Drainage

a method of relieving the soil of surplus water. The method of withdrawing water by open surface ditches or drains is too well known to require discussion. Surface ditches are open to objections. They fill up and must be redug. They are in the way of the farmer's team and implements, and they withdraw land from cultivation. In the older countries, where labor is cheap and land is expensive, tile draining is resorted to. Circular sections of earthenware tiling, shaped like joints of stovepipe, but with much thicker walls, are laid end to end in ditches, and covered with earth again. Such drains, if carefully constructed, last for generations without renewal. They must be laid, however, deep enough to be beyond the reach of tools, or possible pressure from the feet of animals or heavy machinery. A tile drain is supposed to draw the water from a strip of land the width of which is from five to twenty times the depth of the drain, according to the nature of the soil. A drain three feet below the surface of a stiff, heavy clay will drain a strip of ground about fifteen feet in width. A similar drain four feet beneath the surface of sandy soil is quite sufficient for a strip forty, or even eighty feet wide.

The utility of drainage is readily understood. Under favorable conditions, the roots of field crops descend from two to five or even fifteen feet, but never below the level of standing water. Changes from day to day in atmospheric pressure cause the surface of the earth to breathe. When atmospheric pressure increases a little more air enters the surface. When the pressure is less part of the air leaves the earth. This breathing extends down only to the level of standing water, and marks the limit to which root growth descends. For this simple reason drainage is of great benefit to crop growers. There is no danger of the earth becoming too dry; for the soil, however well drained, acts like a sponge, raising as much water from great depths as is needed for ordinary purposes of cultivation. Care must be taken, of course, to give the entire line of pipes a pitch in the same direction, and to provide a free outlet at the lower end.