Page:Household Cyclopedia 1881.djvu/13

 Chapter 1

AGRICULTURE

1.1 The Modern Theory Of Agriculture

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 etc., 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, always evolve ammonia, which plants need and absorb. Thus is explained one of the benefits of manuring, but not the only one as