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.
This text is from the Household Cyclopedia, 1881 which is in Public Domain.