Agricultural Chemistry, like most branches of natural science, may be said to be entirely of modern growth. While it is true we have many old speculations on the subject, they can scarcely be said to possess much scientific value. The great questions which had first to be solved by the agricultural chemist were,—What is the food of plants? and,—What is the source of that food? The second of these two questions more easily admitted of answer than the first. The source of plant-food could only be the atmosphere or the soil. As the composition of the atmosphere, however, was not discovered till the close of last century, and the chemistry of the soil is a question which is still requiring much work ere we shall be in possession of anything like a full knowledge of it, it will be at once obvious that the very fundamental conditions for a solution of the question were awanting. The beginning, then, of a true scientific agricultural chemistry may be said to date from the brilliant discoveries associated with the names of Priestley, Scheele, Lavoisier, Cavendish, and Black—that is, towards the close of last century.
Early Theories on Source of Plant-food.
While this is so, and while we must regard the early attempts made towards solving this question as being, for the most part, of little scientific value, it is not without interest, from the historical point of view, to glance briefly at some of these old interesting speculations.
The Aristotelian doctrine, regarding the possibility of dividing matter into the so-called four primary elements, fire, air, earth, and water, which obtained in one form or another till the birth of modern chemistry, had naturally an important influence on these early theories.
Van Helmont's Theory.
Among the earliest and most important attempts made to solve the problem of plant-growth was that by Jean Baptiste Van Helmont, one of the best known of the alchemists, who flourished about the beginning of the seventeenth century. Van Helmont believed that he had proved by a conclusive experiment that all the products of vegetables were capable of being generated from water. The details of this classical experiment were as follows:—
"He took a given weight of dry soil—200 lb.—and into this soil he planted a willow-tree that weighed 5 lb., and he watered this carefully from time to time with pure rain-water, taking care to prevent any dust or dirt falling on to the earth in which the plant grew. He allowed this to go on growing for five years, and at the end of that period, thinking his experiment had been conducted sufficiently long, he pulled up his tree by the roots, shook all the earth off, dried the earth again, weighed the earth and weighed the plant. He found that the plant now weighed 169 lb. 3 ounces, whereas the weight of the soil remained very nearly what it was—about 200 lb. It had only lost 2 ounces in weight."[1]
The conclusion, therefore, come to by Van Helmont was that the source of plant-food was water.[2]
Some fifty years later an extremely interesting book was published bearing the following title: 'A Discourse concerning the Vegetation of Plants, spoken by Sir Kenelm Digby, at Gresham College, on the 23d of January 1660. (At a meeting of the Society for promoting Philosophical Knowledge by Experiments. London: Printed for John Williams, in Little Britain, over against St Botolph's Church, 1669.)' The author attributes plant-growth to the influence of a balsam which the air contains. This book is especially interesting as containing the earliest recognition of the value of saltpetre as a manure. The following is an extract from this interesting old work:—
"The sickness, and at last the death of a plant, in its natural course, proceeds from the want of that balsamick saline juice; which, I have said, makes it swell, germinate, and augment itself. This want may proceed either from a destitution of it in the place where the plant grows, as when it is in a barren soil or bad air, or from a defect in the plant itself, that hath not vigour sufficient to attract it, though it be within the sphere of it; as when the root has become so hard, obstructed and cold, as that it hath lost its vegetable functions. Now, both these may be remedy'd, in a great measure, by one and the same physick.... The watering of soils with cold hungray springs doth little good; whereas muddy saline waters brought to overflow a piece of ground enrich it much. But above all, well-digested dew makes all plants luxuriate and prosper most. Now what may it be that endues these liquors with such prolifick virtue? The meer water which is common to them all, cannot be it; there must be something else enclosed within it, to which the water serves but for a vehicle. Examine it by spagyric art, and you will find that it is nothing else than a nitrous salt, which is dilated in the water. It is this salt which gives fœcundity to all things: and from this salt (rightly understood) not only all vegetables, but also all minerals draw their origine. By the help of plain salt-peter, dilated in water and mingled with some other fit earthy substance, that may familiarize it a little with the corn into which I endeavoured to introduce it, I have made the barrenest ground far out-go the richest, in giving a prodigiously plentiful harvest. I have seen hemp-seed soaked in this liquor, that hath in due time made such plants arise, as, for the tallness and hardness of them, seemed rather to be coppice-wood of fourteen years' growth at least, than plain hemp. The fathers of the Christian doctrine at Paris still keep by them for a monument (and indeed it is an admirable one) a plant of barley consisting of 249 stalks, springing from one root or grain of barley; in which they counted above 18,000 grains or seeds of barley. But do you think that it is barely the salt-peter, imbibed into the seed or root, which causeth this fertility? no: that would be soon exhausted and could not furnish matter to so vast a progeny. The salt-peter there is like a magnet, which attracts a like salt which fœcundates the air, and gave cause to the Cosmopolite to say there is in the air a hidden food of life."[3]
Duhamel and Hales.
