198 NATURE AND PROPERTIES OF SOILS 



of at least approximating the wilting point from other soil 

 factors. These formulas 1 are arranged in the order of their re- 

 liability, based on the data obtained by the authors : 



„ ™-., . /v. • Moisture equivalent 



1. Wilting coefficient = =-«j 



o wn- & • * Hygroscopic coefficient 



2. Wilting coefficient = — — £ — ^ 



Water-holding capacity — 21 

 ( Hil gar d Method) 



3. Wilting coefficient 



2.9 



While such formulae are only approximate in their applica- 

 tion, they are valuable for rough calculations. They also show 

 in a general way the correlations between the various moisture 

 conditions established by experimental methods. 



105. The availability of the soil-water. — From the dis- 

 cussions already presented regarding the forms of water in 

 the soil, the ways in which they are held, and their movements, 

 it is evident that all moisture present in a soil is not available 

 for plant growth. Three divisions of the soil-water may be 

 made on this basis: unavailable, available, and superfluous. 



It is obvious that all of the moisture below the wilting point 

 is out of reach of the plant and may be classified as unavail- 

 able. It includes all of the hygroscopic and that part of the 

 capillary which is tightly held, the so-called inner capillary 

 water. The amount of the capillary moisture unavailable to 

 plants is much greater with clayey than with sandy soils. For 

 example, a sand with a hygroscopic coefficient of 1.5 per cent. 



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

 Plant Ind., Bui. 230, pp. 56-77, 1912. 



See also, Loughridge, K. H., Investigations in Soil Physics; Calif. Agr. 

 Exp. Sta., Rep. 1892-3-4, pp. 70-100, 1894. Alway, F. J., and Clarke, 

 V. L., Use of Two Indirect Methods for the Determination of the 

 Hygroscopic Coefficient of Soils; Jour. Agr. Ees., Vol. VII, No. 8, pp. 

 345-359, Nov., 1916. 



1 Note that the wilting coefficient, moisture equivalent, water-holding 

 capacity and hygroscopic coefficient are expressed in percentage of 

 water based on dry soil. 



