56 WILTING COEFFICIENT FOE DIFFEEENT PLANTS. 



The highest determination recorded probably accompanies a root 

 distribution such that the water must move through relatively long 

 distances in the soil by capillary action. The lowest determination 

 will be reached only when the root distribution is perfect. It is 

 obvious that in all measurements of the wilting coefficient variations 

 may be found anywhere between these two conditions as limits. 



The tables show that plants having finely divided roots give uni- 

 formly a lower determination than those with larger roots, irrespec- 

 tive of whether they grow normally in arid or humid regions (Table 

 XII). An effort has been made to secure a condition of uniformity 

 in the root distribution by using a small pot and by keeping the roots 

 well within the soil mass as a result of the equalized temperature; but 

 with some plants, notably the dasheen (Colocasia sp.), as well as some 

 of the larger rooted western plants which have given a rather imper- 

 fect root distribution, a wilting-coefficient determination somewhat 

 higher than normal has been obtained. 



As already stated, it is believed that the differences observed in 

 the wilting coefficient for different plants are largely due, not to the 

 ability of one variety to exert a greater pull on the soil moisture than 

 another variety, but to the more perfect root distribution of one vari- 

 ety as compared with another. 



INDIRECT METHODS OF DETERMINING THE WILTING COEF- 

 FICIENT. 1 



In field studies of soil moisture, determinations of the total water 

 content can easily be made. The errors which enter into the deter- 

 mination of the wilting coefficient under field conditions are relatively 

 great, owing to the direct evaporation from the soil, local variation in 

 soil texture, and lack of uniformity in the root distribution, combined 

 with the difficulty of determining the exact wilting point when the roots 

 occupy a large soil mass. Furthermore, it is only during periods of 

 extreme drought that conditions are favorable for wilting-coefficient 

 determinations in the field. In view of these difficulties it becomes 

 important to ascertain whether the soil-moisture content at the wilting 

 point can be determined by an indirect method, based upon the rela- 

 tionship of the wilting coefficient to the moisture retentiveness of the 

 soil as measured by physical methods. 



EELATION OF THE WILTING COEFFICIENT TO THE MOISTURE EQUIV- 

 ALENT. 



The moisture equivalent of the soil is the percentage of water 

 which it can retain in opposition to a centrifugal force 1,000 times 

 that of gravity. 2 In making the determinations the soils are placed 



i Kubahka wheat, G. 1. 1440, was used as the Indicator plant. 



2 Briggs, L. J., and McLane, J. W. The Moisture Equivalents of Soils. Bulletin 45, Bureau of Soils, 

 U. S. Dept. of Agriculture, 1907. 

 230 



