RELATION OF THE SOIL TO WATER 29 



movement of the water begins. The extent to which 

 losses by percolation may occur depends upon the 

 character of the soil and the amount of rainfall. 

 When soils are covered with vegetation the losses by 

 percolation are less than from barren fields. In all 

 soils which have only a limited number of capillary 

 spaces and a large number of non-capillary spaces, the 

 amount of water which can be held above the bottom 

 water is small. From such soils the losses by perco- 

 lation are greater than from soils which have a larger 

 number of capillary spaces, and a smaller number of 

 non-capillary spaces. In coarse sandy soils many of 

 the spaces are too large to be capillary. 



If all of the water which falls on some soils could be 

 retained and not carried beyond the reach of crops by 

 percolation, there would be an ample supply for agri- 

 cultural purposes. To prevent losses by percolation, 

 the texture of the soil may be changed by cultivation 

 and by the use of manures. If the soil is of very fine 

 texture, as a heavy clay, percolation is slow, and before 

 the water has time to sink into the soil, evaporation' 

 begins; with good cultivation the water is able to 

 penetrate to a depth beyond the immediate influence of 

 evaporation. Compacting an open porous soil by 

 rolling, checks rapid percolation and prevents the 

 water from being carried beyond the reach of plant 

 roots. In order to prevent excessive losses by perco- 

 lation, the treatment must be varied to suit the re- 

 quirements of different soils. 