The names of the French writer, Duhamel, and of the English, Stephen Hales, may be mentioned in passing as authors of works bearing on the question of vegetable physiology. Both of these writers flourished about the middle of the eighteenth century. The writings of the former contained much valuable information on the effects of grafting, motion of sap, and influence of light on vegetable growth, and also the results of experiments which the author had carried out on the influence of treating plants with certain substances. 'Statical Essays, containing Vegetable Staticks; or an Account of some Statical Experiments on the Sap of Vegetables, by Stephen Hales, D.D.' (2 vols.), was published in London in 1738; and contained, as will be seen from its title, records of experiments of very much the same nature as those of Duhamel.
Jethro Tull's Theory.
Some reference may be made to a theory which created a considerable amount of interest when it was first published—viz., that of Jethro Tull. The chief value of Tull's contribution to the subject of agricultural science was, that he emphasised the importance of tillage operations by putting forward a theory to account for the fact, universally recognised, that the more thoroughly a soil was tilled, the more luxuriant the crops would be. As Tull's theory had a very considerable influence in stirring up interest in many of the most important problems in agricultural chemistry, and as it contained in itself much, the value of which we have only of late years come to understand, a brief statement of this theory may not be without interest.
According to Tull the food of plants consists of the particles of the soil. These particles, however, must be rendered very minute before they become available for the plant, which absorbs them by means of its rootlets. This pulverisation of the soil goes on in nature independently of the farmer, but only very slowly, and the farmer has therefore to hasten it on by means of tillage operations. The more efficiently these operations are carried on, the more abundant will the supply of plant-food be rendered in the soil. He consequently introduced and advocated the system of horse-hoe husbandry. This theory, he informs us, was suggested to him by the custom, which he had noticed on the Continent, of growing vines in rows, and hoeing the intervals between these rows from time to time. The excellent results which followed this mode of cultivation induced him to adopt it in England for his farm crops. He accordingly sowed his crops in rows or ridges, wide enough apart to admit of thorough tillage of the intervals by ploughing as well as by hand-hoeing. This he continued until the plant had reached maturity. As to the exact width of the interval most suitable, he made a large number of experiments. At first, in the cultivation of wheat, he made this interval six feet wide; but latterly he adopted an interval of lesser width, that finally arrived at being between four and five feet. He likewise experimented on each separate ridge as to which was the best number of rows of wheat to be sown, latterly adopting, as most convenient, two rows at ten inches apart. The great success which he met with in this system of cultivation induced him to publish the results of his experiments in his famous work, 'Horse-Hoeing Husbandry.'
While Tull's theory was based on principles at heart thoroughly sound, he was carried away by his personal success into drawing unwarrantable deductions. Thus he came to the conclusion that rotation of crops was unnecessary, provided that a thorough system of tillage was carried out. Manures also, according to him, might be entirely dispensed with under his system of cultivation, for the true function of all manures is to aid in the pulverisation of the soil by fermentation.
The first really valuable scientific facts contributed to the science were made by Priestley, Bonnet, Ingenhousz, and Sénébier.
Discovery of the Source of Plants' Carbon.
