176 The Plant World. 



four. It measures a real soil property with sufficient accuracy 

 and sufficient ease and the empirical character of the correlations 

 between it and field properties is a fault which seems more 

 serious than it is. 



But the heat of wetting is by no means perfect. It is 

 perhaps the best we have, but it does not follow that it is the 

 bett we can get, and further search is well worth the effort. I am 

 convinced that the most promising line for such search to fol- 

 low is in the devising of a new and practicable method for the 

 determination of the critical motsiure content and there is at 

 least one indication of what such a method might be. In the 

 discussion of the water equivalent it was pointed out that, as 

 the centrifugal force is increased, the percentage of water 

 retained falls toward a limiting or asymptotic value which varies 

 with the physical character of the soil. Now this limiting retenti- 

 vity, or asymptotic water equivalent, is very probably a real 

 proj^erty of the soil and just as much a soil constant as is the 

 critical moisture content. Furthermore, in the few cases so 

 far studied, the critical moisture content and this limiting water 

 equivalent seem to be not only related but identical. So 

 httle experimental work has been done that it is not yet possi- 

 ble to say definitely that this so, but if it is, the whole situation 

 is simplified. We need but to determine the water equivalent 

 with a centrifugal force high enough to attain the asymptotic 

 extraction in all types of soil and — -presto — we have attained 

 the critical moisture content. The problem would be solved 

 and there would be to hand a more capable tool than any which 

 soil investigation has yet possessed. 



The visi(-n is entrancing but as yet it is only a vision, and 

 it behooves us to set rapidly to work uj)on such experimenta- 

 tion as will prove this vision for mirage or for truth. We need 

 more studies of the critical moisture content, of the water equiv- 

 alent, and, especially of the relations of the two. Aside from 

 its organic relations this is (to my mind) the most vital and 

 the most urgent field of sail physics today. The most impor- 

 tant thing about the physical soil constant is the search for it. 



Bureau of Soils, 

 U. S. Department of Agriculture, 



Washington, D. C. 



