Apparently High Service of Water 7 



reference has been made must act as a powerful ad- 

 junct in the retardation of both capillary and gravi- 

 tational movements of water below the reach of deep 

 root feeding ; and if this is true, practically all loss of 

 water by downward percolation is prevented, and the 

 whole rainfall not lost by surface evaporation becomes 

 available for crop production. 



There is another condition, brought about by the 

 presence of the layer of air - dry soil beneath the 

 moisture -bearing zone, which in humid regions only 

 exists in exceptional localities, and which may have an 

 important influence in making a larger part of each 

 year's rainfall available for crop production. I refer 

 to the possibility of the large amount of air stored in 

 the air -dry soil beneath the moist layer contributing 

 to deep soil breathing. By slow diffusion upward, and 

 by movements induced by changes in atmospheric pres- 

 sure, the roots may be supplied with oxygen from be- 

 low as well as from above, and thus have their feed- 

 ing depth lowered on this account beyond what is 

 usual in humid soils. So, too, it appears to be quite 

 possible that nitrification and other biologic processes 

 may be permitted to go forward under these condi- 

 tions, when in humid soils they are largely prohibited 

 for lack of sufficient aeration. 



These suggestions, however, do not appear to offer 

 an adequate explanation of the ability of crops to 

 reach maturity in the arid soils of the West without 

 irrigation, when there is no rain for such long inter- 

 vals ; for, as we approached Merced from the north, a 

 very sandy belt of land was passed which was white 