To Charles Bonnet (1720-1793), a Swiss naturalist, is due the credit of having made the first contribution to a discovery of very great importance—viz., the true source of the carbon, which we now know forms so large a portion of the plant-substance. Bonnet, who had devoted himself to the question of the function of leaves, noticed that when these were immersed in water bubbles were seen, after a time, to collect on their surface. De la Hire, it ought to be pointed out, had noticed this same fact about sixty years earlier. It was left to Priestley, however, to identify these bubbles with the gas he had a short time previously discovered—viz., oxygen. Priestley had observed, about this time, the interesting fact that plants possessed the power of purifying air vitiated by the presence of animal life.[4] The next step in this highly interesting and important discovery was taken by John Ingenhousz (1730-1799), an eminent physician and natural philosopher. In 1779, Ingenhousz published a work in London entitled 'Experiments on Vegetables.' In it he gives the results of some important experiments he had made on the question already investigated by Bonnet and Priestley. These experiments proved that plant-leaves only gave up their oxygen in the presence of sunlight. In 1782 he published another work on 'The Influence of the Vegetable Kingdom on the Animal Creation.'[5]
The source of the gas, which Bonnet had first noticed to be given off from plant-leaves, Priestley had identified as oxygen, and Ingenhousz had proved to be only given off under the influence of the sun's rays, was finally shown by a Swiss naturalist, Jean Sénébier[6] (1742-1809), to be the carbonic acid gas in the air, which the plant absorbed and decomposed, giving out the oxygen and assimilating the carbon.
Publication of First English Treatise on Agricultural Chemistry.
In 1795, a book dealing with the relations between chemistry and agriculture was published. This work was written by a Scottish nobleman, the Earl of Dundonald, and possesses especial interest from the fact that it is the first book in the English language on agricultural chemistry. The full title is as follows: 'A Treatise showing the Intimate Connection that subsists between Agriculture and Chemistry.'
In his introduction the author says: "The slow progress which agriculture has hitherto made as a science is to be ascribed to a want of education on the part of the cultivators of the soil, and to a want of knowledge, in such authors as have written on agriculture, of the intimate connection that subsists between the science and that of chemistry. Indeed, there is no operation or process not merely mechanical that does not depend on chemistry, which is defined to be a knowledge of the properties of bodies, and of the effects resulting from their different combinations."
In quoting this passage Professor S. W. Johnson remarks:[7] "Earl Dundonald could not fail to see that chemistry was ere long to open a splendid future for the ancient art that had always been and always will be the prime supporter of the nations. But when he wrote, how feeble was the light that chemistry could throw upon the fundamental questions of agricultural science! The chemical nature of the atmosphere was then a discovery of barely twenty years' standing. The composition of water had been known but twelve years. The only account of the composition of plants that Earl Dundonald could give was the following: 'Vegetables consist of mucilaginous matter, resinous matter, matter analogous to that of animals, and some proportion of oil.... Besides these, vegetables contain earthy matters, formerly held in solution in the newly-taken-in juices of the growing vegetables.' To be sure, he explains by mentioning in subsequent pages that starch belongs to the mucilaginous matter, and that on analysis by fire vegetables yield soluble alkaline salts and insoluble phosphate of lime. But these salts, he held, were formed in the process of burning, their lime excepted; and the fact of their being taken from the soil and constituting the indispensable food of plants, his lordship was unacquainted with. The gist of agricultural chemistry with him was, that plants 'are composed of gases with a small proportion of calcareous matter; for although this discovery may appear to be of small moment to the practical farmer, yet it is well deserving of his attention and notice.'"
De Saussure.
The year 1804 witnessed the publication of by far the most important contribution made to the science up till this time. This was 'Recherches Chimique sur la Végétation,' by Theodore de Saussure, one of the most illustrious agricultural chemists of the century. De Saussure was the first to draw attention to the mineral or ash constituents of the plant; and thus anticipate, to a certain extent, the subsequent famous "mineral" theory of the great Liebig. The French chemist maintained that these ash ingredients were essential; and that without them plant-life was impossible. He also adduced fresh experiments of his own in support of the theory, based on the experiments of Bonnet, Priestley, Ingenhousz, and Sénébier, that plants obtain their carbon from the carbonic acid gas in the air, under the influence of the sunlight. He was of opinion that the hydrogen and oxygen of the plant were, probably, chiefly derived from water. He showed that by far the largest portion of the plant's substance was derived from the air and from water, and that the ash portion was alone derived from the soil. To Saussure we owe the first definite statement on the different sources of the plant's food. It may be said that the lapse of nearly a century has shown his views to be, in the main, correct.
Source of Plant-nitrogen.
