CHAPTER XIII 



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, varying 

 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 tending 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. 



Transpiration 



Transpiration^ 

 <y 



oft 





mm 



\- > 



W^--^ 



' ' .-*." .-. ' i ''.'' ; 1 : '* Percolation ^ /: -f v ' 



. .. . .'. *-'J:.'\/ r J^,. <. ' ' V/ * 



-^U-^ ' ^^ ' ^ 



t==l-v- 



FIG. 42. Diagram illustrating the ways by which water may be lost 



from a soil. 



264 



