22^ 



BACTERIA AND SOIL FERTILITY 



which, although used by the crop in smaller quantities, is never- 

 theless at times the limiting factor in crop production. 



The total phosphorus of the irrigation waters analyzed varied 

 from traces to 5.47 parts per million. The great majority of 

 them, however, contained less than one part per million. The 

 average in two acre-feet of the stream was 3.46 pounds, for the 

 wells 3.36 pounds, and for the drains 1.82 pounds. 



The phosphorus in two ac^e-feet of the water from the richest 

 stream is sufficient for the production of 175 bushels of corn, 

 120 bushels of wheat, or 33 tons of sugar-beets. In the case of 

 all the other streams, whereas not as high, it undoubtedly plays 

 a part, in maintaining the phosphorus content of the soil. The 

 bringing of thefee qujintaties of potassium and phosphorus to a soil 

 must of necessity increase bacterial activity. 



Even more important than the phosphorus is the nitrogen of 

 the waters, for nitrogen is the limiting factor of crop production 

 in most of the soils of the intermountain region. This varies 

 from traces up to 24.3 parts per million. The average quantity 

 of nitrogen in two acre-feet of the irrigation water is 22.8 

 pounds, while that in the highest is 132.2 pounds per acre. This 

 would be sufficient to produce 186 bushels of corn, 1 14 bushels of 

 wheat,ori7tonsofsugarbeets. 



Nitrogen Fixation. — Moreover, most of the soils of arid and 

 semi-arid America contain a rich nitrogen-£xing flora. This is 

 composed of both aerobic and anaerobic azo£ers. Work already 

 done indicates that these may increase the soil nitrogen twenty to 

 thirty pounds .n acre yearly. The quantity fixed, however, de- 

 pends among other things, upon the water in the soil. 



Per cent of Water- 



Nitrogen Gam 



Per cent of Water'- 



\itrogen Gain 



holding Capacity 



Per Cent 



holding Capacity 



Per Cent 



10 



30 



60 



IS 



20 



25 



70 



100 



30 



25 • 



80 



90 



40 



38 



90 



45 



50 



45 



100 



25 



