THE: SOIL AND WATKR 137 



depends also on the nature of the soil, the temperature, and the 

 humidity of the air. 



Briggs 1 and associates have elaborated methods for the deter- 

 mination of the wilting point of plants, and traced the relation 

 between the moisture content of the soils at the time of wilting, 

 and the moisture equivalents of the soils, their hygroscopic power 

 and their capacity to hold moisture. The moisture equivalent is 

 the percentage of moisture the soil will retain in opposition to a 

 centrifugal force 1,000 times the force of gravity. The relations 

 between these factors he expresses as follows : 



moisture equivalent 

 Wilting coefficient - 



i .84 



hygroscopic co-efficient 



0.68 



moisture-holding capacity 21 

 2.90 



The water which cannot be withdrawn from soils by a plant 

 may be termed unavailable water, and that in excess of this, 

 available water. The California Experiment Station holds the 

 available water to be practically the hygroscopic water, that is, 

 the moisture absorbed from the soil by a damp atmosphere. 



Nature of the Plant. Plants vary in their power of absorbing 

 water from soils. That is to say, some plants will reduce the 

 water in the same soil to a lower percentage than others, before 

 wilting. This may be due in part to difference in the ratios be- 

 tween root area and surface growth, enabling a lower rate of 

 water to supply the requirements of the plant ; or to greater root 

 attraction for water by some plants, causing a larger flow of water 

 into the one plant than in the other, under the same conditions of 

 soil moisture; or to differences in the amount of water transpired. 



During a severe drouth, the California Experiment Station 2 

 determined the amount of water in a large number of soils, where 

 plants were doing well, or were suffering. California soils, it 

 must be recalled, are different from soils of humid climates, there 



1 Proc. Am. Soc. Agr., 1910, p. 138; 1911, p. 250; Bui. 230, Bureau 

 of Plant Industry. 



- Report 1897, p. 95. 

 10 



