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Manures and the principles of manuring. Charles Morton Aikman
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.
Sir Humphry Davy's Lectures.
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