NITRIFICATION 137 



of organic nitrogen compounds, particularly in the acid 

 soils of woods and meadows rich in humus and vegetable 

 debris. 



The first step in the oxidation of ammonium salts is 

 brought about by the agency of bacteria, included by 

 Winogradsky in the genus Nitrosomonas. They produce 

 nitrites, and -pure cultures of them are unable to carry the 

 oxidation further. After being introduced into a nutrient 

 solution of ammonium sulphate (p. 144), an incubation 

 period lasting five or six days occurs, during which little 

 chemical change is effected ; after- 

 wards the formation of nitrites goes 

 on rapidly,- as much as 90 mg. or 

 more per litre of solution being 

 produced in a day. In the early 

 stages of nitrification of such a 

 solution the nitrite organisms are 

 usually found in a zoogloea state, 

 attached to the particles of mae- FIG. 29. i 



, , . ,. Win. (X6oo). 



nesmm carbonate lying on the 



bottom of the flask. The individual cells are difficult 

 to observe at first, but after eight or ten days they 

 frequently separate and become motile for a time before 

 forming a zoogloea again. The organisms isolated from 

 western European soils, and known as Nitrosomonas 

 europcea, Win., are oval, from 1.2 to 1.8 /* long and 

 .9 to i A* thick (Fig. 29), and in a motile state have a 

 single long polar flagellum. The colonies on silica jelly 

 are round, very minute, and at first colourless, highly 

 refractive with a brown edge ; later they become dark 

 brown. Several apparently permanent modifications or 

 varieties have been described by Winogradsky ; some of 

 them from eastern Europe, Brazil, aud Australia being 



