10 WILTING COEFFICIENT FOE DIFFERENT PLANTS. 



between the actual and the nonavailable water at any time. The 

 water content which is available for growth is represented by the 

 difference between the actual moisture content and the wilting 

 coefficient. 



WAX-SEAL METHOD FOR DETERMINING THE WILTING 

 COEFFICIENT. 



The method which we have employed for determining the wilting 

 coefficient consists essentially (1) in the use of an impervious pot, 

 (2) in sealing over the soil surface in the pot with wax so as to 

 prevent all evaporation from the soil, and (3) in keeping the soil 

 mass at an approximately constant temperature. Under these 

 conditions the percentage of water remaining in the soil at the time 

 the plants wilt is termed the wilting coefficient. 



ESSENTIALS IN DETERMINING THE WILTING COEFFICIENT. 



In determining the wilting coefficient of a soil we have found that 

 the following precautions should be observed: 



(1) The soil mass should be as uniform as possible, since the 

 wilting coefficient varies with the texture of the soil and is con- 

 sequently affected by stratification or other nonuniformity of the 

 soil mass. 



(2) The soil should be brought to a uniform moisture content 

 before being placed in the pots. Otherwise, small volumes of soil 

 may remain dry and thus introduce an error in the final moisture 

 determinations. 



(3) All loss of water from the soil should be prevented except that 

 resulting from the transpiration of the plant. Otherwise, the surface 

 soil may dry out below the wilting coefficient before the inner soil 

 mass has reached this limit. 



(4) All sudden fluctuations in temperature should be avoided. 

 Otherwise, condensation will occur on the inner walls of the pot 

 as the result of distillation from the soil, due to the difference of 

 temperature. This condensed water will be absorbed by the roots 

 in contact with the inner walls of the pot and the moisture content of 

 the principal soil mass may thus be reduced below the wilting 

 coefficient. 



DESCRIPTION OF THE METHOD. 



In working out a practical method embodying the above require- 

 ments, which have not been fully complied with in the methods 

 heretofore described, we have adopted the following procedure : 



(1) The air-dried soil is sifted through a 2-millimeter screen to 

 remove gravel and to insure greater uniformity. The soil, after 



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