vvJjLi\.JL X Jl/Xt J\.Xxx 



THE CONTROL OF SOIL MOISTURE 



In the discussion of the water requirements of plants, 

 it was apparent that for a normal yield of any crop the 

 amount used by the plant alone was very great, var;^ing 

 from five to ten acre inches according to conditions. Were 

 this the only loss of water, the question of raising crops 

 with given amounts of rainfall would be a simple one. 

 Three further sources of water loss, however, are usually 

 found functioning in the soil and tendmg to lower the 

 water that would go toward transpiration, a loss absolutely 

 necessary for proper plant growth. The various ways by 

 which water finds an exit from a soil are (1) transpiration, 

 (2) run-off over the surface, (3) percolation, and (4) evap- 

 oration. The following diagram makes clear their re- 

 lationships 



Transpiratm 



Impomtion 



\r/(f off^ 





I J i / Percalatm ^ / 1" 



f 



^hp&^fHmhf 



Fig 42 —Diagram illustrating the ways by which water may be lobt 



from a soil. 

 264 



