WATER OF SOIL IN ITS RELATION TO PLANTS 195 



less than the percentage representing the lento-capillarity. 

 Since the inner capillary water moves very sluggishly if at 

 all, wilting must occur before the plant has drawn to any great 

 extent on this part of the capillary moisture. The hygroscopic 

 water is, therefore, wholly unavailable to plants and generally 

 some of the capillary as well, although Alway 1 has shown that 

 under certain conditions the plant may reduce the moisture 

 down to the hygroscopic coefficient. The wilting coefficient ex- 

 pressed in soil-moisture terms may be located somewhere be- 

 tween the hygroscopic coefficient and the point of lento- 

 capillarity. 



104. The wilting coefficient and its determination. — It 

 has been known for many years that the common plants pos- 

 sess different capacities for resisting drought. This has usu- 

 ally been ascribed to one or more of three causes: (1) differ- 

 ences in root extension; (2) differences in ability to become 

 adjusted to a slow intake of water; and (3) differences in the 

 osmotic pull that plants exert on the soil-water. The last two 

 factors argue for different wilting coefficients for crops on the 

 same soil. 



The extended work of Briggs and Shantz, 2 however, indi- 

 cate that the permanent wilting point, expressed as a soil- 

 moisture percentage, is practically the same for all plants. 

 Later Caldwell 3 demonstrated that this relationship of the 

 physical constants of the soil to the wilting point depends on 

 the rate at which the plant loses water, showing that the soil 

 factors are not entirely dominant in this respect. 



The conclusions of Briggs and Shantz, nevertheless, seem 



1 Alway, F. J., Studies on the Relation of the Non-available Water of 

 the Soil to the Hygroscopic Coefficient; Nebr. Agr. Exp. Sta., Res. Bui. 3, 

 1913. 



2 Briggs, L. J., and Schantz, H. L. The Wilting Coefficient for Dif- 

 ferent Plants and its Indirect Determination, U. S. Dept. Agr., Bur. 

 Plant Ind., Bui. 230, 1912. 



3 Caldwell, J. S., The Relation of Environmental Conditions to the 

 Phenomena of Permanent Wilting in Plants; Physiological Researches, 

 Station N, Baltimore; U. S. Dept. Agr., Vol. 1, No. 1, July, 1913. 