There was one question, which, even at that remote period in the history of the subject, engaged the attention of agricultural chemists—viz., the question of the source of the plant's nitrogen—a question which may be fitly described at the present hour as still the burning question of agricultural chemistry.[8]
As soon as it was discovered that nitrogen was a constituent of the plant's substance; speculations as to its source were indulged in. The fact that the air furnished an unlimited storehouse of this valuable element, and the analogy of the absorption of carbon (from the same source by plant-leaves), naturally suggested to the minds of early inquirers that the free nitrogen of the air was the source of the plant's nitrogen. As, however, no direct experiments could be adduced to prove this theory, and as, moreover, nitrogen was found in the soil, and seemed to be a necessary ingredient of all fertile soils, the opinion that the soil was the only source gradually supplanted the older theory. Little value, however, must be attached to these early theories, as they can scarcely be said to have been based on experiments of serious value. Indeed it may be safely affirmed, in the light of subsequent experiments, that it was impossible for this question to be decided at this early period, from the fact that analytical apparatus, of a sufficiently delicate nature, was then wholly unknown. Indeed it is only within the last few years that it has been possible to carry out experiments which may be regarded as at all crucial. A short sketch of the development of our knowledge of the relation of nitrogen to the plant will be given further on.
A series of lectures on agricultural chemistry, delivered by Sir Humphry Davy during the years 1802-1812, for the Board of Agriculture, and subsequently published in book form in the year 1813,[9] affords us an opportunity of gauging, pretty accurately, the state of knowledge on the subject at the time.
Position of Agricultural Chemistry at beginning of Century.
In his opening lecture Davy says: "Agricultural chemistry has not yet received a regular and systematic form. It has been pursued by competent experimenters for a short time only. The doctrines have not as yet been collected into any elementary treatise, ... and," he adds, "I am sure you will receive with indulgence the first attempt made in this country to illustrate it by a series of experimental demonstrations."
He further on remarks: "It is evident that the study of agricultural chemistry ought to be commenced by some general inquiries into the composition and nature of material bodies, and the law of their changes. The surface of the earth, the atmosphere, and the water deposited from it, must either together, or separately, afford all the principles concerned in vegetation, and it is only by examining the chemical nature of these principles that we are capable of discovering what is the food of plants, and the manner in which this food is supplied and prepared for their nourishment."
Davy goes on further to say: "No general principles can be laid down respecting the comparative merits of the different systems of cultivation and the various systems of crops adopted in different districts, unless the chemical nature of the soil, and the physical circumstances to which it is exposed, are fully known."
He recognises the enormous importance of experiments. "Nothing is more wanting in agriculture than experiments, in which all the circumstances are minutely and scientifically detailed."
In dealing with the composition of plants he says: "It is evident that the most essential vegetable substances consist of hydrogen, carbon, and oxygen, in different proportions, generally alone; but in some few cases combined as carbon and nitrogen. The acids, alkalies, earths, metallic oxides, and saline compounds, though necessary in the vegetable economy, must be considered as of less importance, particularly in their relation to agriculture, than the other principles."
Further on: "It will be asked, Are the pure earths in the soil merely active as mechanical or indirect chemical agents, or do they actually afford food to the plant?"
This question he answers by saying that "water, and the decomposing animal and vegetable matter existing in the soil, constitute the true nourishment of plants; and as the earthy parts of the soil are useful in retaining water, so as to supply it in the proper proportion to the roots of the vegetables, so they are likewise efficacious in producing the proper distribution of the animal or vegetable matter. When equally mixed with it, they prevent it from decomposing too rapidly; and by their means the soluble parts are supplied in proper proportions."
Value of Davy's Lectures.
The chief value of these lectures is due to the fact that they form the first attempt to connect in a systematic manner the various scattered facts, up to that time ascertained, and to interpret their bearing on agricultural practice. We have in them, it is true, a strange mixture of facts belonging rather to botany and physiology than to agricultural chemistry; still they undoubtedly furnished a great impetus to inquiry, and at the same time they did much to popularise the science.
But not merely did Davy summarise and systematise the various results arrived at by others, he also made many valuable contributions to the science himself. The conclusions he drew from the results he obtained were, no doubt, in many cases false, and in other cases exaggerated; still the results possess a permanent interest. He may be said to have worked out many of the most important physical or mechanical properties of a soil, although exaggerating the importance of the influence of these properties on the question of fertility.[10]
These experiments had to do with the heat- and water-absorbing powers of a soil. He experimented on a brown fertile soil, and a cold barren clay, and found at what rate they lost heat. "Nothing," he says, "can be more evident than that the genial heat of the soil, particularly in spring, must be of the highest importance to the rising plant; ... so that the temperature of the surface, when bare and exposed to the rays of the sun, affords at least one indication of the degree of the fertility."
