WATER-RELATION BETWEEN PLANT AND SOIL. 7 



fair to become of considerable use in soil studies. This is a moisture 

 content at which penetrability is least, and its value is nearly the same 

 as that of Briggs's and McLane's moisture equivalent. There are at 

 present, however, rather grave difficulties in its direct experimental 

 determination. 



Another newly studied physical soil characteristic which, under 

 certain conditions, may have great value in connection with ecological 

 (including agricultural) inquiries is brought forward by the work of 

 Briggs and Shantz.^ This is the "wilting coefficient" or the moisture 

 residuum left in the soil when wilting of the plant has so far progressed 

 that recovery can not be attained in a water-saturated atmosphere, 

 without increase in the soil-moisture content. Under some conditions 

 it appears that this is truly a soil constant, uninfluenced by the plant 

 or the aerial conditions, but the w^ork of CaldwelP and that of Shive 

 and Livingston^ bring out the apparent fact that, for most atmospheric 

 conditional complexes encountered in nature, the aerial conditions are 

 decidedly not without influence upon the soil-moisture residuum here 

 dealt with. It appears, in any event, as though this character might be 

 very useful if its value be accompanied by atmometric data. The coeffi- 

 cients obtained by Briggs and Shantz are shown by them to be mathe- 

 matically deducible from other soil constants, at least under certain 

 conditions. For the present this whole matter must be left in abeyance 

 until investigation may be carried further. In the meantime, it is 

 worthy of special note that Briggs and Shantz have made the first 

 logical attempt to express as a single magnitude the several variables 

 which are determined in the mechanical analysis of soils. This appears 

 to be a definite advance in soil physics. 



Still another newly advocated function of certain physical properties 

 of the soil has been shown to be of value in considerations of the water- 

 relations between some soil types and the plants naturally occurring 

 thereon. This is Crump's'* "coefficient of soil humidity," which is 

 the ratio of the water content to the humus content, both expressed as 

 percentage of the dry weight of the soil. This constant is apparently 

 of value with soils of considerable humus content, but is only a directly 

 empirical one at best, and can not be expected to prove generally 

 useful, as is indeed pointed out by its author. 



'Briggs, L. J., and H. L. Shantz, A wax seal method for determining the lower limit of available 

 soil moisture. Bot. Gaz. 51: 210-19. 1911. 



, Apphcation of wilting coefficient determinations in agronomic investigations. Proc. 



Amer. Soc. Agron. 3: 250-60. 1912. 



, The wilting coefficient and its indirect determination. Bot. Gaz. 53: 20-37. 1912. 



, The relative wilting coefficients for different plants. Bot. Gaz. 53: 229-35. 1912. 



, The wilting coefficient for different plants and its indirect determination. U. S. Dept. 



Agric, Bur. Plant Ind. Bull. 230. 1912. 



, Die relativen Welkungskoefizienten verschiedener Pflanzen. Flora 105: 224-40. 1913. 



^Caldwell, J. S., The relation of environmental conditions to the phenomenon of permanent wilt- 

 ing in plants. Physiol. Res. 1 : 1-56. 1913. 

 ^Shive, J. W, and B. E. Livingston, The relation of atmospheric evaporating power to soil- 

 moisture content at permanent wilting in plants. Plant World 17: 81-121. 1914. 

 ^Crump, W. B., Notes on water content and the wilting point. Jour. Ecol. 1 : 96-100. 1913. 



