226 SOIL CONDITIONS AND PLANT GROWTH 



water was '' unfree" in the sense that it took no part in the 

 process. Keen has shown {i^6c) that Bouyoucos' figures are 

 capable of quantitative interpretation, the amount of "unfree 

 water " at any total moisture content being a function of the 

 amounts of " free " and total water. 



These various divisions do not correspond with one another. 

 Briggs and Shantz (55^:) have worked out the relationships 

 between some of them. They find that the hygroscopic co- 

 efficient is on the average about 0*68 times the wilting 

 coefficient. Alway, on the other hand (3(^), finds that the 

 wilting coefficient and the hygroscopic coefficient are ap- 

 proximately equal. 



Further, Briggs and Shantz (55^) have shown that the 

 following relationship exists between the hygroscopic co- 

 efficient and the composition of the soil : — 



Hygroscopic coefficient = 0*007 sand + 0*082 silt + 0-39 

 clay, where sand, silt, and clay are expressed in American 

 units. 



Alway (3(3:) has modified the formula to make it fit the 

 semi-arid or transition soils of Nebraska : — 



Hygroscopic coefficient = 0*005 coarser fractions + 0*07 

 very fine sand + 0*082 silt + 0*39 clay. 



Briggs' " moisture equivalent " is rather different and is 

 about 1*8 times the hygroscopic coefficient. Briggs and 

 Shantz in their paper show its relationship to the other 

 quantities. 



General agricultural experience shows that the excess 

 water of the plant physiologist is approximately the same as 

 the gravitational water of the physicist. 



The fact that these divisions do not exactly correspond is 

 a strong argument against the reality of their existence. It 

 is inconceivable that water could exist in a dozen different 

 states in the soil. Briggs and Shull both regarded the condition 

 of the water as continuous, and Keen has afforded rigid proof 

 that this is the case. In the course of its range frorh o to 20 

 or 40 f>er cent, by weight of the soil the water is held by forces 

 varying in magnitude from 1000 atmospheres or more down 



