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