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458. On comparing the constituents of such a soil as the above, with the mineral ingredients obtained by incineration from the ashes of plants, it is found that plants withdraw from the soil chiefly its alkaline, mineral acid, and earthy ingredients; and if all these were not essential to the very existence of the plants, they would not, of course, be taken up by them; and as the plants constituting our cultivated crops withdraw those ingredients in a varied amount, it follows that, unless the soils we cultivate contain them in ample amount and variety, it will be impossible for the plants placed upon them to arrive at a perfect state of development of all their parts; for, chemically speaking, and rationally speaking too, soils cannot be expected to produce crops abundantly, unless they contain a sufficient supply of every ingredient which all the crops we wish to raise require from them.

459. The practical purpose of all analyses of plants and soils should, therefore, be to make us acquainted with the constituents of every variety of cultivated crop at their different stages of growth; and to ascertain whether or not each soil in use contains a sufficient supply of such ingredients. The analyses of plants should have thus a twofold object-namely, to guide the cultivator in the treatment of the plants at the various stages of their growth, and to instruct him as to what quantity the ripe plant in its healthy state finally carries off of those ingredients from the soil.

460. "The latter only of these two applications of such knowledge," observes Professor Johnston on this subject, "has hitherto been kept in view by chemists; and so little has been done in reference to it, that we scarcely know as yet what any

one entire plant, when fully ripe, carries off from the soil. In reference to the former application, the few imperfect researches detailed in the preceding sections," (of the second edition of his Lectures on Agricultural Chemistry,) "contain all that we yet know. We may well say, therefore," he concludes, "that our knowledge of the inorganic constituents of plants is yet in its infancy, and that our present opinion upon the subject ought, therefore, to be permitted to hang very loosely about us.'

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461. Here, then, the agricultural student will observe, is an extensive and interesting field for exercising the researches of the analytical chemist for years to come, and a most useful subject upon which to expend a proportion of the funds of agricultural societies, until every variety of soil, whether under cultivation or in a state of nature, and every variety of plant, whether under cultivation or in a state of natural pasture, shall have been minutely and rigorously analysed.

462. Per-centage of mineral ingredidients taken from the soil.-As an example merely of the quantity of the mineral ingredients taken from the cultivated soil by some of the cultivated plants, I shall enumerate these instances:By grain crops :

100 lbs. of Wheat, Barley, Oats, Rye, Rice,

Indian corn, Buck-wheat, Millet seed, Field beans, Field pease, Vetches, Lentils, Linseed,

Hemp seed, Mustard seed, Coffee,

Grain.

1.2 to 2.0 2-3 to 3.8

Husk.

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Straw. 3.5 to 18.5

5.2 to 8.5

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* Johnston's Lectures on Agricultural Chemistry and Geology, 2d edition, p. 528.

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466. On the results exhibited in these tables, Professor Johnston makes these observations. "That the quantity of inorganic matter contained in the same weight of the different crops we raise, or of the different kinds of vegetable food we eat, or with which our cattle are fed, is very unlike; and the quantity contained in different parts of the same plant is equally unlike. These results cannot be the result of accident. They are constant on every soil, and in every climate; they must, therefore, have their origin in some natural law. Plants of different species must draw from the soil that proportion of inorganic matter which is adapted to the constitution, and is fitted to supply the wants of each; while of that which has been admitted by the roots into the general circulation of the plant, so much must proceed to, and be appropriated by, each part, as is suited to the functions it is destined to discharge. And as from the same soil different plants select different quantities of saline and earthy matter, so from the same common sap do the bark, the leaf, the wood, and the seed, select and retain that proportion which the healthy growth and development of each requires. It is with the inorganic as with the organic food of plants: some draw more from the soil, some less; and of that which circulates in the sap, only a small portion is expended in the production of the flower, though much is employed in forming the stem and leaves.

467. "On this subject I shall add two other observations," continues the Profes

sor: "from the constant presence of this inorganic matter in plants, given under all circumstances, a doubt can hardly remain that it is an essential part of their substance, and that they cannot live and thrive without it. But that it really is so, is beyond a doubt, by the farther experimental fact, that if a healthy young plant be placed in circumstances where it cannot obtain this inorganic matter, it droops, pines, and dies. But if it be really essential to their growth, this inorganic matter must be considered as part of the food of plants; and we may as correctly speak of feeding or supplying food to plants, when we add earthy and mineral substances to the soil, as when we mix it with a supply of rich compost, or of well fermented farmyard manure.

468. "I introduce this observation for the purpose of correcting an erroneous impression entertained by practical men in regard to the way in which mineral substances act when applied to the soil. By the term manure, they generally designate such substances as they believe to be capable of feeding the plant, and hence reject mineral substances, such as gypsum, nitrate of soda, and generally lime, from the list of manures properly so called. And as the influence of these substances on vegetation is undisputed, they are not unfrequently considered as stimulants only. Yet if, as I believe, the use of a wrong term is often connected with the prevalence of a wrong opinion, and may lead to grave errors in practice, I may be permitted to press upon your consideration the facts above stated-I may almost say demonstrated. that plants do feed upon dead unorganised mineral matter, and that you, therefore, really manure your soil, as well as permanently improve it, when you add to it such substances of this kind as are found by experience to promote the growth of your crops."

