Ture
Liebig and other chemists have, within the last twenty-five years, endeavored to establish a science of agriculture, based upon a knowledge of the constitution of plants and of soils, and their mutual relations. We propose to give a very condensed account of the general conclusions arrived at.
Food of Plants.
Plants derive their food from the air as well as from the earth; the former by their leaves, the latter by their roots. Elements most necessary to them are carbon, hydrogen, oxygen, and nitrogen, with various mineral substances present in the soil. Carbon is the most abundant. This is to a large extent extracted from the atmosphere by the leaves of plants, during the day-time.
Hydrogen and oxygen are in the water contained in the earth and air; and oxygen is in the air mixed with nitrogen. Plants do not seem able, however, to separate much nitrogen from the air as such, but more readily obtain it by the decomposition of ammonia (composed of hydrogen and nitrogen), which is formed in the atmosphere, and washed down into the earth by rain-water, so as to reach the roots. All ordinary waters, it must be remembered, contain substances dissolved in them. Irrigation of land does not act only by the water itself, but by that which is dissolved or diffused in it. Davy calculated that, supposing one part of sulphate of lime to be contained in every two thousand of river water, and every square yard of dry meadow land to absorb eight gallons of water, then, by every flooding, more than one and a half hundred weight of gypsum per acre is diffused by the water - a quantity equal to that generally used in spreading gypsum as a manure or fertilizer; and so, if we allow only twenty-five parts of animal and vegetable remains to be present in a thousand parts of river water, we shall find that every soaking with such water will add to the meadow nearly two tons per acre of organic matter. The extraordinary fertility of the banks and delta of the river Nile is due to the natural annual overflow of the river, extended by artificial irrigation. In China also, the principle of irrigation is carried out very largely, and it is applicable, on a large or small scale, in any country. The water of lakes is usually charged with dissolved or suspended substances even more abundantly than that of rivers.
Humus.
Soils contain a great amount of matter which results from the decay of vegetables and animals; to a compound of which with earthy material the name of humus is given. This was once incorrectly supposed to give the whole nutriment of the plant. Trees and plants, instead of abstracting carbon from the earth, really, by taking it from the air, and subsequently dying and decaying, annually by their leaves, and finally altogether, give carbon and other atmospheric elements to the soil. As above said, all plants by their leaves absorb carbonic acid from the air, and retain carbon, giving out oxygen. It is evident, therefore, that the leaves are of great importance to the plant. So are the roots, for their absorbing office. Thus it is true that the growth of a plant is always proportioned to the surface of its roots and leaves together. Vegetation, in its simplest form, consists in the abstraction of carbon from carbonic acid, and hydrogen from water; but the taking of nitrogen also, from ammonia especially, is important to them, and most of all, to those which are most nutritious, as the wheat, rye, barley, &c., whose seeds contain gluten and other nitrogenous principles of the greatest value for food. Plants will grow well in pure charcoal, if supplied with rain-water, for rain-water contains ammonia.
Animal substances, as they putrefy, alway evolve ammonia, which plants need and absorb.
Thus is explained one of the benefits of manuring, but not the only one, as we shall see presently.
Animal manure, however, acts chiefly by the formation of ammonia. The quantity of gluten in wheat, rye, and barley is very different; and they contain nitrogen in varying proportions.
Even in samples of the same seed the quantity varies; and why? Evidently because one variety has been better fed with its own appropriate fertilizer than another which has been reared on a soil less accurately adapted by artificial means for its growth. French wheat contains 12 per cent. of gluten; Bavarian 24 per cent. Sir H.
Davy obtained 19 per cent. from winter, and 24 from summer wheat; from Sicilian 21, from Barbary wheat 19 per cent. Such great differences must be owing to some cause, and this we find in the different methods of cultivation.
An increase of animal manure gives rise not only to an increase in the number of seeds, but also to a remarkable difference in the propor- (9) tion of gluten which those seeds contain. Among ❘ perceptibly in the soil of grain-fields, because what manures of animal origin there is great diversity.
Cow dung contains but a small proportion of nitrogen. One hundred parts of wheat, grown on a soil to which this material was applied, afforded only 11 parts of gluten and 64 of starch; while the same quantity of wheat, grown on a soil fertilized with human urine, yielded 35 per cent. of gluten, and of course a smaller proportion of less valuable ingredients. During the putrefaction of urine, ammoniacal salts are formed in large quantity, it may be said, exclusively; for under the influence of warmth and moisture, the most prominent ingredient of urine is converted into carbonate of ammonia.
Guano.
