quantity of felspar, previously fused 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 pickles. The plants grew luxuriantly, reaching a height of three feet, and each bearing nine ears, containing 22 pickles. The grain of the twelve plants weighed 2040 grains.

These experiments show that plants can grow and produce seed when the most scrupulous care is taken to deprive them of every trace of humus. But Saussure has gone further, and shown that even when present, humus is not absorbed. He 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 was really due to absorption by the plant. This omission has been supplied by Weigman and Polstorf. They grew plants of mint (Mentha undulata) 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, etc. was found to become gradually paler, and at the end of a month, during which time the plants had increased by 6-1/2 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.

But though not directly capable of affording nutriment to plants, it must not, on that account, be supposed that humus is altogether devoid of importance, for it is constantly undergoing decomposition in the soil, and thus becomes a source of carbonic acid which can be absorbed, and, as we shall afterwards more particularly see, it exercises very important functions in bringing the other constituents of the soil into readily available forms of combination.

It has been already observed that carbon, hydrogen, nitrogen, and oxygen, cannot be absorbed by plants when uncombined, but only in the forms of water, carbonic acid, ammonia, and nitric acid. It is scarcely necessary to detail the grounds on which this conclusion has been arrived at in regard to carbon and hydrogen, for practically it is of little importance whether they can be absorbed or not, as the former is rarely, the latter never, found uncombined in nature. Neither can there be any doubt that water and carbonic acid are the only substances from which these elements can be obtained. Every-day experience convinces us that water is essential to vegetation; and Saussure, and other observers, have shown that plants will not grow if they are deprived of carbonic acid, and that they actually absorb that substance abundantly from the atmosphere. The evidence for the non-absorption of oxygen lies chiefly in the fact that plants obtain, in the form of water and carbonic acid, a larger quantity of that element than they require, and in place of absorbing, are constantly exhaling it. The form in which nitrogen may be absorbed has given rise to much difference of opinion. In the year 1779, Priestley commenced the examination of this subject, and drew from his experiments the conclusion, that plants absorb the nitrogen of the air. Saussure shortly afterwards examined the same subject, and having found, that when grown in a confined space of air, and watered with pure water, the nitrogen of the plants underwent no increase, he inferred that they derived their entire supplies of that element from ammonia, or the soluble nitrogenous constituents of the soil or manure. Boussingault has since re-examined this question, and by a most elaborate series of experiments, in which the utmost care was taken to avoid every source of fallacy, he was led to the conclusion, that when haricots, oats, lupins, and cresses were grown in calcined pumice-stone, mixed with the ash of plants, and supplied with air deprived of ammonia and nitric acid, their nitrogen underwent no increase. It has been objected to these experiments, that the plants being confined in a limited bulk of air, were placed in an unnatural condition, and Ville has recently repeated them with a current of air passing through the apparatus, and found a slight increase in the nitrogen, due, as he thinks, to direct absorption. It is much more probable, however, that it depends on small quantities of ammonia or nitric acid which had not been completely removed from the air by the means employed for that purpose, for nothing is more difficult than the complete abstraction of these substances, and as the gain of nitrogen was only 0·8 grains, while 60,000 gallons of air, and 13 of water, were employed in the experiment, which lasted for a considerable time, it is reasonable to suppose that a sufficient quantity may have remained to produce this trifling increase.

While these experiments show that plants maintain only a languid existence when grown in air deprived of ammonia and nitric acid, and hence, that the direct absorption of nitrogen, if it occur at all, must do so to a very small extent, the addition of a very minute quantity of the former substance immediately produces an active vegetation and rapid increase in size of the plants. Among the most striking proofs of this are the experiments of Wolff, made by growing barley and vetches in a soil calcined so as to destroy organic matters, and then mixed with small quantities of different compounds of ammonia. He found that when the produce from the calcined soil was represented by 100, that from the different ammoniacal salts was—

These experiments not only prove that ammonia can be absorbed, but they also indirectly confirm the statement already made, that humus is not necessary; for in some instances the produce was higher than that obtained from the uncalcined soil with the same manures, although it contained four per cent of humus.

On such experiments Liebig rests his opinion that ammonia is the exclusive source of the nitrogen of plants, and although he has recently admitted that it may be replaced by nitric acid, it is obvious that he considers this a rare and exceptional occurrence. The evidence, however, for the absorption of nitric acid appears to rest on as good grounds as that of ammonia, for experience has shown that nitrate of soda acts powerfully as a manure, and its effect must be due to the nitric acid, and not to the soda, for the other compounds of that alkali have no such effect. Wolff has illustrated this point by a series of experiments on the sunflower, of which we shall quote one. He took two seeds of that plant, and sowed them on the 10th May, in a soil composed of calcined sand, mixed with a small quantity of the ash of plants, and added at intervals during the progress of the experiment, a quantity of nitrate of potash, amounting in all to 17·13 grains. The plants were watered with distilled water, containing carbonic acid in solution, and the pot in which they grew was protected from rain and dew by a glass cover. On the 19th August one of the plants had attained a height of above 28 inches, and had nine fine leaves and a flower-bud; the other was about 20 inches high, and had ten leaves. On the 22d August, one of the plants having been accidentally injured, the experiment was terminated. The plants, which contained 103·16 grains of dry matter, were then carefully analysed, and the quantity of nitrogen contained in the soil after the experiment and in the seed was determined.

Hence, the nitrogen contained in the plants must, in this instance, have been obtained entirely from the nitrate of potash, for the quantity contained in it and in the seeds is exactly equal to that in the plants and the soil, the difference of 0·03 grains being so small that it may be safely attributed to the errors inseparable from such experiments. For the sake of comparison, an exactly similar experiment was made on two seeds grown without nitrate of potash, and in this instance, after an equally long period of growth, the largest plant had only attained a height of 7·5 inches, and had three small pale and imperfectly developed leaves. They contained only 0·033 grains of nitrogen, while the seeds contained 0·032—indicating that, under these circumstances, there was no increase in the quantity of that element.

But, independently of these experimental results, it may be inferred from general considerations, that nitric acid must be one of the sources from which plants derive their nitrogen. It has been already stated, that the humus contained in the soil consists of the remains of decayed plants, and there is every reason to suppose that the primeval soil contained no organic matters, and that the first generation of plants must have derived the whole of their nitrogen from, the atmosphere. If, therefore, it be assumed that ammonia is the only source of the nitrogen of plants, it would follow, that as that substance cannot be produced by the direct union of its elements, the quantity of ammonia in the air could only remain undiminished in the event of the whole of the nitrogen of decaying plants returning into that form. But this is certainly not the case, for every time a vegetable substance is burned, part of its nitrogen is liberated in the free state, and in certain conditions of putrefaction, nitric acid is produced. Now, if ammonia be the only form in which nitrogen is absorbed, there must be a gradual diminution of the quantity contained in the air; and further, there must either be some continuous source of supply by which its quantity is maintained, or there must be some other substance capable of affording nitrogen in a form fitted for the maintenance of plant life. As regards the first alternative, it must be stated that we know of no source other than the decomposition of plants from which ammonia can be derived, and we are therefore compelled to adopt the second alternative, and to admit that there must be some other source of nitrogen, and it cannot be doubted, from what has been already stated, that it is from nitric acid only that it can be obtained.