469. The discovery of the constant existence of inorganic matter in plants, which could have been discovered by chemistry alone, must have, in future, a very important influence in modifying the notions, and regulating the practice of cultivating all our plants. "It establishes a clear relation between the kind and quality of the crop, and the nature and che

mical composition of the soil in which it grows;-it demonstrates what soils ought to contain, and therefore how they are to be improved;-it explains the effect of some manures in permanently fertilising, and of some crops in permanently impoverishing the soil; it illustrates the action of mineral substances upon the plant, and shows how it may be, and really is, in a certain measure, fed by the dead earth: -over nearly all the operations of agriculture, indeed, it throws a new and unexpected light." *

470. The great importance to agriculture of ascertaining the constituents of plants by analysis, is thus estimated by Liebig:-" By an exact examination of the quantity of ashes in cultivated plants," he observes, "growing on various kinds of soils, and by their analysis, we learn those constituents of the plants which are variable, and those which remain constant. Thus, also, we will attain a knowledge of the quantities of all the constituents removed from the soil by different crops. The farmer will then be enabled, like a systematic manufacturer, to have a book attached to each field, in which he will note the amount of the various ingredients removed from the land in the form of crops, and therefore how much he must restore to bring it to its original state of fertility. He will also be able to express, in pounds weight, how much of one or of another ingredient of soils he must add to his own land, in order to increase its fertelity for certain kinds of plants. These investigations are a necessity of the times in which we live; but in a few years, by the united diligence of chemists of all countries, we may expect to see the realisation of these views; and, by the aid of intelligent farmers, we may confidently expect to see established, on an immoveable foundation, a rational system of farming for all countries and for all soils." +

471. Classification of soils.-A correct and intelligible classification of soils would much facilitate their description by writers on agriculture, and would render their characters more easily understood by the readers of agricultural works. For want

of any systematic classification, farmers have established a classification amongst themselves, which seems to answer all practical purposes. Thus, when a soil is described as being clayey or sandy as its fundamental characteristic, it is understood to be strong or light, and when it is said to be sharp or heavy, the kind of crop each is best suited to grow-namely, turnips on the former, and wheat on the latter-is easily understood; but, of course, these conventional terms are only understood by practical men, and convey no definite meaning to others.

472. Were soils as definite in their characters as minerals are, there would be no difficulty in applying to them the external characters employed to describe minerals, and these are quite sufficient to identify them to mineralogists; but as soils are so varied in aspect and texture, definite rules are quite inapplicable to them.

473. And if the external characters cannot be employed to describe soils correctly, much less can their chemical composition be adopted as a basis of classification suited to the wants of practical men. Chemical tests can only be employed when the soil is about to be analysed; and to require an analysis before a soil can be described or understood, is to place a direct barrier against acquiring a scientific knowledge of its characters by practical

men.

474. All I can present on this subject is a sketch of a classification of soils, proposed by M. De Gasparin, who, though employing some chemical tests to ascertain the nature of soils, had previously endeavoured to ascertain it by studying their agricultural characters. The result was, that he was led to adopt the following conclusions in regard to the relative values of the characters of soils. "It is," he says, "only after having destined a particular soil to an appropriate culture, that we can begin to consider the labour and improvement it requires. Those labours and improvements will be without an object and a bearing, if we are still ignorant of the plant to which they would be useful. And,

* Johnston's Lectures on Agricultural Chemistry and Geology, 2d edition, p. 303-7. + Liebig's Chemistry of Agriculture and Physiology, 3d edition, p. 213.

moreover, this investigation of the appropriation of soils to particular kinds of culture, is connected with the most natural classification in a mineralogical point of view; it breaks the smallest number of affinities, and consequently renders the determination of soils more easy and more satisfactory." I cannot help thinking that M. De Gasparin has here hit upon the principle upon which a correct and useful classification of soils may be founded.

475. In his endeavour to reduce this principle to practice, he has placed soils in two great divisions; the first includes those having a mineral or inorganic basis, the second those having an organic one.

476. The first great division, consisting of soils having a mineral or inorganic basis, he divides into four classes, comprehending saliferous soils, siliceous soils, clays, and calcareous and magnesian soils.

477. The character of saliferous soils is, that they have " a salt or styptic taste, containing at least 0.005 parts of hydrochlorate of soda, or sulphate of iron;" and they consist 1st, of saline soils; and, 2d, of vitriolic soils.

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482. The character of fresh mould is, that "the water in which this mould is digested or boiled does not redden litmus paper."