Guano consists of the excrements of sea-fowl, collected during long periods on certain islands in the South Sea. A soil which is deficient in organic matter is made much more productive by the addition of this manure. It consists of ammonia, combined with uric, phosphoric, oxalic, and carbonic acids, with some earthy salts and impurities.
The urine of men and animals living upon flesh contains a large quantity of nitrogen, partly in the form of urea. Human urine is the most powerful manure for all vegetables which contain nitrogen; that of horses and horned cattle contains less of this element, but much more than the solid excrements of these animals. In the face of such facts as these, is it not pitiable to observe how the urine of the stable or cow-shed is often permitted to run off, to sink uselessly into the earth, or to form a pool in the middle of a farm-yard, from which, as it putrefies, the ammonia formed in it rapidly escapes into the atmosphere? Cultivated plants need more nitrogen than wild ones, being of a higher and more complex organization. The result of forest growth is chiefly the production of carbonaceous woody fibre; of garden or field culture, especially the addition of as much nitrogen as the plant can be made to take up.
Solid Manure.
The solid excrements of animals do not contain as much nitrogen as those which are voided in a liquid form, and do not constitute so powerful a fertilizing material. In urine, moreover, ammonia loses a good deal of its volatility by being combined and dissolved in the form of salts. In an analogous manner, one of the uses of sulphate of lime or gypsum, as a manure, is to fix the ammonia of the atmosphere. Charcoal and humus have a similar property.
Mineral Matter in Plants.
Besides the substances already mentioned, others are needed by plants as part of their food, to form their structure. The firmness of straw, for example, is due to the presence in it of silica, the principal constituent of sand and flints. Potassa, soda, lime, magnesia, and phosphoric acid, are contained in plants, in different proportions.
All of these they must obtain from the soil. The alkalies above-named (potassa and soda) appear to be essential to the perfect development of the higher vegetable forms. Some plants require them in one mode of combination, and some in another; and thus the soil that is very good for one, may be quite unfit for others. Firs and pines find enough to support them in barren, sandy soil.
The proportion of silicate of potash (necessary for the firmness of wheat straw) does not vary is removed by the reaper, is again replaced in putrefying straw. But this is not the case with meadow-land. Hence we never find a luxuriant crop of grass on sandy and limestone soils which contain little potash, evidently because one of the constituents indispensable to the growth of the plants is wanting. If a meadow be well manured, we remove, with the increased crop of grass, a greater quantity of potash than can, by a repetition of the same manure, be restored to it. So, grass-land manured with gypsum soon ceases to feel its agency. But if the meadow be strewed from time to time with wood ashes, or soap-boilers' lye made from wood ashes, then the grass thrives as luxuriantly as before. And why? The ashes are only a means of restoring the necessary potash for the grass stalks. So oats, barley, and rye may be made for once to grow upon a sandy heath, by mixing with the scanty soil the ashes of the heath-plants that grow upon it. Those ashes contain soda and potash, conveyed to the growing furze or gorse by rain-water. The soil of one district consists of sandstone; certain trees find in it a quantity of alkaline earths sufficient for their own sustenance. When felled, and burnt, and sprinkled upon the soil, oats will grow and thrive that without such aid would not vegetate.
The most decisive proof of the absurdity of the indiscriminate use of any strong manure was obtained at Bingen, a town on the Rhine, where the produce and development of vines were highly increased by manuring them with animal matters, such as shavings of horn. After some years, the formation of the wood and leaves decreased perceptibly. Such manure had too much hastened the growth of the vines: in two or three years they had exhausted the potash in the formation of their fruit leaves and wood; so that none remained for the future crops, as shavings of horn contain no potash. Cow-dung would have been better, and is known to be better.
Conditions of Vegetation.
The sun's heat and light, air, water, and the common elements of the earth are necessary to the existence of plants. But a greater or less abundance of certain elements, and their existence in more or less favorable states of combination, determines the magnitude and fertility, or, in a word, the whole productiveness, of the vegetable growth.
The rules of agriculture should then, if rationally perfected, enable us to give to each plant what it requires for the attainment of the special object of its culture; namely, the increase of certain parts which are used as food for men and animals.
One instance may illustrate this idea. The means to be resorted to for the production of fine pliable straw for hats and bonnets are the very opposite to those which would tend to produce the greatest possible amount of seed or grain from the same plant.
Sand, clay, and lime, as has been said, are the principal constituents of soils. Clay and marl always contain potash and soda. Pure and, or pure limestone, would alone constitute absolutely barren soils. All arable land contains an admixture of clay, although an excess of it, in propor. tion, is of course disadvantageous.
Rotation of Crops.
The exhaustion of alkalies in a soil by successive crops is the true reason why practical farmers suppose themselves compelled to suffer land to lie fallow. It is the greatest possible mistake to