The Disk Harrow.—The purpose of the plow is to break up the soil so that it will be crumbly and mellow. The frequency with which land should be thoroughly stirred to full plow-depth depends upon the condition of the soil and the character of the crops. Oftentimes a disk or cutaway harrow may replace the plow. Its action is the same as that of the plow, loosening and turning the soil over. When land has had a good plowing within the year, and has not become compact, stirring to a depth of four inches may give a better seed-bed for some crops than could be made by use of a plow. This is true of land that has produced a cultivated crop and is being prepared for a fall-seeding. The gain in time of preparing ground for oats in the spring makes the use of the disk or cutaway harrow profitable on mellow corn-stubble land.
There is temptation to carry the substitution of the disk harrow for the breaking-plow too far. Its use alone would have the same effect as poor plowing, reducing the depth of the soil. The surface soil, down to plow-depth, is the chief feeding-ground for plants because it is kept in good tilth by organic matter and tillage. The depth of this soil affects the amount of available plant-food and water. The duration of time between deep plowings depends upon the soil and the crops. Experience shows that when land has been broken for corn or potatoes or beans or similar crop, the one plowing may be sufficient for a succeeding crop. If grass is not seeded with the succeeding crop, it is best to give another thorough plowing before seeding to grass in August if the soil is heavy, but in naturally loose soils a disk harrow makes a better seed-bed.
Two influences favor such undue dependence upon a disk harrow that a soil may become shallow: the cost of preparing the seed-bed is reduced, and the saving in moisture may give a better stand of plants when the harrow takes the place of the plow. The immediate productiveness of a crop is not an assurance that the method is right: consideration for the good of the land must be shown. Depth of soil is a requirement of a good agriculture, and deep plowing is a means to that end. The looseness of the soil and the character of the season may make substitution right in one instance and wrong in another. Deep soils, well filled with organic matter, will bear shallow preparation of a seed-bed more frequently than thin soils, and yet it is the latter that may profit most by having its best part kept near the surface at the time a new sod must be made. The disk harrow has some place as a substitute for a plow, but when its use results in making a soil more shallow, the harm is a most serious one.
Cultivation of Plants.—If a soil would remain mellow throughout the season, there usually would be no reason to disturb the roots of plants by any deep stirring, and all tillage would be only deep enough to make a mulch of earth for the retention of moisture and to destroy all weeds. Soils containing enough clay to make them retentive of moisture become too compact when rains beat upon the ground, as usually happens after the planting of spring crops. A deep and close cultivation of corn and potato plants after they appear in the row helps to restore the condition created by the plow and harrow, and often is the best practice. There is some sacrifice of roots, but the gain far exceeds the loss. It may be necessary to give a second such cultivation when a clay soil is deficient in organic matter, but the root-pruning is a handicap.
Controlling Root-growth.—The exception to the rule that plant-roots should not be pruned by deep cultivation is found in the case of a close soil in a wet season. The plants extend their roots only in the soil at the surface because the ground is soaked with water nearly all the time. They cannot form far enough below the surface to withstand a drouth that may follow the wet weather. Good tillage in such a case demands the pruning of the roots and the airing of the soil when the ground is dry enough to permit such stirring, and the plants then extend their roots in the lower soil where they rightly belong. Judgment is required to decide when such tillage is desirable, but judgment is needed all the time in farming. When a continued period of wet weather affects the position of the plant-roots, it rarely is advisable not to risk deeper tillage than is given in a normal season. Underdrainage helps to prevent such ill-effect of continued rains in the early part of a plant's life-time.
Elimination of Competition.—Weeds pump the water out of the soil, use up available plant-food, and compete for the sunlight. Tillage is given for several reasons, and one is the destruction of weeds. A weeder which stirs the soil only an inch or two deep is an excellent destroyer of weeds when they are starting, but after the weeds are well-rooted, the weeder acts only as a cultivator for the plants that should be destroyed. Modern cultivators have fine teeth that let the surface remain nearly level, and they do their best work when the weeds are small. The use of "sweeps" should be more general. The blades are so placed that they slip under the surface, letting the soil fall back so that a mulch is formed.
Length of Cultivation.—Most tilled crops grow rapidly until they shade and mulch the soil. Tillage should continue, if possible, until this occurs. The exception is in the case of orchard trees and other plants that should not have their period of growth extended late in the fall. Good tillage tends to increase the lateness of a crop by encouraging growth. The new wood of trees may not become hardy enough to withstand the frost of winter if tillage is continued. Early maturity is hastened by exhaustion of soil moisture and plant-food.
CHAPTER XXII
CONTROL OF SOIL MOISTURE