483. That of acid mould being, that, under the same circumstances, it "reddens litmus paper."

484. It is intimated that M. De Gasparin has laid" down rules for the description of species, and with examples of all the methods of description. In reading these, we at once perceive how precise an idea of soils is conveyed in a manner that cannot be misunderstood by any agriculturist. The possibility of transmitting these clear and pointed descriptions to a distance, follows as a matter of course; and we shall in this manner be freed from all that vagueness which has been so long a just cause of complaint." This is all that can be desiderated on the subject; but, useful as all M. De Gasparin's services to agriculture have been in the right direction, he has not yet succeeded in establishing a faultless description of soils, for, let me apply some of the rules he has offered above, and test his own application of them. For example, he says, that clay soils are characterised by "not yielding effervescence with acids, and affording by levigation less than 0.70 of the first portion;" and the character of siliceous soils he gives in these words, "producing no effervescence with acids, affording by levigation at least 0.70 of large particles." Surely the mere difference of affording 66 at least" and "less" than 0.70 of any ingredient, is not sufficient to account for the great difference existing in agriculture betwixt clay and sandy soils. He does not, however, confound loams with clays, as some writers have done, the loams containing clay only a little "more than 0.10 of the weight of the soil;" whereas clays afford only a little "less than 0.70 of the first portion" of the matter separated by levigation, thereby establishing a great difference of character betwixt them.

385. There is no doubt, however, of the truth of the opinion expressed by M. De Gasparin, were a correct nomenclature and classification of soils established, when he says, "that the study of agricultural

treatises would be greatly facilitated; the different methods of culture which are followed in distant countries will no longer appear so marvellous, and will become more intelligible; we shall comprehend better the considerations which limit or extend the several cultures; and a necessary link being established between the science of agriculture and the other natural sciences, it will become more intelligible to all, and will more readily profit by the progress of all the other branches of human knowledge." *

486. Origin of soils.-On endeavouring to establish a relationship between geology and agriculture, it seems incumbent to give the agricultural student some idea of the origin of the soil upon the cultivation of which he is about to exercise his skill, and from which he is to extract his future subsistence; and, in consulting geological and other writers on this subject, some seem to regard the existing soils as not only exceedingly simple in their origin, but certainly indicative of the rock on which they rest. Thus, Mr Morton says, that "the surface of the earth partakes of the nature and colour of the subsoil or rock on which it rests. The principal mineral of the soil of any district is that of the geological formation under it; hence we find argillaceous soil resting on the various clay formations- calcareous soil over the chalk-and oolitic rocks and siliceous soils over the various sandstones. On the chalk the soil is white; on the red sandstone it is red; and on the sands and clays the surface has nearly the same shade of colour as the subsoil. The lime, potash, and iron, existing in various proportions in the rock, are acted on by the atmosphere, and the rock is decomposed; some of it will form impalpable matter, some into sand, and some into coarse gravel or rubble. The surface is composed of the same materials as the subsoil, with the addition of vegetable and animal matter, in every state of decay, intimately mixed with it; and we perceive a change in the external appearance of the surface whenever there is a change in the subsoil below."t

Here the direct derivation of the soil from the subjacent rock is fully stated.

487. A subsequent author, Mr Whitley, gives his view of the formation of soils in these words: "The ordinary effect of atmospheric influence does not appear sufficient to produce such changes, (those observable on the surface of the globe;) for, if we examine the granitic tors, we shall find that mosses and lichens grow around their bases, and creep up their sides, forming, by their decay, a light vegetable mould on which the weather produces little effect. Much less would atmospheric influence be able to produce such extensive changes as those we have described. We are therefore led to the conclusion that some more powerful and effective agent has been at work; and the phenomena connected with the facts we have reviewed, are only consistent with the theory of a vast body of water having, by its violent action, broken and comminuted the earth's surface to a considerable depth-thus holding in mechanical suspension the materials of which the soil and subsoil are composed - the coarser and heavier parts of which first subsided, then the clays, and lastly, the fine earthy matter.

In endeavouring to establish this view of the formation of soil, the decomposition of rocks by atmospheric and chemical agencies must not be overlooked. These causes had probably produced extensive changes before that catastrophe whose effects we have just been describing; and to the present time their operation tends to improve and deepen the soil. The crumbling down of rocks by decomposition may be regarded as conservative of the soil, by supplying fresh portions to replace those which are constantly being washed away." Yet after a review of "the violent action of a body of water" having swept away "large portions of the earth's surface from their original position, and deposited them at lower levels," Mr Whitley arrives, rather unexpectedly, at a conclusion of the origin of the present soil, which is much in unison with, and even in the same words as, the

* Comptes Rendues de l'Académie des Sciences, tome viii., No. 8, p. 285, 1839, as translated in Jameson's Edinburgh New Philosophical Journal, vol. xxvii., p. 84. I may here mention that M. De Gasparin is engaged in a voluminous work on Agronomy, volumes of which have already appeared in France.

+ Morton On Soils, p. 1. 1838. I have not seen the subsequent editions of this work.

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