Again he says: "The power of soils to absorb water from air is much connected with fertility.... I have compared the absorbent powers of many soils, with respect to atmospheric moisture, and I have always found it greatest in the most fertile soils; so that it affords one method of judging of the productiveness of land."
Where he erred was in overestimating the functions of the mechanical properties of a soil, and in considering fertility to be due to them alone.
During the next thirty years or so, little progress seems to have been made in the way of fresh experimentation.
In 1834, Boussingault,[11] the most distinguished French agricultural chemist of the century, began that series of brilliant chemico-agricultural experiments on his estate at Bechelbronn, in Alsace, the results of which have added so much to agricultural science. It was the first instance of the combination of "science with practice," of the institution of a laboratory on a farm; a combination peculiarly fitted to promote the interests of agricultural science, and an example which has been since followed with such magnificent results in the case of Sir John Lawes's famous Rothamsted Experiment Station, and other less known research stations.
Boussingault's first paper appeared in 1836, and was entitled, "The amount of nitrogen in different kinds of foods, and on the equal value of foods founded on these data."
In the year following other papers were published on such subjects as the amount of gluten in different kinds of wheat; on the meteorological considerations of how far various agricultural operations—such as extensive clearings of wood, the draining of large swamps, &c.—influence of climate on a country; and on experiments on the culture of the vine.
Boussingault was the first observer to study the scientific principles underlying the system of rotation of crops. In 1838 he published the results of some very elaborate experiments he had carried out on this subject. He also was the first chemist to carry out elaborate experiments with a view to deciding the question of the assimilation by plants of free atmospheric nitrogen. His first contribution to the subject was published in 1838, but can scarcely be regarded as possessing much scientific value, except in so far as it stimulated further research. Some thirteen years later he returned to this question; and during the years 1851-1855 carried out most elaborate experiments, the results of which, until quite recently, were generally regarded as having, along with the experiments of Messrs Lawes, Gilbert, and Pugh, definitely settled the question.[12]
In 1839 Boussingault was elected a member of the French Institute, an honour paid to him in recognition of his great services to agricultural chemistry.[13]
The foregoing is a brief epitome of the history of the development of agricultural chemistry up to the year 1840, the year which witnessed the publication of one of the most memorable works on the subject, which has appeared during the present century—Liebig's first report to the British Association, a work which may be described as constituting an epoch in the history of the science. Liebig's position as an agricultural chemist was so prominent, and his influence as a teacher so potent, that a few biographical facts may not be out of place before entering upon an estimate of his work.
Liebig.
Liebig was born at Darmstadt in the year 1803. He was the son of a drysalter, and early devoted himself to the study of chemistry in the only way at first at his disposal—viz., in an apothecary's shop. Soon finding, however, his opportunities of study limited, he left the apothecary's shop for the University of Bonn. He did not remain long at Bonn, but in a short time left that university for Erlangen, where he studied for some years, taking his Ph.D. degree in 1822. His subsequent studies were carried on at Paris under Gay-Lussac, Thénard, Dulong, and other distinguished chemists. Through the influence of A. Humboldt, who was at that time in Paris, and whose acquaintance he was fortunate enough to make, he was received into Gay-Lussac's private laboratory. In 1824—that is, when he was only twenty-one years of age—he was appointed Professor Extraordinarius of Chemistry at the University of Giessen. Two years later he was appointed to the post of Professor Ordinarius—an appointment which he held for twenty-five years. In 1845 he was created Baron, and in 1852 appointed Professor at Munich. He died in 1873.
His First Report to British Association.
The report above referred to was made by Liebig at the request of the Chemical Section of the British Association. It was read to a meeting of the Association held in Glasgow in 1840, and was subsequently published in book form, under the title of 'Chemistry in its Application to Agriculture and Physiology,' Liebig's position, past training and experience were such as to peculiarly fit him for the part of pioneer in the new science. As Sir J. H. Gilbert has remarked,[14] "In the treatment of his subject he not only called to his aid the previously existing knowledge directly bearing upon his subject, but he also turned to good account the more recent triumphs of organic chemistry, many of which had been won in his own laboratory."
