170 THE STUDY OF PLANT COMMUNITIES * Chapter VII 



conditions. This conclusion was not immediately acceptable to 

 everyone, and numerous studies were made to check its validity 

 with different kinds of plants of different ages under a variety of 

 conditions. It is now generally agreed that permanent wilting of 

 any species occurs at the same water content of a soil of a certain 

 texture regardless of the age of the plant or environmental condi- 

 tions under which it grew. Uniformity of results is assured if non- 

 cutinized herbaceous plants are used and if permanent wilting of 

 the lowest pair of leaves is used as an end point. This eliminates the 

 problem of recognizing the onset of permanent wilting and varia- 

 tions related to the ability of some plants to live much longer than 

 others after the onset of wilting. 



Briggs and Shantz also concluded that it was possible to calcu- 

 late the wilting point from the moisture equivalent because the 

 following relationship held in their soils : 



... ~ . moisture equivalent 



wilting coefficient == 3 



5 1.84 ±0.013 



Although this often holds true, it does not apply to all soils. Studies 

 in different parts of the country indicate that the ratio ranges at 

 least from 1.4 to 5.65. Attempts to relate the moisture content at 

 the time of wilting to other variables have been equally unsatis- 

 factory, and it, therefore, appears that its determination is most 

 reliable when observed directly. Because the expression, "wilting 

 coefficient" has been so often associated with calculated values, it 

 is logical to restrict it to that usage and to apply the term, "per- 

 manent wilting percentage'/ to determinations made by direct ob- 

 servation. 



Readily available water is that which can be used by plants for 

 growth and is, therefore, the moisture above the permanent wilt- 

 ing percentage. This includes gravitational water, but its rapid 

 drainage makes it of little consequence. The remaining usable 

 water is in the range from field capacity or moisture equivalent 

 down to the permanent wilting percentage. This range is narrow 

 in sand and wide in clay. Obviously, the wider the range, the 

 longer plants can resist drought and, in cultivation, the less fre- 

 quently irrigation is necessary. The rate at which water moves 

 from soil to an absorbing surface is strongly indicative of plant- 

 soil moisture relationships at the time. An indication of the avail- 



