B. A. Keen 



401 



The present writer has shown^ that the course of the evaporation 

 of water from the soil can only be explained on the assumption 

 that an intimate connection exists between the soil and its moisture 

 content over a wide range. The evaporation was quite different from that 

 shown by moist sand. The latter could be readily explained from known 

 laws of diffusion, but in the evaporation from soil other factors were 

 distinguished. A definite mathematical expression was found for this 



24 



20 



16 



12 



-1 -2 -3 -4 2 4 6 8 



Freeziug-point depression in ° C. Time in liours 



Fig. 1. Comparison for sand and soil at varying moisture contents, 

 of freezing-point depression and evaporation. 



relationship, and all the available data pointed to the importance of the 

 soil colloids in controlling it. These differences in the relations of sand 

 and soil to their water content can be very well seen by comparing the 

 writer's curves showing the evaporation from sand and soil, with 

 Bouyoucos' curves for the depression of the freezing-point at various 

 moisture contents (Fig. 1). It is significant that two such diverse methods 

 of examining the soil solution should both show a fundamental difference 

 in its relation to a group of inert particles such as quartz sand on the 

 one hand, and to soil on the other. 



Bearing these facts in mind one can readily understand the incom- 

 plete and sometimes conflicting results obtained from all methods of 

 ' B. A. Keen. Journ. Agric. Sci. 6 (1914), p. 456. 



