6o 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 sph'erical 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 
-~ r > * *\| produced by ammoniacal decomposition, 
Sjj t / an d convert them into nitrites (Fig. 18, A). 
They are incapable, however, of forming 
nitrates from any nitrogen compound 
except ammonium salts, and hence the 
FIG. 18. Nitrifying bacteria, proteid compounds of decaying bodies 
A is a nitrous and B and C J 
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 
