218 BOTANICAL GAZETTE [September 



amounts upon vegetation, or of the amount and range of the soil 

 moisture in different plant associations. This has been largely due 

 to the difficulty in relating the amount pi moisture actually present 

 in the soil to the production of vegetation. It is clear that there 

 can be no direct relation between the percentage of water present in 

 soils and the amount available for plant growth, for a sandy soil 

 with 15 per cent of moisture is at or near saturation, while a stiff 

 clay with 15 per cent of water is so dry that all plants wilt in it, 

 even with a humid atmosphere. 



Efforts have been made to establish a standard by which the 

 actual water content of soils could be related to plant production. 

 Clements (14) determined the amount of water remaining in soils 

 when pronounced wilting occurred, and regarding this as non- 

 available termed it the echard, while the difference between the 

 amount actually present in the soil and the echard was the available 

 water, or chresard. Livingston (15) recognized that the water- 

 holding capacity of soils varied and had a fairly constant relation 

 to the soil moisture conditions. Then Briggs and McLane (16) 

 determined the moisture equivalent of soils by the application of a 

 centrifugal force of 1000 times that of gravity, providing a method 

 of measuring and comparing the retentiveness of different soils for 

 moisture acted upon by a definite force. This had the advantage of 

 being measured in absolute terms and of being reproducible within 

 narrow limits of error. It remained for Briggs and Shantz (17) 

 to refine the methods of determining the percentage of water in 

 soils when permanent wilting occurs in such a plant as the standard 

 Kubanka wheat, giving the wilting coefficient, and further to show 

 that a constant relation exists between the moisture equivalent 



. ., ... ~ . . . moisture equivalent ... 

 and the wilting coefficient; that is, ~ == wilting 



coefficient. 



1.84 



demonstrated 



from 



g coefficient 



reached. The writer, believing 



that none of the water absorbed 

 from soil whose moisture content is below the wilting: coefficient 



organism 



coefficient 



