i6 THE BIOLOGY OF FLOWERING PLANTS 



is proportional to the amount of fine particles with a diameter 

 of less than 0*5 mm. Crump (1913a) established a relation 

 between the amount of humus and the water content of 

 various English soils. He calculates a coefficient of humidity 

 by dividing the weight of water by the weight of humus. 

 Table IV gives some of his results. 



TABLE IV 



The approximate equality of the coefficients indicates that the 

 humus content largely controls the amount of water. The 

 coefficient for other types of soil, e.g. for those containing 

 clay, is different. 



Briggs and Shantz (1912a) have attempted to work 

 out a more exact relation between soil moisture and 

 all the constituents of the soil. They express the water- 

 retaining power as the moisture equivalent which is the 

 percentage of water retained against a centrifugal force of 

 1000 g. This quantity is more easily determined and is 

 less liable to experimental error than the amount of water 

 retained against gravity. The constitution of the soil is 

 expressed as the percentages of sand, silt, and clay, defined 

 as particles between 2 and 0*05 mm., 0*05 and 0*005 J^i^m 

 and less than 0*005 '^"^- i^ diameter, respectively. They 

 give the equation 



Moisture equivalent =o"02X sand +0*22 X silt +i'05 Xclay 



as relating the water-retaining power to the soil constitution. 

 The equation is empirical, and the factors would require 

 modification to suit different series of soils. It should be 

 noted that the large factor for the finest particles expresses 

 the controlling influence of these. 



