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of water equal to 1.75 inches in excess of an adjoining plot which was not plowed. Quiroga, in a thesis presented to the College of Agriculture, Ohio State University, reports that the moisture content of the early plowed plots was higher than the late plowed throughout the season. He found also that the available nitro
gen was much higher, in the early plowed plots, and that the yield of corn was greater. All evidence indicates that the soil should be stirred as early in the spring as can be done without injury to its texture, either by plowing or by the use of some form of cultivator or harrow.
Tillage Destroys Weeds.—Lastly, tillage is useful in destroying weeds. Weeds should not be permitted to grow because they rob the crop of its moisture and plant food. During growth all plants pump up water by means of their roots, and give it off through the leaves. It has been shown that at the best the supply of water in the ground is seldom sufficient for a maximum crop so that any withdrawal of water from the soil by the weeds works a positive injury to the desirable plants. While it is probable that the weeds do the greatest injury to the crop by depriving it of water, they also rob it of nitrogen and mineral food. Some farmers argue that if the weeds remain on the ground they are removing no fertility, but it must be remembered that they are using that portion of the plant food that could be used by the crop and that the weeds must decay before this food is again rendered available, so that so far as the present crop is concerned the food is as completely removed as it would be if taken from the field. The destruction of weeds was formerly regarded as the only reason for tillage after seeding. It is now known that stirring the soil has a distinct value in itself, and that the killing of the weeds is really secondary. In fact if the farmer so tills his farm as to reap the maximum benefits to be derived from this process he will have no need to worry about the weeds.
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