In his dedication to the British Association at the beginning of the book, Liebig says: "Perfect agriculture is the true foundation of all trade and industry—it is the foundation of the riches of States. But a rational system of agriculture cannot be formed without the application of scientific principles; for such a system must be based on an exact acquaintance with the means of nutrition of vegetables, and with the influence of soils and actions of manure upon them. This knowledge we must seek from chemistry, which teaches the mode of investigating the composition and of studying the characters of the different substances from which plants derive their nourishment."
His criticism of the "Humus" Theory.
The first subject which Liebig discusses is the scientific basis of the so-called "humus" theory. The humus theory seems to have been first promulgated by Einhof and Thaer towards the close of last century. Thaer held that humus was the source of plant-food. He stated in his published writings that the fertility of a soil depended really upon its humus; for this substance, with the exception of water, is the only source of plant-food. De Saussure, however, by his experiments—the results of which he had published in 1804—had shown the fallacy of this humus theory; and his statements had been further developed and substantiated by the investigations of the French chemist Braconnot and the German chemist Sprengel. Despite, however, the experiments of Saussure, Braconnot, and Sprengel, the belief that plants derived the carbonaceous portion of their substance from humus still seemed to be commonly held in 1840.
While Liebig, therefore, can scarcely be said to have been the first to controvert the humus theory, he certainly dealt it its death-blow. He reasserted de Saussure's conclusions, and by some simple calculations showed very clearly that it was wholly untenable. One of the most striking of the arguments he brought forward was the fact that the humus of the soil itself consisted of the decayed vegetable matter of preceding plants. This being so, how, he asked, could it be the original source of the carbon of plants? To reason thus was simply to reason in a circle. He pointed out, further, that the comparative insolubility of humus in water, or even in alkaline solutions, told against its acceptance as correct.
His Mineral Theory.
Having thus controverted the humus theory, he then goes on to deal with the question of the source of the various plant constituents. In treating of the relation of the soil to the plant, he puts forward his "mineral" theory. It cannot be doubted that, while the advance of science since Liebig's time has induced us to considerably modify his mineral theory, it contained the statement of one of the most important facts in the chemistry of plant physiology. He was the first to fully estimate the enormous importance of the mineral portion of the plant's food, and point the way to one of the chief sources of a soil's fertility. Up to this period the ash constituents had been generally considered to be of minor importance. By emphasising the contrary opinion, and insisting upon their essentialness to plant-life, he gave to agricultural research a fresh impetus upon the right lines. His statement of his mineral theory was in the main true, but was not the whole truth.
De Saussure, as has already been pointed out, to a certain extent, anticipated Liebig's mineral theory. He was of the opinion that whatever might be the case with some of the mineral constituents of plants, others were necessary, inasmuch as they were always found in the ash. Of these he instanced the alkaline phosphates. "Their small quantity does not indicate their inutility," he sagaciously remarks. Sir Humphry Davy, as has already been pointed out, missed recognising the true importance of the ash constituents. It was left to Liebig, then, to restate the important doctrine of the essentialness of the mineral matter, already implied to some extent by de Saussure.
Liebig says: "Carbonic acid, water, and ammonia are necessary for the existence of plants, because they contain the elements from which their organs are formed; but other substances are likewise necessary for the formation of certain organs destined for special functions, peculiar to each family of plants. Plants obtain these substances from inorganic nature."
While insisting on the importance of the mineral constituents, he did so in a more or less general way not sufficiently distinguishing one mineral constituent from another.
As all plants contained certain organic acids, and as these organic acids were nearly always found in a neutral state—i.e., in combination with bases, such as potash, soda, lime, and magnesia—the plant must be in a position to take up sufficient of these alkaline bases to neutralise these acids. Hence the necessity of these mineral constituents in the soil. According to him, however, the exact nature of the bases was a point of not so much importance. He assumed, in short, as has been pointed out by Sir J. H. Gilbert, a greater amount of mutual replaceability amongst the bases than can be now admitted.
Passing on to a consideration of the difference of the mineral composition of different soils, he attributes this to the difference in the rocks forming the soils. "Weathering" is the great agent at work in rendering available the otherwise locked-up stores of fertility. He attributes the benefits of fallow exclusively to the increased supply of these incombustible compounds which were thus rendered available to the plant. Treating of this subject, he says: "From the preceding part of this chapter" (in which he has been explaining weathering) "it will be seen that fallow is that period of culture when the land is exposed to progressive disintegration by the action of the weather, for the purpose of liberating a certain quantity of alkalies and silica, to be absorbed by future plants."
