CHANGES BROUGHT ABOUT BY XOX-PATHOGEXIC ORGANISMS 69 



great iron beds of northern Minnesota have been deposited by 

 the activity of such organisms. 



Oxidation of Ammonia. In most soils there are numerous 

 bacteria that oxidize free ammonia to nitrous acid, and by neutral- 

 ization with the soil bases form nitrites. These organisms do not 

 develop well in the laboratory in the presence of organic matter. 

 It seems evident that they utilize the energy secured from the 



Fig. 36. Bacteria that oxidize ammonia and nitrous acid to nitrous 

 and nitric acid respectively: a, Xitrosomonas europea; b, N. javet 

 Xitrobacter (Winogradsky) . 



oxidation of the ammonia to build up their protoplasm out of 

 simple materials. They are among the best examples of the 

 strictly prototrophic bacteria. Organisms capable of bringing 

 about this change are called nitroso-bacteria. The reaction may be 

 represented as follows: 



XH, + 20 2 = HX0 3 + H 2 0. 



This is the first of the two steps in the process called nitrification 

 in the soil. 



Oxidation of Nitrous Acid. The nitrous acid formed in the 

 soil and in water, etc., by the preceding group is further changed 

 by another group of organisms called the nitrate bacteria. Like 

 the preceding, they are widely distributed in water and soil, 

 and complete the process called nitrification or, better, oxidation 

 of nitrogen. The nitrates produced by their activity are the 

 source of nitrogen for green plants. A few of the latter are able 

 to make use of nitrogen in the form of ammonia compounds, but 

 in nature this rarely occurs. The reaction may be represented as 

 follows : 



2HNO 2 4- O 2 - 2HXO 3 . 



It is probable that the fertility of the average soil is more largely 

 determined by the maintenance of conditions favorable to the 



