1 1 2 TILL A GEMO VEMENTS OF SOIL WA TER [chap. 



rainfall percolating through and evaporating from 

 the surface of either gauge are identical, except for 

 certain minor differences explicable on other grounds. 

 Thus the water remaining in the deeper layers of the 

 6-foot gauge, even when saturated after a rainfall, has 

 never been lifted to the surface and evaporated. Again, 

 there would generally seem to be water enough in the 

 layer of soil and subsoil traversed by the roots of the 

 plant to supply the crop with the water it actually uses. 

 The fact that has to be explained is the capacity of a 

 soil to maintain the crop during drought, even to carry 

 it through all its stages of growth without any rainfall, 

 as in certain regions of North America and Australia. 

 On reference to Alway's figures (p. 97) it will be seen 

 that the fallowed land down to a depth of 6 feet had 

 an average of 12-9 per cent of "free water." Assuming 

 the soil to weigh about three million pounds per acre 

 foot, there would have been approximately 1000 tons 

 of free water down to the 6-foot depth, equivalent to 

 10 inches of rainfall. This of itself would be sufficient 

 for far more than the average growth made under these 

 conditions. Further, Alway has shown that after a 

 crop, however thoroughly the soil may have been dried 

 out down to the depth (4 to 6 feet) to which the roots 

 of the crop penetrate, yet the layer below will still be 

 moist and possess the same proportion of water which 

 the similar layer contains below uncropped land along- 

 side. Leather has adduced similar evidence from India, 

 and these considerations lead one to conclude that in 

 the main the crop draws for its water supply only upon 

 the zone penetrated by the roots of the plant, and that 

 the movements of water from still lower zones are so 

 small as to be a negligible factor in crop production. It 

 cannot, however, be said that the evidence for this view is 

 complete, and the question merits further investigation. 



