6 4 



RELATIONS OF SOIL TO WATER 



plants required slightly more water than the grasses and 

 cereals. 



TABLE VII 



RELATION OF PLANTS TO HYGROSCOPIC WATER (HEINRICH) 



From what has now been said it seems clear that if we 

 desire to know how much water available for plant use a soil 

 contains, we shall arrive at the result best by ascertaining 

 how much water the soil loses when exposed to ordinary air till 

 it ceases to lose weight. Water which can only be expelled 

 by heat is incapable of assimilation by ordinary crops. 



Although a soil containing only hygroscopic water cannot 

 support a crop, the possession of hygroscopic power is under 

 some conditions of distinct advantage to a soil. Hilgard has 

 observed that in the arid districts of California it is only 

 on soils capable of absorbing 4-8 per cent, of hygroscopic 

 water that crops can successfully resist drought. He thinks 

 that the surface soil is cooled during the extreme heat of the 

 day by the evaporation of a portion of the hygroscopic water, 

 this water being regained during the night either from the 

 atmosphere, or from the vapour rising from the subsoil. 



Water Capacity of Soils. A soil contains the largest pos- 



