A need of drainage is indicated by the curling and wilting of the leaves of corn and other crops during dry, hot weather. This curling and wilting is due to the fact that during the early growth of the crop free water stands so high in the soil that the crop roots are confined to a shallow layer of soil. When dry, hot weather comes, the free water recedes, the upper soil dries out, and the roots cannot get sufficient water to supply the demands of transpiration, hence the curling and wilting of the leaves. If drains are placed in this soil, the free water will be kept at a lower level in the spring and the plant roots will develop deeper in the soil, where there will be constant supply of film water during the dryer and warmer summer weather. The wiry and spindling growth of grass and grain crops may indicate too much water. The growth of moss on the surface of the ground and the cracking of the soil in dry weather are also indications of too much water. DRAINS How can we get rid of this surplus free water? We can make passageways through the soil to a lower level and then let gravity pull the water through them to lower ground below. These passageways are called drains. Drains may be classed as: Surface drains which are shallow, open channels made in the soil with a plow, hoe or other tool, to carry off surface water. They are temporary and need frequent renewing. Open-ditch drains are deeper, more permanent water passageways around or across the fields. Surface and open-ditch drains take only surface water. They also carry off surface soil and manures washed into them. They frequently become choked or stopped by trash and soil, and are in the way of cultivation and harvesting operations. Covered drains, under drains or blind ditches are water passageways made of brush, poles, stones, tiles, etc. (Figs. 80-81), placed in the bottoms of ditches and then covered with soil. INFLUENCE OF COVERED OR UNDER DRAINS ON FERTILITY _Influence on soil water._ Covered or under drains take not only surface water, but also remove free water from the soil beneath down to nearly the level of the bottom of the drains, and thus increase the area of film water. Removing the free water enables the soil to absorb more readily rain water falling on the surface and therefore checks surface wash and the gullying of fields. _Influence on soil ventilation._ Lowering the free water allows a deeper penetration of air and, therefore, a deeper root development and enables crops to better resist dry periods. _Influence on soil temperature._ Lowering the free water in the soil influences soil temperature: By diminishing the amount of water to be heated. By checking evaporation. By letting warm showers sink down into the soil. By increasing ventilation and therefore permitting the circulation of warm air in the soil. The cropping season is lengthened by causing the soil to be warmer and drier earlier in the spring and later in the fall.