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in such condition of tilth that the air may freely circulate through it.
Tillage Increases Amount of Available Water.Tillage not only increases the amount of surface on which the plants can feed, but at the same time enlarges the water supply by giving the soil greater capacity for holding moisture. Attention has been called to the fact that each soil-grain is surrounded by a film of water which is called the capillary water or film moisture. The plant is dependent upon this film moisture for its supply, and it is readily seen that the amount of capillary water that the soil can retain depends upon the aggregate surface area presented by the particles of which it is composed. The rate at which this area, and the consequent amount of available moisture increase, is strikingly brought out by King in his book entitled “The Soil” from which the following is quoted: “Suppose we take a marble exactly one inch in diameter. It will just slip inside a cube one inch on a side, and will hold a film of water 3.1416 square inches in area. But reduce the diameters of the marbles to one-tenth of an inch, and at least 1,000 of them will be required to fill the cubic inch, and their aggregate surface will be 31.416 square inches. If, however, the diameters of these spheres be reduced to onehundredth of an inch 1,000,000 of them will be required to make a cubic inch and their total surface area will be 314.16 square inches. Suppose, again, the soil particles to have a diameter of one-thousandth of an inch. It will then require 1,000,000,000 of them to fill completely the cubic inch, while their aggregate surface must measure 3141.59 square inches.” Another way of stating the same fact is that if an acre of ground is so tilled as to reduce the average diameter of the soil particles to one-tenth the original diameter, the plant now has ten acres from which to draw its supply of water and mineral food for each acre it had before;
and the soil is enabled to hold as film moisture ten times as much water as it could in the first instance.
Tillage to Conserve Moisture.—From what has been said regarding the importance of water to the plant it must be apparent that one of the chief problems of agriculture is to maintain a proper degree of moisture in the soil. It seldom happens that a crop can obtain from the soil the amount of water necessary for a maximum yield, and great skill is required to keep it from suffering for lack of moisture during the hot summer period of scanty rainfall. While man can do nothing in the way of distributing the rainfall through
out the growing season, he can, by a judicious use of tillage methods, do much toward saving the excess of moisture precipitated in the early spring, for the use of the plant during the drier weather of the summer. One way in which tillage accomplishes this end is by increasing the capacity of the soil for storing water, as described in the preceding paragraph. It must also be evident that the loosening of the ground incident to tillage makes it easier for the rain to enter the soil, and tends to prevent loss by surface washing, as the water sinks into the soil instead of running away. Special mention might be made here of the “earth mulch” and late fall, and early spring plowing, as methods of tillage especially recommended for the conservation of soil moisture.
The Earth Mulch to Conserve Moisture.—During dry weather water is constantly being evaporated from the surface of the ground. Under ordinary conditions, where the soil is somewhat firm, water is drawn up from below by capillary attraction to replace that removed by evaporation. As this may be very rapid in the hot dry weather of midsummer the result is that the water is virtually pumped out of the soil until it is too dry for good plant growth. If something is done to break this capillarity the water can not be brought up from below. This is the end accomplished by the earth mulch, which is simply a layer two or three inches deep of very dry soil, so dry and loose that it can not take up the water from the layer next beneath it. The same end can be attained by covering the ground with loose straw or other similar material, the principle underlying both kinds of treatment being the same. To make an effective earth mulch the cultivation should be shallow and frequent, the aim being to make the layer as dry as possible. A rain, of course, will again compact the loose earth, and renew the capillarity, so that the cultivation should be repeated as soon as may be after a rain. Even in the absence of rain the mulch will sooner or later become compact of itself if left too
long without stirring. It is desirable to loosen the soil more frequently in the spring than is necessary later in the season. A mulch about three inches deep has been found to be most effective in conserving moisture, and it has also been shown that mulches produce relatively better results in sandy soils than in clay or loam.
Late Fall Plowing to Conserve Moisture.—Plowing the ground late in the fall tends to save the moisture, as the loose ground turned up by the plow prevents loss of water by evaporation. The broken uneven
surface also makes it possible for the soil to absorb more of the water from the winter rain and snow. An experiment reported from Wisconsin shows that a plot plowed in the fall contained 1.15 acre inches more water than an adjacent plot not so plowed. It must be borne in mind, however, that fall plowing is not a practice
capable of universal application, for there are certain hard soils with a low humus content which may be badly puddled if fall plowed. Here, as everywhere in farming, good judgment is called for on the part of the farmer.
Early Spring Plowing to Conserve Moisture.Plowing the ground very early in the spring is a rational practice, for there is no other season when tillage is so effective in conserving the moisture of the soil. King reports one experiment where early plowed ground, seven days after plowing, contained an amount