BACTERIA 



21 



Fig. 10— BACTERIA ARE USED TO STIMULATE CROP GROWTH 



A comparison between a red clover plant which has been inoculated with the proper nodule- 

 forming bacteria and another plant of the same species which has not been inoculated 

 From a photograph by the Department of Agricultural Bacteriology, University of Wisconsin 



their own protoplasm. Root nodules on certain leguminous 

 plants contain colonies of another bacillus (Bacillus radicicola) , 

 which likewise obtains its nitrogen directly from the air. This 

 useful function of these nitrogen-fixing bacteria has been turned 

 to good account in agriculture. Not only is "green manuring," 

 or plowing in of leguminous crops (rye, clover, vetch, cow peas, 

 etc.), commonly practiced, but the marketing of pure cultures of 

 nodule-forming bacteria under different trade names is now 

 common. 



Another form of bacteria, known as the denitrifying bac- 

 teria, perform the function, perhaps not so useful, of converting 

 some of the ammonia into free nitrogen once more (Fig. 9). 



Other types of bacteria (such as Nitrosomonas and Nitroso- 

 coccus) are causative agents in the oxidation of free ammonia 

 to nitrous acids and nitrites, salts that are not available for 

 plants, but the bacillus Nitrobacter completes the oxidation by 



