8 WILTING COEFFICIENT FOE DIFFERENT PLANTS. 



arid and arid regions, to determine the variation exhibited in their 

 ability to reduce the moisture content of the soil before permanent 

 wilting takes place. The results of these investigations have led us 

 to conclude that the variation exhibited by different plants is much 

 less than has heretofore been supposed and that it is insignificant 

 compared with the range in moisture retentiveness exhibited by 

 different soils. 



As the work developed it became evident that many precautions 

 which had not heretofore been observed were necessary in determin- 

 ing accurately the moisture content of the soil corresponding to the 

 wilting point of plants. This led to the development of the wax-seal 

 and balancing methods. 



In connection with the study of the moisture requirements of 

 plants in semiarid regions it is necessary to be able to determine 

 quickly the soil-moisture content at the wilting point. This consti- 

 tutes the datum from which the moisture available for growth can be 

 calculated and "without which field determinations of soil moisture 

 are of little value. Owing to the difficulty of making direct determi- 

 nations in the field of the moisture content corresponding to the 

 wilting point, we have compared the wilting-point determination 

 with the moisture retentiveness of the soil as measured by physical 

 methods, which has resulted in the development of several indirect 

 methods applicable to field conditions. These methods are described 

 in the last part of this paper. 



WILTING COEFFICIENT. 



If the roots of a plant are well established in a mass of soil the plant 

 gradually reduces the water content until permanent wilting occurs. 

 The water remaining in the soil under this condition has been termed 

 "nonavailable" by previous writers. We have found, however, that 

 plants can reduce the soil-moisture content below the point corre- 

 sponding to the permanent wilting of the leaves, so that at the 

 wilting point a part of the soil moisture must still be available. In 

 fact, as we shall show later, this loss of water from the soil to the air 

 goes on through the plant tissues even after the death of the plant, 

 and appears to be limited only by the establishment of a state of 

 equilibrium between the soil and the air. The plant during the 

 drying stage acts simply as a medium for the transference of water, 

 and, while the rate of loss is reduced, the final result is the same as if 

 the air and soil were in direct contact. 



The nonavailable water in a soil is then, strictly speaking, the soil- 

 moisture content when in equilibrium with the moisture of the air. 

 Such a condition of equilibrium can at best be only approximate, 

 since the saturation deficit of the air is constantly changing with the 



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