SOIL WATER 



133 



the water content of the soil is increased 25 per cent above the optimum 

 percentage, plants begin to suffer as a result of too much moisture, and 

 when the moisture falls 25 per cent below the optimum, they suffer from 

 drought. 



The amount of capillary water in the soil is determined chiefly by 

 its texture. The following table shows the percentage of water held by 

 soils ranging in texture from coarse sand to clay, when subjected to a 



Effect op Little, Medium, and Much Water on Wheat. 1 



centrifugal force 2940 times that of gravity. A coarse sand held only 

 4.6 per cent of moisture, while clay held 46.5 per cent or ten times as 

 much. The water held under natural conditions by the several classes 

 of soil given in the table would be much larger, but the relative amounts 

 would be the same. 



Capillary water is also influenced to some extent by the structure of 

 the soil, and to somewhat greater extent by its content of humus or 



1 Courtesy of The Macmillan Company, N. Y. From " Principles of Irrigation Practice," by Widtsoe. 



