258 



SOILS AND SOIL WATER RELATIONS 



the plant. Since no water is added to the soil the plants eventually pass into 

 a state of permanent wilting due to the loss of water by transpiration. As soon 

 as this occurs a sample of the soil is removed and its water content is deter- 

 mined by the oven-drying method. 



Prior to extensive determinations of wilting percentages it was supposed 

 that plants differed very markedly in their capacity to reduce the water 

 content of a soil. It was assumed, for example, that species which could 

 endure drought conditions could deplete the moisture content of a soil to a 

 lower percentage before showing permanent wilting than could those species 

 which were soon injured or killed when subjected to drought. Extensive 

 investigations have shown, however, that h3'drophytes, mesophjtes, and xero- 

 phytes all reduce the water content of a given type of soil to about the same 

 value before the condition of permanent wilting is induced (Table 25). 



TABLE 25 RELATIVE WILTING PERCENTAGES FOR DIFFERENT SPECIES OF PLANTS (dATA OF 



BRIGGS AND SHANTZ, 19121^) 



While the wilting percentage for a given soil shows no appreciable varia- 

 tion when measured by means of different plants growing in it, the value varies 

 greatly with the type of soil. The percentage of water remaining in a soil 

 when permanent wilting of the plants growing in it occurs ranges from ap- 

 proximately 5 to 10 per cent for sandy loams, from about lO to 15 per cent 

 for silty loams, and from about 15 to 20 per cent in clay loams. 



According to Caldwell (1913) and Shive and Livingston (191 4) a greater 

 percentage of water is left in the soil at permanent wilting if the plants are 

 transpiring rapidly than if they are transpiring slowly. Veihmeyer and Hen- 

 drickson (1934), however, record that the wilting percentage of a given soil 



