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. 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. Nile is due to the natural annual overflow of the river, extended by artificial irrigation.