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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
SURFACE OF GROUND
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 growth, and, hence, are not so easily affected by the extreme drying of the surface of the ground that takes place in times of scanty rainfall. It will readily be seen that tile draining determines the highest point the water-table can reach, but that in dry weather the level of the ground water my be much below the drain. It is sometimes thought, for this reason, that part of the
Good drainage encourages the roots to strike downward and when the
drouth comes the plants do not suffer
water from summer rains, which would otherwise be absorbed by the soil below the drain, may be lost through the tile. Experience has shown, however, that the water does not percolate into the drain as some suppose, and that it is only when the rainfall is sufficient to raise the water-table to the level of the drain, that any water is removed by it. It is a matter of common observation that, except in the case of quite low lands, it is only the very heavy summer rains that cause the drains to run. It will thus be seen that it is simply the excess of water which is removed by underdraining, and not that part which is of most importance to the plant. Although the crop probably makes little direct use of the free water, one must not lose sight of the fact that it may be drawn into the upper layers of the soil by capillarity to replace that lost by evapora
tion, and for this reason the underdraining should not be so deep as to interfere seriously with capillary action.
Drainage Sometimes Beneficial on High Lands.Strangely enough, experience has shown that it is not merely low lying soils which are benefited by underdraining. In many cases heavy clay soils in elevated positions, especially if underlaid by rather impervious subsoils, are greatly improved in tilth by tiling. In such soils the percolation is so slow that practically the same effect is produced as would be expected if the general level of the ground water were near the surface. These soils are made more mellow by drainage and respond more readily to early tillage. Clay soils are often puddled by the fine particles in the soilwater being deposited in the spaces between the soilgrains, thus cementing them together. The use of tile will often prevent this by causing the water to sink more rapidly through the soil. Tile-drained fields are not so likely to be injured by hauling heavy loads over them as are those not so treated.
Irrigation in Humid Climates in Experimental Stage.—There are large areas in the western part of this country which, for lack of sufficient water, produce very scanty vegetation, although in many instances the soil is well supplied with the other materials essential to the plant. The results to be derived from irrigation on such soils is too well known to call for comment here. The work of investigators, notably that of King, in the so-called humid climates east of the Mississippi has shown that even here the total rainfall is seldom sufficient for a maximum yield of the staple crops, and the precipitation is distributed so unevenly throughout the season that a comparatively small part of it can be used by the plants.
Many market gardeners recognize the fact that some system of irrigation is necessary for the most profitable returns, and are in the habit of supplying water artificially to their more valuable crops. Marshall P. Wilder when asked to name the three things most essential to successful strawberry culture is said to have replied: "First, plenty of water; second, more water; third, still more water.”
At the present time irrigation of the staple farm crops is not practiced to any large extent in the humid