SOIL CHANGES DUE TO IRRIGATION 105 



distinct effect on the living organisms in the soil. The 

 detailed relations that exist between soil life and varying 

 soil moisture are yet to be determined, and will furnish 

 another and most important chapter in irrigation practice. 

 Very few investigations have been made on this phase of 

 irrigation, although the field is full of promise. 



It is well known that bacteria and other forms of low 

 life flourish best when in the presence of an abundance of 

 water, and the statement is commonly made that the 

 greatest effects of bacterial life are obtained when an 

 excess of water is available. While these findings are 

 generally true, it must be observed that few studies of 

 bacterial activity have been made under an environment 

 similar to that which prevails in the soil. Low forms of 

 life, like higher ones, require various foods in addition to 

 water; and these substances must be in solution at a 

 certain concentration. Under irrigation, as already shown, 

 the concentration of the soil solution may be varied con- 

 siderably. When over-irrigation is practised, the soil 

 solution is kept very dilute; when no irrigation is prac- 

 tised, during rainless summers, it may be kept very con- 

 centrated. This phase of the subject, in relation to soil 

 life, is yet to be studied. 



Stewart and Greaves have studied, at the Utah Sta- 

 tion, the effect of varying applications of water on the 

 nitrifying organisms. Series of field plats were grown to 

 different crops. Each series received irrigation from 25 

 inches to none. The soil was ideally adapted to rapid 

 bacterial action. The work was continued over eight 

 years, so that the conclusions may be accepted with con- 

 siderable assurance of their truth. It was found that 

 the nitric nitrogen content never exceeded 300 pounds to 

 a depth of 10 feet. The application of irrigation water 



