82 J. W. SHIVE AND B. E. LIVINGSTON 



of wilting have little or no effect upon the residual water content 

 here considered, and they also conclude that this soil moisture 

 residue (which they term the "wilting coefficient"^) for any given 

 soil, remains constant, for all species of plants grown in it, and 

 for various stages of their development. Furthermore, Briggs 

 and Shantz have found certain definite relations to exist between 

 this so-called wilting coefficient, on the one hand, and (1) the 

 hygroscopic coefficient, (2) the moisture equivalent, (3) the 

 moisture holding capacity, and (4) the numerical data expressing 

 the mechanical analysis of the soil, on the other. They have 

 given mathematical expression to these relations, making it 

 apparently possible to calculate the soil moisture residue at 

 permanent wilting from any one of these physical constants of 

 the soil. Considering the numerous wilting tests carried out by 

 these authors, and the data collected by them from a large num- 

 ber of experiments extending over considerable time, there can 

 be no doubt that their conclusions are correct for the conditions 

 under which their experiments were carried out. But Briggs 

 and Shantz made no attempt at quantitative description of envir- 

 onmental factors other than those of the moisture residue in 

 question and the physical properties of the soil employed; they 

 describe the atmospheric conditions under which their experiments 

 were carried out only in the most general way, so that their re- 

 sults are not available for quantitative comparison with those 

 obtained by other authors under possibly different aerial con- 

 ditions. 



Brown's^ work on the conditions determining the beginning 

 of actual wilting (temporary wilting of Briggs and Shantz) 

 showed clearly enough that this phenomenon depends upon the 

 evaporating power of the air as well as upon the physical charac- 

 ters of the soil; with high evaporation rates the residual soil 

 moisture at the time of temporary wilting is much higher, for 

 any given soil, than is the case with low evaporation rates. 



' As Caldwell has well remarked (in his paper about to be cited), this term is 

 unfortunate in that it logically should apply to the plant, which wilts, and not 

 to the soil, which does not wilt. The term will be avoided here on this account. 



^ Brown, W. H., The relation of evaporation to the water content of the soil 

 at the time of wilting. Plant World 15: 121-34. 1912. 



