CHAPTER VII

DRAINAGE AND IRRIGATION

Film Moisture and Ground Water.-An important method for increasing the fertility of some classes of soils is that of underdraining by the use of tile or

other means. Water exists in the soil in two principal forms, viz: as the film or capillary moisture previously discussed, and in the form known indiscriminately as free water, ground water, or hydrostatic water. In the latter condition the water occupies the spaces between the soil grains, and is not held by the attraction of

these particles. The surface level of this free water is known as the "water-table," and is situated in some soils very near the surface while in others it is many feet below. The exact height of the water-table can be readily ascertained by sinking a hole to such a depth that water will stand in it, the level of the water in the hole being practically that of the water-table. It is this free or ground water that supplies shallow wells and the ordinary springs. In some cases the watertable may be at the level of the ground or above it, as is obviously the case where marshes and lakes exist.

High Water-Table Objectionable.-When the level of the free water is near the surface of the ground, the soil will be greatly benefited by some system of underdrainage, as this hydrostatic water is, for several reasons, injurious to the crop. Ground water limits the feeding space available to the plant, and, consequently, the amount of food it can obtain. Those plants that are of importance to agriculture must have their roots. supplied with air, and investigations have shown that such plants do not send their roots below the watertable, because the spaces between the soil particles below this level are filled with water, thus preventing the entrance of air. In other words, the depth to which the plant will send its roots is determined by the position of the water-table.

Free water makes the soil cold. A great deal more heat is necessary to warm water a certain number of degrees, than is required to raise the temperature of an equal weight of the dry matter of the soil to the same amount. A soil, therefore, that contains much water is harder to heat than one that is comparatively

dry. A very wet soil causes plant-food to become. locked up in unavailable forms, and in some cases compounds are produced which are actually poisonous to the desirable plants. An excessive amount of water in the soil also dilutes the plant-food in solution and makes it more difficult for the plant to procure sufficient nourishment.

One of the most important considerations in this connection is the fact that the presence of free water

in the soil prevents nitrification and promotes denitrification. In water-logged soil nitrates are rapidly decomposed, the nitrogen being given off to the air in the free, or elemental condition; and for this reason not only is the nitrogenous food in the soil destroyed, but the application of nitrogen fertilizers to such a soil results in great waste of this valuable element of fertility.

Drainage Aerates and Warms the Soil.-Underdraining the field results in lowering the water-table

to the level of the drain, the water flowing off through the tile instead of standing near the surface as stagnant water. A few ways in which this is of benefit to the soil may be indicated. The removal of the free water from the soil above the drain allows the entrance of air, and for that reason increases the depth to which the roots will penetrate. The entrance of air with its oxygen and carbonic acid, and the consequent greater depth reached by the roots and earthworms, are factors of importance in improving the texture of the soil. The

Diagram showing the level of the ground water in a tile-drained field a few hours after a heavy rain

rains will now soak down through the soil rather than run off the surface, and in this way the nitrogen in the rainwater is added to the soil, and surface washing is to a certain extent prevented. Rain in the spring is warmer than the ground, and as it percolates through the soil has a beneficial effect in warming it, thus putting it in condition to promote plant growth much earlier in the season. Evaporation of water from the surface of the soil tends to keep it cool, and as the amount of water near the surface is decreased by drainage, evaporation is also lessened. Well drained. lands, therefore, maintain a higher temperature throughout the season than do those containing much free water. Drainage lengthens the season of plant

growth and promotes nitrification and other processes by which the plant food is made available.

Drainage increases Available Water and prevents Injury from Drouth.-Paradoxical as it may seem, underdraining increases the amount of water available to the plant. The crop depends almost entirely on the capillary or film moisture for its supply of water, and

In cold, undrained lands, where the water-table in the spring is high, the plants are shallow rooted, and when the drouth of summer lowers the water-table they suffer for lack of moisture

as has been said, the roots do not enter that part of the soil containing free water. Lowering the water-table greatly increases the total amount of film moisture, as all that part of the soil from which the free water has been removed is capable of holding capillary water. Thus it will be seen that while the total amount of water in the soil is decreased by drainage, that which is of use to the plant is made much greater.

Drainage prevents injury from drouth, for by means of it the plants are encouraged to make deeper root