60 NITRIFICATION AND DENITRIFICATION. 



The Nitrifying Bacteria. It thus appears that there are two 

 types of nitrifying bacteria. The first converts ammonium com- 

 pounds into nitrites, and, hence, are called the nitrite bacteria 

 (Fig. 1 8). They have been found in soils of very widely separate 

 localities, and probably live in all soils. Two slightly different 

 varieties have been recognized, both spherical bacteria, and named 

 Nitrosomonas and Nitrosococcus, names that will probably soon go 

 out of use. They appear to be able to form nitrites from almost any 

 kind of ammonium salt, and, since they are quite universally dis- 

 tributed in all decaying organic matter, 

 as well as in all humus, they will evi- 

 dently seize the ammonium compounds 

 _ ^ ^^- - * -^,w 



-*" *^- - v w produced by ammoniacal decomposition, 















and convert them into nitrites (Fig. 18, A) . 



-A c They are incapable, however, of forming 



*/A>j3 nitrates from any nitrogen compound 



**Kv except ammonium salts, and hence the 



FIG. 18 Nitrifying bacteria, proteid compounds of decaying bodies 



A is a nitrous and B and C 



nitric bacteria. cannot be nitrified till they are reduced 



to the form of ammonia. The second 



type of nitrifying bacteria is called the nitrate bacteria, since they 

 oxidize the nitrites into nitrates. Only a single type of this class 

 has been found, and it was named Nitrobacter (Fig. 18, B and C). 

 It is smaller than most nitrite organisms and of a slightly elongated 

 shape. It is also widely distributed, probably in all soils, and is 

 able to convert any kind of nitrite into nitrate. It cannot, how- 

 ever, act upon any nitrogen compounds except nitrites, and hence 

 its action must be preceded by that of the nitrite bacteria. 



In ordinary soil these two kinds of nitrifiers act together and 

 simultaneously. So closely connected is their action that it is 

 difficult to find any traces of nitiites in the soil, since they are 

 converted into nitrates as rapidly as they are formed. The whole 

 nitrification may be very rapid. If ammonium salts are added to 

 soil, they cannot commonly be found in the drainage- water from the 

 soil, since the nitrification progresses so rapidly that they become 

 completely converted into nitrates before draining away. But 