It must be admitted, then, that carbonic acid, ammonia, nitric acid, and water, are the great organic foods of plants. But while they have afforded to them an inexhaustible supply of the last, the quantity of the other three available for food are limited, and insufficient to sustain their life for a prolonged period. It has been shown by Chevandrier, that an acre of land under beech wood accumulates annually about 1650 lb. of carbon. Now, the column of air resting upon an acre of land contains only about 15,500 lb. of carbon, and the soil may be estimated to contain 1 per cent., or 22,400 lb. 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 indirectly 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 a continuous circulation of these constituents through both plants and animals, there are various changes which tend to liberate in the free state a certain quantity both of the carbon and nitrogen of plants, and these being thus removed from the sphere of organic life, there would be a gradual diminution in the amount of vegetation at the earth's surface, unless this loss were counterbalanced by some corresponding source of gain. In regard to carbonic acid the most important source is volcanic action, but the loss of nitrogen, which is far more important and considerable, is restored by the direct combination of its elements. The formation of nitric acid during thunder storms has been long familiar; but it would appear from the recent experiments of Clöez, which, should they be confirmed by farther enquiry, will be of much importance, that this compound is also produced without electrical action when air is passed over certain porous substances, saturated with alkaline and earthy compounds. Fragments of calcined brick and pumice stone were saturated with solution of carbonate of potash, with carbonates of lime and magnesia and other mixtures, and a current of air freed from nitric acid and ammonia passed over them for a long period, at the end of which notable quantities of nitric acid were detected.

Source of the Inorganic Constituents of Plants.—The inorganic constituents of plants being all fixed substances, it is sufficiently obvious that they can only be obtained from the soil, which, as we shall afterwards see, contains all of them 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 from the small quantities in which they are found in plants, imagined that they were there merely accidental impurities 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 in nearly the same proportions, in the ash of each individual plant, has been further refuted by the experiments of Prince Salm Horstmar, who has established 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, weak but upright; and without sulphuric acid, though normal in form, the plant was feeble, and produced no fruit.

Manner in which the Constituents of Plants are absorbed.—Having treated of the sources of the elements of plants, it is necessary to direct attention to the mode in which they enter their system.

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 principally absorbed by the roots, and passes into the tissues of the plant, where a part of it is decomposed, and goes to the formation of certain of its organic compounds; while by far the larger quantity, in place of remaining in it, 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 much diminished during the night. Saussure made similar experiments, and observed that the quantity of water exhaled by a sunflower amounted to about 220 lb. in four months. The exhalation of 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 lb. 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:—

It further appears, that the exhalation is not uniform, but increases during the active growth of the plant, and diminishes again when that period is passed. These variations are shown by the subjoined tables, of which the first gives the total exhalation, and the second the average daily loss of water during certain periods.

Table I.—Showing the Number of Grains of Water given off by the Plants during stated divisional Periods of their Growth.

Table II.—Showing the average daily Loss of Water (in Grains) by the Plants, within several stated divisional Periods of their Growth.

Similar experiments were made with the same plants in soils to which certain manures had been added, and with results generally similar. Calculating from these experiments, we are led to the apparently anomalous conclusion that the quantity of water exhaled by the plants growing on an acre of land greatly exceeds the annual fall of rain; although it is obvious 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. Of course this large exhalation must depend 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 the larger part of the water which a plant requires is absorbed by its roots, the reverse is the case with carbonic acid. A certain proportion no doubt is carried up through the roots by the water, which always contains a quantity of that gas in solution, but by far the larger proportion is directly absorbed from the air by the leaves. A simple experiment of Boussingault's illustrates this absorption very strikingly. He took a large glass globe having three apertures, through one of which he introduced the 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 was 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 and Nitric Acid.—Little is known regarding the mode in which these substances enter the plant. It is usually supposed that they are entirely absorbed by the roots, and no doubt the greater proportion is taken up in this way, but it is very probable that they may also be absorbed by the leaves, at least the addition of ammonia to the air in which plants are grown, materially accelerates vegetation. It is probable, however, that the rain carries down the ammonia to the roots, and there is no doubt that that derived from the decomposition of the nitrogenous matters in the soil is so absorbed.

Inorganic Constituents.—The inorganic constituents of course are entirely absorbed by the roots; and it is as a solvent for them that the large quantity of water continually passing through the plants is so important. They exist in the soil in particular states of combination, in which they are scarcely soluble in water. But their solubility is increased by the presence of carbonic acid contained in the water, and which 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. And phosphate of lime is also taken up by water containing carbonic acid, or even common salt in solution. 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 and Gilbert, 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.

It is worthy of notice, however, that the absorption of the elements of plants takes place even though they may not be in solution in the soil, the roots apparently possessing the power of directly acting on and dissolving insoluble matters; but a distinction must be drawn between this and the view entertained by Jethro Tull, who supposed that they might be absorbed in the solid state, provided they were reduced to a state of sufficient comminution. It is now no longer doubted that, whatever action the roots may exert, the constituents of the plant must be in solution before they can pass into it—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.

CHAPTER II.

THE PROXIMATE CONSTITUENTS OF PLANTS.

The substances absorbed by the plant, which are of simple composition, and contain only two elements, are elaborated within it, and converted into the many complicated compounds of which its mass is composed. Some of these, as, for example, the colouring matters of madder and indigo, the narcotic principle of the poppy, &c., are confined to a single species, or small group of plants, while others are found in all plants, and form the main bulk of their tissues. The latter are the only substances which claim notice in a treatise like the present. They have been 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 they contain being always in the proportion required 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 there is no evidence, but only that its elements are present in the proportion to form it.

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 received the name incrusting matter, because 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—

It is represented chemically by the formula, C₂₄H₂₁O₂₁, which shows it to be a compound of 24 atoms of carbon with 21 of hydrogen and 21 of oxygen.

Incrusting matter.—Large quantities of this substance enter into the composition of all plants. Of its chemical nature little is known, as it cannot be obtained separate from cellulose, but it is analogous to that substance in its composition, and probably contains hydrogen and oxygen in the proportion to form water.

