244 AGEICULTUEAL AND INDUSTEIAL BACTERIOLOGY 



fixed in discussions of soil bacteriology that it probably 

 cannot be replaced by a better one. 



The process of nitrification as it occurs in soils usually 

 takes place in two distinct steps; first, the oxidation of 

 ammonia to nitrous acid or nitrites, a process which may 

 be termed nitrosation, and second, an oxidation of the 

 nitrous acid or nitrites to nitrates, a process termed nitra- 

 tation. 



The process of nitrosation is brought about by a peculiar 

 group of prototrophic bacteria. All of them belong to the 

 genus Nitrosomonas. Some are short rods and motile. 

 Others are spherical and nonmotile. They do not grow at 



FIG. 56. NITROSOMONAS. A. Nonmotile spherical series. B. Motile 

 rod-shaped form. 



all readily in the laboratory upon the commonly used media 

 and may be isolated and studied only by the use of special 

 methods. They require for their growth the presence of 

 ammonia and free atmospheric oxygen. For the most 

 part they do not grow well in the presence of an 

 abundance of organic matter, particularly of amino acids 

 and peptone. They are found in the laboratory not to 

 require any organic matter whatever for their development. 

 Apparently they oxidize the ammonia in accordance with 

 the following equation : 



2NH 3 + 30 2 = 2HN0 2 + 2H 2 



and secure by this means energy for their growth and for 

 the manufacture of food. They are capable of securing 



