190 



THE PRINCIPLES OF SOIL MANAGEMENT 



Table XXXV. Loss of Water by Evaporation from Below 

 Columns of Different Air-Dried Soils 



It appears from these figures that the thermal move- 

 ment of water by simple diffusion is determined: (1) 

 By the size of the individual pores. (2) By the total 

 amount of pore space in the soil. (3) Upon the thickness 

 of the soil layer. When equally dry the fine-textured 

 soil retains moisture as vapor more effectively than does 

 coarse-textured soil. In so far as the structure of the 

 soil modifies either the size of the pores or their total 

 volume, it may modify the loss of water. A coarsely 

 cloddy mulch would therefore be ineffective. Particu- 

 larly striking is the small depth of soil which is effective 

 to prevent the loss of water. Even the one-inch mulch 

 has a wonderfully high efficiency. 



IV. CONTROL OF SOIL WATER 



In the control of soil moisture it is desired to accom- 

 plish one of two things: (a) The average water content 