Starch.—Starch is one of the most abundant constituents of plants, and is found in most seeds, as those of the cereals and the leguminous plants; in the tubers of the potatoe, the bulbs of tulips, &c. &c. It is obtained by placing a quantity of wheat flour in a bag, and kneading it under a gentle stream of water. When the water is allowed to stand, it deposits the starch as a fine white powder, which, 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. They were formerly believed to be composed of an external coating of a substance insoluble in water, and containing in their interior a soluble kernel; but this opinion has been refuted, and distinct evidence been brought to show that the exterior and interior of the globules are identical in chemical properties. Starch is insoluble in cold water, but by boiling, it dissolves, forming a thick paste. By long continued boiling with water containing a small quantity of acid, it is completely dissolved and converted into dextrine, and eventually into sugar. The same change is produced by the action of fermenting substances, such as the extract of malt; when 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—

and its composition is represented by the formula C₁₂H₁₀O₁₀, so that it differs but little from cellulose in composition, although 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.

Inuline.—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 C₂₄H₂₁O₂₁.

Gum is excreted from various plants as a thick fluid, which dries up into transparent masses. Its composition is identical with that of starch. It dissolves readily in cold water, and is converted into sugar by long continued boiling with acids. Its properties are best marked in gum arabic, which is obtained from various species of acacia; that from other plants differs to some extent, although 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. It 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 produced, but its properties more nearly resemble those of gum. It plays a very important part in the process of germination, and may be converted into sugar on the one hand, and apparently also into starch on the other.

Sugar.—Under this name are included four or five distinct substances, of which the most important are, cane sugar, grape sugar, and the uncrystallisable sugar found in many plants.

Cane Sugar.—This variety of sugar, as its name implies, is found most abundantly in the sugar cane, but it occurs also in the maple, beet-root, and various species of palms, from all of which it is extracted on the large scale. It is extremely soluble in water, and can be obtained in large transparent prismatic crystals, as in common sugar-candy. It swells up, and is converted into a brown substance called caramel, when heated, and by contact with fermenting substances, yields alcohol and carbonic acid. It contains—

and its chemical formula is C₁₂H₁₁O₁₁.

Grape Sugar is met with in the grape, and most other fruits, as well as in honey. It is produced artificially 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—

and its formula is C₁₂H₁₂O₁₂; but when crystallised it contains two equivalents of water, and is then represented by the formula C₁₂H₁₂O₁₂ + 2H₂O.

The uncrystallisable sugar of plants is closely allied to grape sugar, and, so far as at present known, has the same composition, although, from the difficulty of obtaining it quite free from crystallised sugar, this is still uncertain.

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 C₂₄H₁₉O₁₉, or C₁₂H₁₀O₁₀; and in the latter case it must be identical with starch and gum.

It will be observed that all the substances belonging to this class are very closely related in chemical composition, some of them, as starch and gum, though easily distinguished by their properties, being identical in constitution, while others only differ in the quantity of water, or of its elements which they contain. In fact, they may all be considered as compounds of carbon and water, and their relations are, perhaps, more distinctly seen when their formulæ are written so as to show this, as is done in the following table, in the second column of which those containing twelve equivalents of carbon are doubled, so as to make them comparable with cellulose:—

The relation between these substances being so close, it is not difficult to understand how one may be converted into another by the addition or subtraction of water. Thus, cellulose has only to absorb an equivalent of water to become grape sugar, or to lose an equivalent in order to be converted into starch, and we shall afterwards see that such changes do actually occur in the plant during the process of germination.

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 in containing more 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. Oily or Fatty Matters.—The oily constituents of plants form a rather extensive group of substances all closely allied, but distinguished by minor differences in properties and constitution. Some of them are very widely distributed throughout the vegetable kingdom, 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 much less oxygen 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; and they 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 olive oil be exposed to cold, it is seen to become partially solid; and if it be then pressed, a fluid flows out, and a crystalline substance remains; the former is oleine, though not absolutely pure, and the latter margarine. The perfect 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 having a sweet taste, and which has received the name of glycerine, or the sweet principle of oil. Glycerine, as it exists in the fats, appears to be a compound of C₃H₂O, 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 Acid is best obtained pure by boiling olive oil with an alkali until it is saponified, and decomposing the soap with an acid, expressing the margaric acid, which separates, and crystallising it from alcohol. It is a white crystalline fusible solid, insoluble in water, but soluble in alcohol and in solutions of the alkalies. Its composition is—

and its formula C₃₄H₃₄O₄.

Stearic Acid.—Although this acid exists in many plants, it is most conveniently extracted from lard. It is a crystalline solid less fusible than margaric acid, but closely resembling it in its other properties. Its formula is C₃₆H₃₆O₄.

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 C₃₆H₃₄O₄, while that from linseed oil is C₄₆H₃₈O₆, but this is still doubtful.

Other fatty acids have been detected in palm oil, cocoa-nut oil, &c. &c., which so closely resemble margaric and stearic acids as to be easily confounded with them. Though presenting many points of interest, it is unnecessary to describe them in detail here.

Wax is a substance closely allied to the oils. It consists of two substances, cerine and myricine, which are separated from one another by boiling alcohol, in which the former is more soluble. They are extremely complex in composition, the former consisting principally of an acid similar to the fatty acids, called cerotic acid, and containing C₅₄H₅₄O₄. The latter has the formula C₉₂H₉₂O₄. 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 C₄₈H₅₀O₂. It is probable that other plants contain different sorts of wax, but their investigation is still so incomplete, 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. The fatty matters contained in animals are identical with those of plants.

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 when its solution is heated to about 150°, it coagulates into a flocky 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 in its properties from vegetable albumen.

It is a substance of very complicated composition, and chemists are not agreed as to the formula by which its constitution is to be expressed, a difficulty which occurs also with most of the other nitrogenous compounds. The results of the analyses of albumen from different sources are however quite identical, as may be seen from those subjoined—

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, with which its composition completely agrees.

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, the 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 from coagulated albumen.

Animal Fibrine exists in the blood and the muscles, and agrees in all its characters and composition with vegetable fibrine, as is shown by the subjoined analyses—

Caseine.—Vegetable caseine exists abundantly in most plants, especially in the seeds, and remains in the juice after albumen has been precipitated by heat, from which it may be separated 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. When prepared from the pea it has been called legumine, from almonds emulsine, and from oats avenine; 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 use of rennet, and the process of curding milk is simply the coagulation of its caseine. It is soluble in alkalies, and 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—

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—

The most cursory examination of these analytical numbers is sufficient to show 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 already mentioned, 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 sulphur and phosphorus they contain, and which, though very small in quantity, are indubitably essential to them. Much importance has been attributed to these constituents by various chemists, and especially by Mulder, who 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—

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, the more especially as 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, diastase being 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.

