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912

FOOD OF PLANTS-VARIETIES OF SOIL.

manure supplied restores exactly those substances which previous crops had removed. There are few substances which more nearly fulfil this purpose than farm-yard manure, because the salts and mineral ingredients present in the food pass off almost wholly into the excretions of the animal; and thus the ingredients restored, so far as the supply extends, are nearly identical with those which have been removed: since, if the ash obtained from a given weight of any specific crop be compared with the ash obtained by burning the excreta of animals fed upon an equal quantity of the same crop, the composition of the ash in the two cases will be nearly identical.

It is, however, well known, that the recent droppings of cattle are far more effectual as manure than the ashes of such manure when burned; partly, because the recent manure contains carbon in a form which, during its decay within the soil, gradually supplies carbonic anhydride to the roots of the plant; but chiefly, as it appears, because it contains a large proportion of nitrogen, which, during its putrefaction, furnishes ammonia, or some other azotised compound, in a form suited to the wants of the growing

crop.

It is owing to similar causes that night-soil and urine possess so high a value as manure. They are much richer in nitrogenised compounds, and in phosphates, than farm-yard manure; and, indeed, they contain all the saline constituents of the corn and animal food, as well as almost all the nitrogen of the azotised constituents of the food which had been conveyed into the stomach, whether those azotised constituents had been assimilated or not. The Chinese have long been aware of the importance of these materials as manure, and they collect and carefully restore to their fields that which we in the pride of superior civilization discharge into sewers, and wastefully consign to the ocean, after allowing it to accumulate in offensive mud-banks within our tidal rivers, upon the shores of which, by the ebb and flow of the tide, it is alternately exposed to the heat of the sun and to the action of the water, under circumstances the most favourable to its decomposition; in consequence of which it is constantly contaminating the atmosphere of our large towns with its sickening effluvia.

(1714) Varieties of Soil.-In order to render the chemical action of manures intelligible, it will be necessary to take a rapid survey of the principal kinds of soil, and of the nature of the saline ingredients required by ordinary farm crops during their growth.

Soils vary greatly in physical characters, as well as in chemical composition. A soil may, for example, be described as clayey,

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sandy, marly, or calcareous, according as alumina, silica, or lime is the prevailing ingredient. The land will also be stiff or porous, moist or dry, partly owing to these peculiarities, and partly to its geological position, according as it rests upon clay, upon sand, upon chalk, or upon gravel; its character will also vary, in part, with its site, according as it lies high or low, and as its surface is inclined or horizontal. But these considerations, though of high importance to the agriculturist, lie beyond the province of the chemist, whose principal attention must primarily be directed to the chemical peculiarities of the soil. In order that a soil be fertile it must not only be well drained and sufficiently watered, but it must possess a certain quantity both of organic and of saline ingredients.

Seeds, it is true, in some cases may be made to germinate and develope plants upon flannel, or in well-washed siliceous sand, if duly moistened from time to time with distilled water. The plants thus obtained may even sometimes flower; but they never produce fertile seed; and, if these plants be burned, the weight of the ash which they yield never exceeds that of the ash yielded by a quantity of seed equal to that from which the plants were raised. By adding suitable saline mixtures to washed siliceous sand, Wiegmann and Polstorf, however, obtained tobacco, oats, barley, and other plants, in healthy growth, which furnished perfect seed. In these cases the plants had been supplied with those materials, the absence of which had in the previous experiments prevented their mature development. But it must not be hastily concluded from these results, that organic manures can advantageously be dispensed with in agriculture; experience has fully proved that a judicious combination both of organic and of saline manures is essential, and it is owing to ignorance of this fact that many of the patent mineral manures, when used alone, have so signally failed.

Different classes of plants, in order to the production of a luxuriant growth, require a soil possessed of physical qualities which must vary according to the kind of crop; and, in addition to this, each species of plant requires a variation in the chemical composition of the soil, suited to its peculiar habits. The prevailing and most abundant components of the soil, whether they be siliceous, aluminous, or calcareous, are not those which exert the greatest influence upon the development of the plant. They simply constitute the matrix in which the plant grows; and this matrix is more or less favourable to its growth according as its

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CLASSIFICATION OF CROPS BY THEIR ASHES.

physical character is more or less conformed to the wants of the plant; but the chemically active constituents of the soil consist of certain soluble saline compounds which are generally found in it in comparatively small quantities. In natural and fertile soils these salts are already present, either in an active or soluble form, or else in the passive or insoluble condition, stored up in rocks which have not as yet become disintegrated by exposure to the weather.

