SOIL STERILITY 133 



as are required by plants. Fungi and putrefying bacteria 

 reduce the vegetable proteins to a form which can be acted 

 upon by the ammonifying bacteria which finally leave the 

 nitrogen in the form of ammonia; it may then be either 

 combined into an ammonium salt and utilized by the 

 plant or oxidized by other organisms into nitrous, and 

 then nitric, acid. The latter combines with bases in the 

 soil to form nitrates. Where the proper organisms are 

 in the soil in sufficient numbers to carry this process 

 smoothly to a finish the soil is usually highly productive. 



Desirable organisms other than of the class mentioned 

 above are the symbiotic nitrogen-fixing bacteria which 

 live in the nodules of legume roots and synthesize at- 

 mospheric nitrogen into forms which can be utilized by 

 the host plant. A number of different kinds of bacteria 

 fix atmospheric nitrogen without symbiosis with higher 

 plants; still other organisms are known to break up and 

 make available certain insoluble compounds in the soil 

 which are essential to profitable crop production. 



The desirable microorganisms do best under practically 

 the same soil conditions as do crop plants. They thrive 

 or grow most luxuriantly in soils rich in organic matter, 

 well aerated, and with about the optimum moisture content 

 for most crops. Where the soil is water-logged, puddled, 

 or contains injurious matter, the more desirable nitrify- 

 ing and nitrogen-fixing bacteria are largely replaced by 

 denitrifying and putrefying organisms which rapidly 

 deplete the soil of available nitrogen. 



Biological Inactivity and Soil Sterility. Alkali salts 

 which injure or prevent the production of crops on certain 

 lands also injure the activities of the desirable soil organ- 

 isms. Taylor (18) found that at least part of the sterility 

 of certain Bengal soils was due to scarcity of bacteria and 