CHAPTER III.

THE CHANGES WHICH TAKE PLACE IN THE FOOD OF PLANTS DURING THEIR GROWTH.

The simple compounds which the plant absorbs from the atmosphere and soil are elaborated within its system, and converted into the various complex substances of which its tissues are composed, by a series of changes, the details of which are still in some respects imperfectly known, although their general nature is sufficiently well understood. They may be best rendered intelligible by reference, in the first instance, to the changes occurring during germination, when the young plant is nourished by a supply of food stored up in the seed, in sufficient quantity to maintain its existence until the organs by which it is afterwards to draw its nutriment from the air and soil are sufficiently developed to serve that purpose.

Changes occurring 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, and though probably not absolutely essential, the absence of light is favourable in the early stages. The temperature required for germination varies greatly in different seeds, some germinating readily at a few degrees above the freezing point, and others requiring a tolerably high temperature. The rapidity with which it takes place appears to increase with the temperature; but this is true only within very narrow limits, for beyond a certain point heat is injurious, and when it exceeds 120° or 130° Fahrenheit, entirely prevents the process. The presence of oxygen is also essential, for it has been shown that if seeds are placed in a soil exposed to an atmosphere deprived of that element, or if they be buried so deep that the air does not reach them, they may lie without change for an unlimited period; but so soon as they are exposed to the air, germination immediately commences. Illustrations of this fact are frequently observed where earth from a considerable depth has been thrown up to the surface, when it often becomes covered with plants not usually seen in the neighbourhood, which have sprung from buried seeds. When all the necessary conditions for germination are fulfilled, the seed absorbs moisture, swells up, and sends out a shoot which rises to the surface, and a radicle which descends—the one destined to develop the leaves, the other the roots, by which the plant is afterwards to derive its nutriment from the air and the soil. But until these organs are properly developed, the plant is dependent on the matters contained in the seed itself. These substances are mostly insoluble, but are brought into solution by the atmospheric oxygen acting upon the gluten, and converting it into a soluble substance called diastase, which in its turn reacts upon the starch, converting it first into dextrine, and then into cellulose, and the latter is finally deposited in the form of organised cells, and produces the first little shoot of the plant. At the first moment of germination, the oxygen absorbed appears simply to oxidize the constituents of the seed, but this condition exists only for a very limited period, and is soon followed by the evolution of carbonic acid, water being at the same time formed from the organic constituents of the seed, which gradually diminishes in weight. The amount of this diminution is different with different plants, but 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 never sufficient to counterbalance the diminished 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 observed 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; and as these substances may be viewed as compounds of carbon and water, the change was very simply explained by supposing that the carbon was oxidised and converted into carbonic acid and its water eliminated. But this hypothesis is incapable of explaining all the phenomena observed; for woody fibre, which is one of the chief constituents of the young plant, contains more carbon than the starch and sugar from which it must have been produced, and we are, therefore, forced to admit that the action must be more complicated. There is every reason to believe that the nitrogenous constituents of the seed are most abundantly oxidized, for they are remarkably prone to change; but the action of the air is not confined to them, and it appears most probable that all the substances take part in the decomposition, 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 occurring during the After-growth of the Plant.—When the plant has developed its roots and leaves, and 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, ammonia, and nitric acid, and to decompose and convert them into the different constituents of its tissues. These changes take place slowly at first, and more rapidly as the organs fitted for the elaboration of its food are developed. The roots and the leaves are equally active in performing this duty, the former absorbing the mineral matters along with the carbonic acid, ammonia, nitric acid, and moisture in the soil, or the manure added to it; the latter gathering the gaseous substances existing in the air. Each of these undergoes a series of changes claiming our consideration.

Decomposition of Carbonic Acid.—Carbonic acid, which appears to be absorbed with equal readiness by the roots, leaves, and stems, undergoes immediate decomposition, its carbon being retained, and its oxygen, in whole or in part, evolved into the air. This decomposition occurs only under the action of the sun's rays, and has been found to be proportionate to the amount of light to which the plant is exposed. It takes place only in the green parts of plants, for though the roots absorb carbonic acid, they cannot decompose it, or evolve oxygen; and the coloured parts, the flowers, fruits, etc., have an entirely opposite effect, absorbing oxygen and giving off 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 six days, during each of which the apparatus was exposed for six hours to the sun's rays. The air was analysed both before and after the experiment, and the results obtained were—

In this experiment the whole of the carbonic acid, amounting to 431 volumes, was absorbed, but only 292 volumes of oxygen were given off. Had the carbonic acid been entirely decomposed, and all its oxygen eliminated, its volume would have been equal to that of the acid, or 431, so that in this instance 139 volumes of the oxygen of the carbonic acid have been retained to form part of the tissues of the plant. On the other hand, the nitrogen is found to be increased after the experiment. It might be supposed that the nitrogen evolved had been derived from the decomposition of the nitrogenous constituents of the plant, but this cannot be the true explanation, because in this particular case it greatly exceeded the whole nitrogen contained in the plants experimented on. Its source is not well understood, but Boussingault supposes it to have existed in the interstices of the plant, and to have escaped during the course of the experiment. Saussure found that the oak, the horse-chesnut, and other plants, absorb oxygen and give off carbonic acid in less volumes than the oxygen, while the house-leek and the cactus absorb oxygen without evolving carbonic acid. The absorption and decomposition of carbonic acid takes place only during the day, and matters are entirely reversed during the night, when oxygen is absorbed and carbonic acid eliminated from all parts of the plants.

Although the action occurring during the night is the reverse of that which takes place during the day, it is in no degree to be attributed to a re-oxidation of the carbon which had been deposited in the tissues of the plant. It appears, on the contrary, to be a purely mechanical, and not a chemical process. During the night the sap continues to circulate through the vessels of the plant, and moisture, carrying with it carbonic acid in solution, is absorbed by the roots; but when it reaches the leaves, where the sun's light would have caused its decomposition during the day, it is again exhaled unchanged. The oxygen absorbed during the night must, however, take part in some chemical processes, for if it were merely mechanical, the absorption would not be confined to that gas alone, but would be participated in by the other constituents of the air. Moreover, the amount of absorption varies greatly in different plants—being scarcely appreciable in some, and very abundant in others. Plants containing volatile oils, which are readily converted into resins by the action of oxygen, or those containing tannin or other readily oxidizable substances, take up the largest quantity. This is remarkably illustrated by an experiment in which the leaves of the Agave americana, after twenty-four hours' exposure in the dark, were found to have absorbed only 0·3 of their volume of oxygen, while those of the fir, in which volatile oil is abundant, had taken up twice, and those of the oak, containing tannin, eighteen 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 of the flower 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 nutriment, 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. It has been already observed that starch, sugar, and the other allied substances, may be considered as compounds of carbon with water; and they might be supposed to owe their origin to the carbonic acid losing the whole of its oxygen, and direct combination then ensuing between the residual carbon and a certain proportion of water; but this would imply that the latter substance undergoes no decomposition, and though undoubtedly 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 separated, and replaced by hydrogen, produced by the decomposition of a certain quantity of water into its elements. 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:

It must not be supposed that we are in a condition to assert that sugar is really produced in the manner here shown, the illustration being given merely for the purpose of pointing out how it may be supposed to occur, and on a similar principle it is possible to explain the formation of most other vegetable compounds; and this subject has been very fully discussed by the late Dr. Gregory, in his "Handbook of Organic Chemistry." That water must be decomposed, is evident from the fact, established by analysis, that the hydrogen of the plant generally exceeds the quantity 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, must manifestly be obtained in a similar manner.

Decomposition of Ammonia.—The nitrogenous or albuminous compounds of vegetables must necessarily obtain their nitrogen from the decomposition either of ammonia or nitric acid, 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. No doubt carbonic acid and water take part with it in these changes, which must be of a very complex character, and in the present state of our knowledge it seems hopeless to attempt any explanation of them.

Decomposition of Nitric Acid.—Chemists are not entirely at one as to whether nitric acid is directly absorbed by the plant, or is first converted into ammonia. But there are certain facts connected with the chemistry of the soil, to be afterwards referred to, which seem to us to leave no doubt that it may be directly absorbed; and in that case it must be decomposed, its oxygen being eliminated, and the nitrogen taking part with carbon and hydrogen in the formation of the organic compounds. 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 intention 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.

CHAPTER IV.

THE INORGANIC CONSTITUENTS OF PLANTS.

When treating of the general constituents of plants, it has been already stated that the older chemists and vegetable physiologists, misled by the small quantity of ash found in them, entertained the opinion that mineral matters were purely fortuitous components of vegetables, and were present merely because they had been dissolved and absorbed 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 Salm 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.

Of late years the analysis of the ash of different plants has formed the subject of a large number of laborious investigations, by which our knowledge of this subject has been greatly extended. From these it appears that the quantity of ash contained in each plant or part of a plant is tolerably uniform, differing only within comparatively narrow limits, and that there is a special proportion belonging to each individual organ of the plant. This fact may be best rendered obvious by the subjoined table, showing the quantity of ash contained in a hundred parts of the different substances dried at 212°. Most of these numbers are the mean of several experiments:—

Table showing the quantity of inorganic matters in 100 parts of different plants dried at 212°.

On examining this table it may be observed that, notwithstanding the very great variety in the proportion of ash in different plants, some general relations may be traced. A certain similarity may be observed between those belonging to the same natural family, the seeds of all the cereal grains, for instance, containing in round numbers two per cent of inorganic matters. Leguminous seeds (peas and beans) contain 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 with the exception of a few extreme cases, the quantity of ash in general approaches pretty closely to five per cent. Still more diversified results are 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 are peculiarly rich in ash, and a leafy plant must necessarily yield a higher total percentage of ash, although, if stems and leaves were separately examined, they might not show so conspicuous a difference.

The leaves surpass all other parts of plants, in the proportion of inorganic constituents they contain, the table showing that in some instances, as in the maple and Jerusalem artichoke, they exceed one-fourth of the whole weight of the dry matter. In other leaves, and more especially in those of the coniferæ, the proportion is much smaller. Taking the average of all the analyses hitherto made, it appears that leaves contain about thirteen per cent of ash, but the variations on either side are so large that little value is to be attached to it except as an indication of the general abundance of mineral matters.

In roots and tubers the variations are less, and all, except the potato and the turnip, contain about seven per cent of ash.

The smallest proportion of mineral matter is found in wood. In one case only does the proportion reach five per cent, while the average scarcely exceeds one, 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.

The general proportion of ash found in different parts of plants is given in round numbers in the subjoined table:—

The differences in the quantity of ash contained in different parts of plants are obviously intended to serve a useful purpose, and it is interesting to observe that the wood which is destined to remain for a long period, sometimes for several centuries, a part of the plant, contains the smallest proportion, and it is not improbable that what it does contain is really due, not to the actual woody matter itself, but to the sap which permeates its vessels. By this arrangement but a small proportion of these important mineral matters, which the soil supplies in very limited quantity, is locked up within the plant, and those which are absorbed, after circulating through it, and fulfilling their allotted functions, are accumulated in the leaves, and annually returned to the soil.

The different proportions of mineral matters contained in the individual organs of plants is most strikingly illustrated when parallel experiments are made on the same species; but the number of instances in which a sufficiently extensive series of analyses has been made to show this, is comparatively limited, and is confined to the oat, the orange-tree, and the horse chesnut—each of which has formed the subject of a very elaborate investigation. The following table gives the results obtained on the oat:—

The specimens of oats on which these analyses were made were from different districts of country, grown on soils of different quality, and were, further, of different varieties; and yet they show, on the whole, a remarkable similarity in the proportion of ash in each part, and indicate that there is a normal quantity belonging to it. Such a series of analyses also affords the most convincing proof that the inorganic matters cannot be fortuitous, and merely absorbed from the soil along with their organic food, as the old chemists supposed, because, in that case, they ought to be uniformly distributed throughout the entire plant, and not accumulated in particular proportions in each individual organ.

Not only does the proportion of ash vary in the different parts of a plant, but even in the same part it is greatly influenced by its period of growth. The laws which regulate these variations are very imperfectly known, but in general it is observed 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. Saussure 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:—

On the other hand, the quantity of ash in the leaves of trees increases considerably in autumn, as shown by this table:—

In general, the proportion of ash appears to increase as the plant reaches maturity, and this is particularly seen in the oat, of which very complete analyses have been made at different periods of its growth:—

Proportion of Ash in different parts of the Oat at different periods of its growth.

The increase is here principally confined to the leaves and chaff, while the stalks, which owe their strength to a considerable extent to the inorganic matters they contain, are equally supplied at all periods of their growth. In the grain only is there a diminution, but this is apparent and not real, and is due to the fact that the determination of the quantity of ash, as made on the grain with its husk, and the former, which contains only a small quantity of mineral matters, increases much more rapidly in weight than the latter, when it approaches the period of ripening, and it is accordingly during the last three weeks of its growth that this diminution becomes apparent.

