66 NITRIFICATION AND DENITRIFICATION 
step was the oxidation of the nitrites into miiraies. ‘The two steps 
are not only independent, but they are brought about by two differ- 
ent species of bacteria. One organism has the power of producing 
nitrite out of ammonia, but can carry the oxidation no farther, 
failing to produce nitrates. The second species can act upon the 
nitrites, carrying their oxidation up to the form of nitrates, but 
it has no power to act upon ammonia. The two together can 
produce the complete nitrification of both ammonium and nitrite 
compounds. 
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 
oL- wie called the nitrite bacteria (Fig. 17), 
ae A %: B They have been found in soils of very 
fen Mm, e% widely separate localities, and probably 
"i °, €*" live in all soils. ‘Two slightly different 
ye ef o varieties have been recognized, and 
3 hy named Nitrosomonas and Nitrosococcus 
wi, By by their discoverer, Winogradski. 
Fic. 17.—Nitrifying bacteria. They appear to be able to form 
4 eS merous and Band C nitrites from almost any kind of am- 
monium. salt, and, since they are quite 
universally distributed in all decaying organic matter, as 
well as in all humus, they will evidently seize the am- 
monium compounds produced by ammoniacal decomposition, 
and convert them into nitrites (Fig. 17, A). They are 
incapable, however, of forming nitrates from any nitrogen 
compound except ammonium salts, and hence the proteid com- 
pounds of decaying bodies cannot be nitrified till they are reduced 
to the form of ammonia. The second type of nitrifying bacteria 
is called the nztrate bacteria, since they oxidize the nitrites into 
nitrates. Only a single type of this class has been found, and it 
was named Niirobacter (Fig.17, Band C). Itis smaller than most 
nitrite organisms and of a slightly elongated shape. It is also 
