MOVEMENT OF SOIL WATER 165 



Evaporation takes place at the surface, and from 

 the plants growing in the soil. The rate of such loss 

 depends on the climatic conditions. In those regions 

 where the rainfall comes in frequent small showers, 

 which wet the soil to a depth of only a few inches, a 

 very large proportion of this water is immediately 

 returned to the surface by capillarity, and lost by evapo- 

 ration. On the other hand, if the rainfall occurs at long 

 intervals and in large amounts, so that it percolates 

 deeply into the subsoil, it may be held there by appro- 

 priate surface tillage. 



III. MOVEMENT OF SOIL WATER 



Soil moisture is subject to movement in three ways. 

 This movement may be injurious if it facilitates the 

 loss of moisture, which should be retained for the crop; 

 it may be beneficial when it serves to replenish the 

 moisture supply upon which the plant is dependent. 

 In the discussion of the moisture content and capacity 

 of soils, it was pointed out that no soil retains within 

 the surface four feet enough water to meet the needs 

 of a full-crop yield under average field conditions. 

 This indicates the necessity for the movement into the 

 root zone of moisture, to take the place of that removed 

 by the plant and lost in other ways. The movement 

 of moisture from adjacent supply in the soil, as the 

 deep subsoil is just as useful as the direct addition 

 of water to the soil by rainfall. The three types of 

 movement of soil moisture are (a) gravitational, (6) 

 capillary and (c) thermal. 