The nature of the soil has also a very important influence on the proportion of mineral matters, and of this an interesting illustration is given in the following table, which shows the quantities found in the grain and straw of the same variety of the pea grown on fourteen different soils:—

Although those differences are very large, especially in the straw, and must be attributed to the soil, it has hitherto been found impossible to ascertain the nature of the relation subsisting between it and the crops it yields; indeed, it must obviously be dependent on very complicated questions, which cannot at present be solved, for it may be observed that the increase in the grain does not occur simultaneously with that in the straw, and in several cases a large proportion of ash in the former is associated with an unusually small amount in the latter. A priori, it might be expected that those soils which are especially rich in the more important constituents of the ash should yield a produce containing more than the average quantity, but this is very far from being an invariable occurrence, and not unfrequently the very reverse is the case. In some instances the variations may be traced to the soil, as in the following analyses of the fruit of the horse-chesnut, grown on an ordinary forest soil, and on a rich soil, produced by the disintegration of porphyritic rock, in which the latter yields a much larger quantity of ash:—

In the majority of instances we fail to establish any connection between the nature of the soil and the plants it yields, chiefly because we are still very deficient in analyses of those grown on uncultivated soils; and on cultivated land it is impossible to draw conclusions, because the nature of the manure exerts an influence quite as great, if not greater, than that of the soil itself.

The relative proportion in which the different mineral matters enter into the composition of the ash varies within very wide limits, as will be apparent from the following table, containing a selection of the best analyses of our common cultivated and a few uncultivated plants.

Table of the Composition of the Ash of different Plants in 100 Parts.

Note.—Alumina and oxide of manganese occur so rarely, that separate columns have not been introduced for them, but their quantity is stated in notes at the end of the table.

A simple inspection of this table leads to various interesting conclusions. It is particularly to be observed that some of the constituents of the ash are not invariably present, and two at least—namely, alumina and manganese—are found so rarely as to justify the inference that they are not indispensable. Of the other substances, iodine is restricted exclusively to sea-plants, but to them it appears to be essential. Oxide of iron, which occurs only in small quantities, has sometimes been considered fortuitous, but it is almost 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 appears to be a comparatively unimportant constituent of the ash, of which it generally forms but a small proportion, although the instances of its entire absence are rare. In the cruciferous plants (turnip, rape, etc.) it is found abundantly, and to them it appears indispensable, but in most other plants it admits of replacement by potash. It seems probable that where the soil is rich in the latter substance, plants will select that alkali in preference to soda; but as they must have a certain quantity of alkali, the latter may supply the place of the former where it is deficient. Cultivation, probably by enriching the soil in that element, increases the proportion of potash found in the ash of plants, as is remarkably seen in the asparagus, which gave the following quantities of alkalies and chlorine:—

The soda having almost entirely disappeared in the cultivated plant, while a corresponding increase had taken place in the quantity of potash.

Potash is one of the most important elements of the ash of all plants, rarely forming less than 20, and sometimes more than 50 per cent of its weight. The latter proportion occurs chiefly in the roots and tubers, but it is also abundant in all seeds and in the grasses. The straw, and particularly the chaff of the cereals, and the leaves of most plants, contain it in smaller quantity, although exceptions to this are not unfrequent, one of the most curious being the case of poppy-seed, which contains only about 12 per cent, while the leaves yield upwards of 37 per cent.

The proportion of lime varies within very wide limits, being sometimes as low as 1, and in other plants reaching 40 per cent of their ash. The former proportion occurs in the grains of the cerealia, and the latter in the leaves of some plants, and more especially in the Jerusalem artichoke. The turnip and some of the leguminous plants also contain it abundantly.

Magnesia is generally found in small quantity. It is largest in the grains, amounting in them to about 12 or 13 per cent of the ash, but in other plants it varies from 2 to 4 per cent. Although small in quantity, it is an important substance, and apparently cannot be dispensed with; at least there is no instance known of its entire absence.

Chlorine is by no means an invariable constituent of the ash, although it is generally present, and sometimes in considerable quantity. It is most abundant when the proportion of soda is large, and exists in the ash principally in combination with that base as common salt. The relation between these two elements may be traced more or less distinctly throughout the whole table of analyses, and conspicuously in that of mangold-wurzel, where the common salt amounts to almost exactly one-half of the whole mineral matter. The analyses of the cultivated and uncultivated asparagus also show that a diminution in the soda is accompanied by a reduction in the proportion of chlorine.

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 partly derived from the sulphur contained in the albuminous compounds, which is oxidised and converted into sulphuric acid during the process of burning the organic matter, and remains in the ash. The quantity of sulphuric acid found in the ash is, however, no criterion of that existing in the plant, for a considerable quantity of it escapes during burning. The extent to which this occurs in particular instances is well illustrated by reference to the case of white mustard, which yields an ash containing only 2·19 of sulphuric acid, equivalent to 0·9 of sulphur; and if calculated on the seed itself, this will amount to no more than 0·039 per cent, while experiments made in another manner prove it to contain about thirty times as much, or more than 1 per cent. For the purpose of determining the total quantity of sulphur which the plants contain in their natural state, it is necessary to oxidise them by means of nitric acid; and from such experiments the following table, showing the total amount of sulphur contained in 100 parts of different plants, dried at 212°, has been constructed:—

Phosphoric acid, which may be looked upon as the most important mineral constituent of plants, is found to be present in very variable proportions. The straws, stems, and leaves contain it in comparatively small quantity, but in the seeds of all plants it is very abundant. In these of the cereals it constitutes nearly half of their whole mineral components, and it rarely falls below 30 per cent.

Carbonic acid occurs in very variable quantities in the ash. It is of comparatively little importance in itself, and is really produced by the oxidation of part of the carbonaceous matters of the plant; but it has a special interest, in so far as it shows that part of the bases contained in the plant must in its natural state have been in union with organic acids, or combined in some way with the organic constituents of the plant.

Silica is an invariable constituent of the ash, but in most plants occurs but in small quantity. The cereals and grasses form an exception to this rule, for in them it is an abundant and important element. It is not, however, uniformly distributed through them, but is accumulated to a large extent in the stem, to the strength and rigidity of which it greatly contributes. The hard shining layer which coats the exterior of straw, and which is still more remarkably seen on the surface of the bamboo, consists chiefly of silica; and in the latter plant this element is sometimes so largely accumulated, that concretions resembling opal, and composed entirely of it, are found loose within its joints. The necessity for a large supply of silica in the stems of other plants does not exist, and in them it rarely exceeds 5 or 6 per cent, but in some leaves it is more abundant.

