THE INFLUENCE OF IRRIGATION 225 



Maximum nitrogen fixation occurs when the soil contains be- 

 tween 60 and 70 per cent of its water-holding capacity, and when 

 the soil becomes filled with water the actual gain is only one- 

 fourth what it is when the optimum moisture content is main- 

 tained. 



These results probably help to explain the remarkable fertility 

 of many of the irrigated soils of the arid regions. Some of them 

 have been producing crops undiminished in quantity for upward 

 of fifty years, and there is no reason why a limited few soils on 

 which the richer irrigation waters are being used cannot continue 

 for another fifty or one hundred years to produce maximum 

 crops. 



Alkali of Water — In addition to potassium, nitrogen, and 

 phosphorus irrigation water carries varying quantities of alkalies 

 which may at times be concentrated enough to become a menace 

 when the water is used for irrigation purposes. For instance, of 

 the fifty-eight streams examined, the majority of which were ex- 

 tensiyely used for irrigation purposes, thirteen of them carried 

 alkali, which is dangerous if interpreted in the light of Hilgard's 

 criterion that irrigation water should not contain over forty grains 

 per million (571.2 parts per million). 



The total soluble-salt content of these fifty-eight streams varied 

 from 60 parts per million to 13 12 parts per million. Moreover, 

 it was found that a stream may be comparatively free from al- 

 kali during part of the year, but at other times it may be heavily 

 charged with alkali. The melting of snow in the mountainous 

 region usually has the effect of freshening the water, but local 

 rains often have the opposite effect. In addition, drainage water, 

 especially from alkali soils, greatly increases the alkaline content 

 of the stream. 



The magnitude of the problem which confronts the users of 

 these waters and the speed with which they may yield barren soils 

 is made clear by two examples. For instance, one stream carried 

 sufllicient salts in it that one acre-foot would deposit on the soil 

 3,581 pounds of soluble salts, or twenty such irrigations would 

 reach the enormous sum of 71,600 pounds — suiEcient, if allowed 



