SOIL WATER 67 



between gravitational water and capillary water, and while 

 it is desirable to have as much capillary water as possible 

 in the soil at all times, it is equally important that the gravi- 

 tational water shall be removed. 



The factors that determine the rate of flow of gravitational 

 water in soil are texture, structure, and cracks and openings 

 produced by freezing, by drying, by roots and by the bur- 

 rowing of sundry forms of animal life, like worms and 

 insects. Another, and very important factor, is the means 

 for the escape of water from the subsoil, since without that a 

 soil will become saturated no matter how favorable the 

 conditions may be for escape of water from the surface 

 soil. For this purpose tile drainage must often be used. 



A sandy soil allows the escape of gravitational water more 

 rapidly than does a loam or clay soil. Soil in good tilth is 

 better in this respect than is compact soil. It is better 

 that water should run through a soil than that it should run 

 off the surface. The latter generally causes erosion with the 

 loss of much good soil, and may leave the subsoil too dry. 

 For this reason a loam or clay soil should always have a 

 loose surface when no crop is on the ground. 



72. The water table. — The gravitational water that 

 passes through the ground accumulates, in humid regions, 

 in the lower depths of soil, or possibly in underlying sand 

 or gravel, which it saturates. The surface of this mass of 

 water is called the water table, the depth of which below 

 the surface of the ground varies from a few inches to a great 

 many feet, depending on the opportunity it has to escape. 

 This is the water that furnishes the supply for shallow wells 

 and for springs. In some places the water table is sufficiently 

 near the surface to be of use to plants owing to its capillary 

 rise during dry periods. 



73. Relations of soil water to plants. — The quantities 

 and movements of the several forms of water in soils are of 