A knowledge of the composition of the ash of plants is of considerable importance in a practical point of view, and enables us in many instances 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, and hence it follows that on such a soil plants cultivated for the sake of their stems, roots, or leaves, in which the quantity of phosphoric acid is small, may yield a good return; while others, cultivated for the sake of their seed, in which the great proportion of that constituent of the ash is accumulated, may yield a very small crop. It is obvious also 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 individual element, may, in the course of time, so far reduce the amount of that substance as to render the soil incapable of any longer producing that plant, although, 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 also, attempts have been made to explain the rotation of crops, which has been supposed to depend on the cultivation in successive years of plants which abstract from the soil preponderating quantities of different mineral matters. But though this has unquestionably a certain influence, we shall afterwards see reason to doubt whether it affords a sufficient explanation of all the observed phenomena.

It may be observed, on examining the table of the percentage and position of the ash, that some plants are especially rich in alkalies, while in others lime or silica preponderate, and it would therefore be the object of the farmer to employ, in succession, crops containing these elements in different proportions. In carrying out this view, attempts have been made 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 is at present possible:—

The special application of these facts must be reserved till we come to treat of the rotation of crops.

It is manifest that, as the crops removed from the soil all contain a greater or less amount of inorganic matters, they must be continually undergoing diminution, and at length be completely exhausted unless their quantity is maintained from some external source. In many cases the supply of these substances is so large that ages may elapse before this becomes apparent, but 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 America, from which the early settlers reaped almost unheard-of crops, but, by injudicious cultivation, they were soon exhausted and abandoned, new tracts being brought in and cultivated only to be in their turn abandoned. The knowledge of the composition of the ash of plants assists us in ascertaining how this exhaustion may be avoided, and indicates the mode in which such soils may be preserved in a fertile state.

FOOTNOTES:

[A] Apparently a species of Sinapis.

[B] Oxide of Manganese, 0·42.

[C] Oxide of Manganese, 0·92.

[D] Alumina, 1·02.

[E] Alumina, 0·63.

[F] Iodide of Potassium, 0·44; Sulphuret of Sodium, 3·66.

[G] Iodide of Potassium, 0·23.

[H] Iodide of Potassium, 1·68.

CHAPTER V.

THE SOIL—ITS CHEMICAL AND PHYSICAL CHARACTERS.

No department of agricultural chemistry is surrounded with greater difficulties and uncertainties than that relating to the properties of the soil. When chemistry began to be applied to agriculture, it was not unnaturally supposed that the examination of the soil would enable us to ascertain with certainty the mode in which it might be most advantageously improved and cultivated, and when, as occasionally happened, analysis revealed 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. But the expectations naturally formed from the facts then observed have been as yet 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. Although it is occasionally possible to predicate from its composition that a particular soil will be incapable of supporting vegetation, it not unfrequently happens that a fruitful and a barren soil are so similar that it is impossible to distinguish them from one another, and cases even occur in which the barren appears superior to the fertile soil. The cause of this apparently anomalous phenomenon lies in the fact that analysis, however minute, is unable to disclose all the conditions of fertility, and that it must be supplemented by an examination of its physical and other chemical properties, which are not indicated by ordinary experiments. Of late years very considerable progress has been made in the investigation of the properties of the soil, and many facts of great importance have been discovered, but we are still unable to assert that all the conditions of fertility are yet known, and the practical application of those recently discovered is still very imperfectly understood.

It must not be supposed that a careful analysis of a soil is without value, for very important practical deductions may often be drawn from it, and when this is not practicable it is not unfrequently due to its being imperfect or incomplete, for it is so complex that the cases in which all the necessary details have been eliminated are even now by no means numerous. In fact, the want of a large number of thorough analyses of soils of different kinds is a matter of some difficulty, and so soon as a satisfactory mode of investigation can be determined upon, a full examination of this subject would be of much importance.

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 has been already discussed: 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, aided by the powerful agency of frost, it is seen to become soft, and gradually to disintegrate, until it is finally converted into an uniform powder, in which the structure of the original rock is with difficulty, if at all distinguishable. The rapidity with which these changes take place is very variable; in the harder rocks, such as granite and mica slate it is so slow as to be scarcely perceptible, while in others, such as the shales of the coal formation, a very few years' exposure is sufficient for the purpose. These actions, operating through a long series of years, are the source of the inorganic constituents of all 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 formed. But they have been produced by a succession 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 overlying them being formed immediately by their decomposition, while those found above 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 different soils must manifestly depend on that of the crystalline rocks from which they have been derived. Their number is by no means large, and they all consist of mixtures in variable proportions of quartz, felspar, mica, 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, several different minerals commonly classified under the name of felspar, which have been distinguished by mineralogists by the names of orthoclase, albite, oligoclase, and labradorite; and there are at least two sorts of mica, two of hornblende, and many varieties of zeolites.

Quartz consists of pure silica, and when in large masses is one of the most indestructible rocks. It occurs, however, intermixed with other minerals in small crystals, or irregular fragments, and forms the entire mass of pure sand.

The four kinds of felspar which have been already named are compounds of silica with alumina, and another base which is either potash, soda, or lime. Their composition is as follows, two examples of each being given—

It is obvious that soils produced by the disintegration of these minerals must differ materially in quality. Those yielded by orthoclase must generally abound in potash, while albite and labradorite, containing little or none of that element, must produce soils in which it is deficient. The quality of the soil they yield is not however entirely dependent on the nature of the particular felspar which yields it, but is also intimately connected with the extent to which the decomposition has advanced. It is observed that different felspars undergo decomposition with different degrees of rapidity but after a certain time they all begin to lose their peculiar lustre, acquire a dull and earthy appearance, and at length fall into a more or less white and soft powder. During this change water is absorbed, and, by the decomposing action of the air, the alkaline silicate is gradually rendered soluble, and at length entirely washed away, leaving a substance which, when mixed with water, becomes plastic, and has all the characters of common clay. The nature of this change will be best seen by the following analysis of the clay produced during this composition, which is employed in the manufacture of porcelain under the name of kaolin, or china clay—

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 rarely complete, and the soils produced from it frequently contain a considerable proportion of the undecomposed mineral, which continues for a long period to yield a supply of alkalies to the plants which grow on them.

Mica is a very widely distributed mineral, and two varieties of it are distinguished by mineralogists, one of which is characterised by the large quantity of magnesia it contains. Different specimens are found to vary very greatly in composition, but the following analyses may represent their most usual composition:

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 are two widely distributed minerals, which are so similar in composition and properties that they may be considered together. Of the former two varieties, basaltic and common have been distinguished, and their composition is given below:—

In these minerals alkalies are entirely absent, and their decomposition is due to the presence of protoxide of iron, which readily absorbs oxygen from the air, when the magnesia is separated and a ferruginous clay left.