(1715) Liebig's Classification of Crops by their Ashes.-In order to ascertain the nature of the salts which are essential to the growth of any plant, Liebig insisted on the importance of an analysis of the ashes obtained by burning such plants, grown upon a soil which is known to suit them; and in accordance with this suggestion, a great number of careful analyses of the ashes of the more important varieties of cultivated plants have been made both on the continent and in this country. From the results thus obtained it appears, 1. that the number of the mineral constituents of plants is comparatively small ; 2. that the rature of the mineral constituents varies in different tribes of plants; and 3. that it varies even in different parts of the same plant.

Potash, soda, lime, magnesia, with now and then small quantities of alumina, oxide of iron, and oxide of manganese, are the bases found in vegetable tissues; and they occur in combination with sulphuric, silicic, and phosphoric acids, as well as with hydrochloric, and with various organic acids; the carbonates of these bases, which are so frequently found in the ashes of plants, are derived from the compounds of the organic acids with the alkalies which the plant contains, and which organic acids are destroyed when it is burned.

Liebig has proposed to class vegetables for agricultural purposes according to the composition of their ashes, and the following table will give an idea of the general principle which he has adopted. It will be observed that the same plant may appear in two different columns, as though it belonged to two different classes. The ash of the tubers of the potato, for example, is rich in alkaline salts, whilst the haulm abounds in salts of calcium; hence, as is shown in the second subdivision of the table, the potato should be regarded as belonging to the alkalino-calcareous class:

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The results of such analyses suggest a probable reason why plants of different species do not all thrive equally well in a soil which may be highly productive of a particular crop; since a soil which abounds in the saline matters required by one crop may be deficient in the ingredients necessary for the full development of plants of a different species. Root crops, and succulent plants generally, require abundance of alkaline and calcareous salts ; and experience has shown that the green shoots of all plants are those which contain these constituents in the largest proportion; the grasses, and plants with stiff but not woody stems, are rich in silica; while the grain crops, and seeds in general, as well as those plants which are richest in azotised nutritive materials, abound in the phosphates of the metals of the alkalies and of the earths.

Not only does the composition of the ash vary in different plants, but its quantity varies also within wide limits. According to the estimate of Johnston, a ton of each of the following substances, in its undried condition, contains upon the average the under-mentioned proportion of ash :—

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It is evident that all plants must, to a certain extent, deprive the soil of its saline components; since no plant adds anything to these materials which already exist in the soil, but on the con

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MODES OF ACTION OF MANURES.

trary, every crop that is carried off removes with it a corresponding amount of mineral material. Owing, however, to this important and undeniable fact, to which public attention was first strongly directed by Liebig, many persons have been tempted to overlook the necessity of organic materials in the manure; and considerable disappointment has often been experienced in finding that the restoration of mineral matters to the soil was not the only condition required to renew, or to preserve its fertility. Messrs. Lawes and Gilbert have rendered essential service to agriculture by their admirable researches upon this point, which have recalled public attention to a circumstance of cardinal importance to the agriculturist.

(1716) Modes of Action of Manures.-It would be a serious mistake to suppose that a supply of the proper manure can always be predicated from a knowledge of the composition of the ashes of a plant, or from the analysis of the soil to be manured. Manures may act in one of three principal ways:-1. as direct food for the plant; 2. as what, for want of a better term, may be viewed as stimulants to its growth at a particular stage; and 3. as chemical agents which modify the constitution of other substances already present in the soil. Liebig has specially called attention to the first of these modes of the action of manures. The second of these methods has been particularly elucidated by Lawes, who showed that in the early stage of the growth of turnips, no manure is to be compared in efficacy with superphosphate of lime; although the plant itself contains so small a proportion of phosphates that it was precisely the plant fixed upon by Liebig, on theoretical grounds, as the one which, from its non-requirement of phosphates, was particularly well calculated to be grown upon land which might have been supposed to have been exhausted of its phosphates by a previous wheat crop. It is now well known to the practical farmer, that the superphosphate, when drilled in judiciously with the turnip seed, stimulates the growth of the plant in the earliest and most critical stage of its existence, and thus secures a far more abundant return than when the supply of superphosphate is omitted.*

The experiments of the same patient and indefatigable agri

It is, however, probable that no manure acts simply as a stimulant, but that all such manures enter into the composition of the plant itself. These stimulating manures appear to supply through certain stages of the plant's growth a constituent or constituents, which, though present in the soil, may not be so in a sufficiently concentrated form, or which, owing to the imperfect development of the root, may not be sufficiently within the reach of the plant at the particular time when they are most needed.

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