loo TILLAGE— MOVEMENTS OF SOIL WATER [chap. 



through the very fine pore spaces that the upward lift 

 cannot keep pace with the loss by transpiration and 

 evaporation. The soils which are least affected by 

 drought are the deep loamy sands of very uniform tex- 

 ture, fine-grained enough to possess a considerable lift- 

 ing surface, and yet not too fine to interfere with the 

 free movement of soil water. The western soils which 

 the American writers describe as capable of growing 

 wheat with a winter rainfall of 10 to 12 inches and an un- 

 broken summer drought of three months' duration, are 

 deep, fine-grained, and uniform, with practically no 

 particles of the clay order of magnitude to check the 

 upward lift by capillarity. 



The following table illustrates how the subsoil acts 

 as a regulator to the amount of water contained in the 

 surface layer, absorbing the water which descends by 

 percolation during rainy periods, and giving it up again 

 by capillarity to the surface soil during periods of 

 drought The first line shows the rainfall during the 

 periods indicated, the second line the amount of 

 evaporation during the same period, while the third line 

 shows the changes in the water content of the top foot 

 of soil. As this change is not represented by the 

 difference between the rainfall and the evaporation, it 

 is clear that water must have been in some cases passed 

 down to the subsoil, in others lifted from it, in quantities 

 shown by the last set of figures. 