The minerals just referred to, constitute the great bulk of the mountain masses, but they are associated with many others which take part in the formation of the soil. Of these the most important are the zeolites which do not occur in large masses but are disseminated through the other rocks in small quantity. They form a large class of minerals of which Thomsonite and natrolite may be selected as examples—

They are chiefly characterized by containing their silica in a soluble state, and hence may yield that substance to the plants in a condition particularly favourable for absorption.

It is obvious from what has been stated that all these minerals are capable, by their decomposition, of yielding soft porous masses having the physical properties of 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 labradorite, 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, and the quality of the soil it yields depends on whether the variety of felspar present be orthoclase or albite. 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 difficulty 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 and trap are types, and which are very widely distributed, differ greatly 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 labradorite, 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 labradorite, 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 alkalies 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 has been 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, labradorite with a considerable proportion of soda, 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. They are also greatly affected by the mechanical influences to which the rocks are exposed; and being 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 felspathic debris.

The sedimentary or stratified rocks are formed of particles carried down by water and deposited at the bottom of the primeval seas from which they have been upheaved in the course of geological changes. The process of their formation may be watched at the present day at the mouths of all great rivers, where a delta composed of the suspended matters carried down by the waters is slowly formed. The nature of these rocks must therefore depend entirely on that of the country through which the river flows. If its course runs through a country in which lime is abundant, calcareous rocks will be deposited, and if it passes through districts of different geological characters the deposit must necessarily consist of a mixture of the disintegrated particles of the different rocks the river has encountered. For this reason it is impossible to enter upon a detailed account of their composition. It is to be observed, however, that the particles of which they are composed, though originally derived from the crystalline rocks, have generally undergone a complex series of changes, geology teaching that, after deposition, they may in their turn undergo disintegration and be carried away by water, to be again deposited. Their composition must therefore vary not merely according to the nature of the rock from which they have been formed, but also according to the extent to which the decomposition has gone, and the successive changes to which they have been exposed. They may be reduced to the three great classes of clays, including the different kinds of clay slates, shales, etc., sandstone and limestone. It must be added also, that many of them contain carbonaceous matters produced by the decomposition of early races of plants and animals, and that mixtures of two or more of the different classes are frequent.

The purest clays are produced by the decomposition of felspar, but almost all the crystalline rocks may produce them by the removal of their alkalies, iron, lime, etc. Where circumstances have been favourable, the whole of these substances are removed, and the clay which remains consists almost entirely of silica and alumina, and yields a soil which is almost barren, not merely on account of the deficiency of many of the necessary elements of plants, but because it is so stiff and impenetrable that the roots find their way into it with difficulty. It rarely happens, however, that decomposition has advanced so far as to remove the whole of the alkalies, which is exemplified by the following analyses of the fire clay of the coal formation, and of transition clay slate:—

The sandstones are derived from the siliceous particles of granite and other rocks, and consist in many cases of nearly pure silica, in which case their disintegration produces a barren sand, but they more frequently contain an admixture of clay and micaceous scales, which sometimes form a by no means inconsiderable portion of them. Such sandstones yield soils of better quality, but they are always light and poor. Where they occur interstratified with clays, still better soils are produced, the mutual admixture of the disintegrated rocks affording a substance of intermediate properties, 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 as they occur in the early geological formations:—

These limestones are hard and possess to a greater or less extent a crystalline texture. They are replaced in later geological periods by others which are much softer, and often purer, of which the oolitic limestones, so called from their resemblance to the roe of a fish, and chalk are the most important. Other limestones are also known which contain an admixture of clay. The soils produced by the disintegration of limestone and chalk are generally light and porous, but when mixed with clay, possess a very high degree of fertility, and this is particularly the case with chalk, which yields 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 often prejudicial to vegetation, and sometimes yield barren or 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 work. It is necessary, however, to remark, that in many instances one variety passes into the other, or, more correctly speaking, sedimentary rocks occur, which are mixtures of two or more of the three great classes. In fact, the name given to each really expresses only the preponderating ingredient, and many sandstones contain much clay, shales and clay slates abound in lime, and limestones in sand or clay, so that it may sometimes be a matter of some difficulty to decide to which class they belong. Such mixtures usually produce better soils than either of their constituents separately, and accordingly, in those geological formations in which they occur, 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.

The fertility of the soils formed from the stratified rocks is also increased by the presence of organic remains which afford a supply of phosphoric acid, and which are sometimes so abundant as to form a by no means unimportant part of their mass. They do not occur in the oldest sedimentary rocks, but as we ascend to the more recent geological epochs, they increase in abundance, until, in the greensands and other recent formations, whole beds of coprolites and other organic remains are met with. Great differences are observed in the quality of the soils yielded by different rocks. In general, those formed by the disintegration of clay slates are cold, heavy, and very difficult and expensive to work; those of sandstone light and poor, and of limestone often 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, remarkable exceptions being offered by the lower chalk and some of the shales of the coal formation. Little is at present known regarding the peculiar nature of many 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.—Reference has been already made to the division of the constituents of the soil into the two great classes of organic and inorganic. And 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. But though the parts then stated lead to the inference that, as a direct source of these substances, humus is unimportant, it has other functions to perform which render it an essential constituent of all fertile soils. These functions are dependent partly on the power which it has of absorbing and entering into chemical composition with ammonia, and with certain of the soluble inorganic substances, and partly on the effect which the carbonic acid produced by its decomposition exerts on the mineral matters of the soil. In the former way, its effects are strikingly seen in the manner in which ammonia is absorbed by peat; for 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. On the other hand, the air and moisture which penetrate the soil cause its decomposition, and the carbonic acid so produced attacks the undecomposed minerals existing in it, and liberate the valuable substances they contain.

In considering the composition of a soil, it is important to bear in mind that it is a substance of great complexity, not merely because it contains a large number of chemical elements, but also because it is made up of a mixture of several minerals in a more or less decomposed state. The most cursory examination shows that it almost invariably contains sand and scales of mica, and other substances can often be detected in it. Now it has been already observed that the minerals of which soils are composed, differ to a remarkable extent in the facility with which they undergo decomposition, and the bearing of this fact on its fertility is a matter of the highest importance, for it has been found that the mere presence of an abundant supply of all the essential constituents of plants is not always sufficient to constitute a fertile soil. Two soils, for instance, may be found on analysis to have exactly the same composition, although in practice one proves barren and the other fertile. The cause of this difference lies in the particular state of combination in which the elements are contained in them, and unless this be such that the plant is capable of absorbing them, it is immaterial in what quantity they are present, for they are thus locked up from use, and condemn the soil to hopeless infertility.