REVIEWS 



The Moisture Withholding Power oe Soils 



Soil moisture is one of the dominant factors governing all vegeta- 

 tion, and its importance lies not in the total amount of water which the 

 soil contains, but in the amount which the plant can draw out of the 

 soil. A heavy soil containing 20 per cent of moisture may afford the 

 plant less water than a sandy soil containing only 5 per cent. All 

 soils withhold certain amounts of water from the plant because the 

 soil particles, after the water has been reduced to a certain point, tend 

 to exert a force or "back-pull" against further withdrawals of water. 

 This force is the key to the moisture relations between the plant and 

 the soil. Therefore ShuU's^ recent investigations dealing with this 

 force are of the utmost value to all whose work depends on a knowl- 

 edge of the moisture relations of the soil. 



Most of the researches in soil moisture have until recently been 

 directed toward the fundamental relationships between the soil and 

 its water, independently of plant growth. This is of course the logical 

 procedure in getting a basis for further work. Naturally, there- 

 fore, most of the contributions to our knowledge of soil moisture have 

 come from the fields of physics and chemistry.^ 



It was not until after the physicists and chemists had begun to 

 discover some of the fundamental properties of soils that others 

 could begin to unravel the relationships between the soil and plant 

 growth. The physical and chemical properties of soils are by no means 

 known yet; in fact, only the first steps have been taken, but enough 

 is known to permit the botanist to begin his work. In this connec- 

 tion, it is interesting that the first great contribution to our knowl- 

 edge of soil moisture in relation to vegetation, the wilting coefficient,^ 

 came from the researches of a physicist and a plant physiologist work- 

 ing together. 



1 ShuU, Charles A. "Measurement of the Surface Forces in Soils." Botanical 

 Gazette, 52: 1-31, 1916. 



2 For example : 



Briggs, L. T. "The Mechanics of Soil Moisture." U. S. Dept. Agr., Div. of 

 Soils, Bui. 10, 1907. 



Briggs, L. J., and McLane, J. W. "The Moisture Equivalent of Soils." U. S. 

 Dept. Agr., Bureau of Soils, Bui. 45, 1907. 



Free, E. E. "Studies in Soil Physics." Plant World, 14: 29-39; 59-66; 

 110-119; 164-176; 186-190; 1911. 



3 Briggs, L. J., and Shantz, H. L. " The Wilting Coefficient for Different Plants 

 and its Indirect Determination." U. S. Dept. Agr., Bureau of Plant Industry, 

 Bui. 230, 1912. 



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