SOIL WATER 85 



grow in well-water or water from a tile drain, if it is 

 renewed often enough. Such water is free soil water. 



87. Movement of Water in Soil. The chief ways in 

 which water exists in the soil are as film water and as 

 free water. The particles can hold a certain amount of 

 water on their surfaces, just as one's hands remain wet 

 when removed from water. Only a limited amount can 

 be held in this way. If too much water is present, it will 

 drop off. If more water is present in the soil than can be 

 held as film moisture, it will fill the pore spaces between 

 the particles. If there is an outlet, the free water will 

 drain away and leave the film or capillary water. 



88. Conservation of Moisture. The free water moves 

 downward by gravity. The capillary water can move in 

 any direction, because the force of adhesion between 

 the soil particles and the water is strong enough to lift 

 water, just as oil is lifted in a lamp- wick. After a heavy 

 rain the soil may be filled with water. Gradually the 

 free water drains away and leaves capillary water only. 

 The surface soil loses some of its water by evaporation. 

 This leaves it drier than the soil below. Some of the water 

 of the lower layer of soil is then drawn up by capillarity 

 to take its place, just as more oil is drawn up in the lamp- 

 wick when that at the end of the wick is removed by burn- 

 ing. In this way the water may be removed from the soil 

 very rapidly, particularly when the weather is dry, warm 

 and windy. 1 



If there is not an abundance of rainfall, it is desirable 

 to stop this movement of water to the surface to be evap- 



1 Water also evaporates within the soil into the soil air. There is a 

 constant movement of this air in and out of the soil, so that this aids in 

 drying a soil. 



