The Ventilation Of The Soil

The spaces between the soil grains are filled either with water, or air, or both. This soil air is somewhat different from the free air above the surface, containing less oxygen, more carbonic acid gas and more ammonia gas. Part of its oxygen is used by the plant roots; the other gases are absorbed from the vegetable matter decaying in the soil. Practically all of our farm crops need a well ventilated soil. The roots of plants, except certain bog, marsh and water plants, must have air to breathe. If it is denied them, because the interspaces of the soil are filled with water, the plants will die. Corn is "drowned out" in low, wet places, chiefly because the roots cannot breathe. Furthermore, air is needed in the soil to make more plant food. The air penetrates deeply into the soil and its oxygen, carbonic acid and ammonia dissolve the minerals and make the soil more fertile. The nitrogen of this air may be used as a food by certain plants (See Chapter XII). The oxygen of the soil air combines with the nitric acid produced by the decay of plants, making it a nitrate, which is a plant food.

Manure which is piled loosely, so that air penetrates it readily, heats quicker and stronger than tightly packed manure; likewise a soil that is well drained and open, so that air passes into it freely, has more life, fermentation and fertility in it than a close-grained, air-tight soil. Air may penetrate the soil to a depth of many feet, depending upon its openness. Soil air changes in temperature like surface air, and continually passes up and down in currents. Methods of Improving the Ventilation of Soil. — Any kind of tillage which stirs and loosens the soil, like plowing and harrowing, promotes a better aeration, or ventilation, of the soil. Plowing under farm manure, green manure or stubble also has the same desirable effect, since the humus thus produced separates the particles of soil and renders it more porous, hence more open to the downward passage of air. Under-draining, however, is the chief means of ventilating a heavy soil. Remove the water and the air will rush in. When the water table is lowered two or three feet, as it may be by under-draining, the roots of plants grow deeper, when they decay, they leave little channels in the soil and through these air penetrates. Earthworms and ants still further deepen and aerate the soil by following these channels. When land is tile-drained, the tiles themselves provide a system of underground ventilation of far reaching influence. The soil of a tile-drained field is ventilated much more thoroughly than the soil of another field of the same character in which the water table stands naturally at the same height. The air in tile drains is largely surface air.

The roots of most farm crops deepen and aerate the soil, but the roots of leguminous plants, especially of clover and alfalfa, are particularly useful in soil ventilation. This is partly because clover roots are large and bore straight down into the subsoil for several feet, leaving much larger and more effective channels for the passage of air and water than the roots of grains ; and also, in a very slight measure, because these plants absorb nitrogen from the soil air, thus making it necessary for more surface air to be forced into the soil to replace that which is lost. Fortunately for the farmer, most soils are able to absorb various gases, notably ammonia, which is very valuable for the nitrogen which it contains. Advantage is taken of this fact when decaying animal matter is buried to remove the offensive smell, and when sandy loam is used behind cows in the stable. The soil acts much like a charcoal filter which is used to remove objectionable odors from water.