His Theory of Manures.
Treating of manures, he showed how the most important constituents of manures were potash and phosphates. In the first edition of his work he also insisted on the value of nitrogen in manures, condemning the want of precautions, in the treatment of animal manures, against loss of nitrogen.
In the later editions of his work he seems to have receded from that opinion, and considered that there was no necessity for supplying nitrogen in manures, since the ammonia washed down in rain was a sufficient source of all the nitrogen the plant required. It was here that Liebig went astray, first in denying the importance of supplying nitrogen as a manure; and secondly, in overestimating the amount of ammonia washed down in rain, which has subsequently been shown to be entirely inadequate to supply plants with the whole of their nitrogen.[15]
His Theory of Rotation of Crops.
In explaining the benefits of the rotation of crops, Liebig propounded a very ingenious theory, but one which was largely of a speculative nature, and which has since been shown to be unfounded on any scientific basis. It was to the effect that one kind of crop excreted matters which were especially favourable to another kind of crop. He did not say whether he considered such excretion positively injurious to the crop which excreted them; but he inferred that what was excreted by the crop was what was not required, and what could, therefore, be of little benefit to a crop of the same nature following it.
The second portion of Liebig's report dealt with the processes of fermentation, decay, and putrefaction.
Publication of Liebig's Second Report to British Association.
In 1842 Liebig contributed his second famous report to the British Association, subsequently published under the title of 'Animal Chemistry; or, Organic Chemistry in its Applications to Physiology and Pathology.' The publication of this report created even greater interest than the publication of his first work. In it he may be said to have contributed as much to animal physiology, as, in his first, he did to agricultural chemistry. His subsequent principal works on agricultural chemistry were—'Principles of Agricultural Chemistry,' published in 1855, and 'On Theory and Practice in Agriculture,' 1856.
Liebig's services to Agricultural Chemistry.
An attempt has been made to sketch in the very briefest manner some of the main points in Liebig's teaching, as contained in his famous report to the British Association in 1840. Agricultural chemistry up till that year can scarcely be described as having a distinct existence as a branch of chemistry. Much valuable work, it is true, had already been done, especially by his two great predecessors, de Saussure and Boussingault; but it was, down to the year 1840, a science made up of isolated facts. Liebig's genius formed it into an important branch of chemistry, supplied the necessary connection between the facts, and by a series of brilliant generalisations formed the principles upon which all subsequent advance has been built.
As has already been indicated, Liebig's chief claim to rank as the greatest agricultural chemist of the century does not rest upon the number or value of his actual researches, but on the formative power he exercised in the evolution of the science. His master-mind surveyed the whole field of agricultural chemistry, and saw laws and principles where others saw simply a confusion of isolated, and, in many cases, seemingly contradictory facts.
But great as the direct value of Liebig's work was, it may be questioned whether its indirect value was not even greater. The publication of his famous work had the effect of giving a general interest to questions which up till then had possessed a special interest, and that for comparatively few. Both on the Continent and in England a very large amount of discussion took place regarding his various theories.
Development of Agricultural Research in Germany.
It was especially in Germany, however, that Liebig's work bore its greatest and most immediate fruit. Thanks to the great chemist, the German Government recognised the importance of forwarding scientific research by State aid. Agricultural Departments were added to some of the universities, largely at State expense, while agricultural research stations were, one after another, instituted in different parts of the country.
The first of the agricultural research stations to be founded was the now famous one of Möckern, near Leipzig. It was instituted in the year 1851. Others followed, until at the present day there are some seventy to eighty of these Versuchs-Stationen scattered throughout Germany, all well equipped and doing excellent work. Some idea of the activity of the German stations may be inferred when it is stated that up to the year 1877 the total number of papers embodying the results of their experiments published by them amount to over 2000.[16]
To trace the development of agricultural chemistry, subsequent to Liebig's time, in the way it has been done prior to the year 1840, is no longer possible. This is due to the enormous increase in the number of workers in the field, as also to the overlapping nature of their work, which renders a strict chronological record wellnigh an impossibility. It will be better, therefore, to attempt to give a brief statement of our present knowledge on the subject, naming the chief workers in the various departments of the subject.