nitrifying organisms from the hosts of other micro-organisms which 

 teem in ordinary fertile soil. The first organism thus isolated was 

 only capable of oxidising ammonia to the state of nitrite, and was 

 only responsible for the first stage of the process of nitrification. Wi- 

 nogradsky completed his discovery in 1891 by the isolation of a second 

 type of organism which possessed the power of transforming tfie par- 

 tially oxidised compound —the nitrite -into the fully oxidised product 

 — nitrate. 



The biological agents in this cycle of change had now been isolated 

 and d^' scribed, and nitrification was raised from the status of a myste- 

 rious and elusive phenomenon to that of a definite outcome of bacterial 

 activity. 



The Nitrifying Organisms. 



Stage I Conversion of Ammonia into a Nitrite. 



The organisms responsible for this, the first stage in the general 

 process of nitrificition^ belonging to the familia • class of minute vege- 

 table organisms known as bacteria. Although severtil distinct species 

 have been isolated which possess the power of converting ammonia 

 into an alkaline nitrite under sui'able conditions, they may be conve- 

 niently classified under the two groups (a) nitrosomonaa, and (b) 

 nitrosococcus. 



Nitro8omona». — This type is peculiar to the soils of the old world — 

 Europe, Asia and \frica, and is distinguished as possessing marked 

 powers of locomotion through the activity of a * cilium' or long motile 

 appendage. Only one species has so far been isolated from European 

 soils and this appeirs in the form of minute, briskly motile cells pro- 

 vided with a ' tail' or flagellum. For bacteria, these organisms are of 

 large size and vary from 20.^00 *° ¥TT.\)oTy °^ ^^ ^^^^ i^ length and 

 about two-thirds as much in breadth. If cultivated in a suitable liquid 

 medium, the individual cells eventually become quiescent and undergo 

 a distention of cell- wall which causes them to collect at the bottom of 

 the liquid in adherent masses of a grey, gelatinous appearance. 

 (Zoogloea). 



Nitrosococcus. — Varieties of this type occur in South American and 

 Australian soils and differ from the nitrosomonas class in two imp -rtant 

 features ; (1) absence of cilia or organs of locomotion, (2) non-forma- 

 tion of gelatinous masses of zoogloea. 



Distinct species of nitrosococcus from Quito and Brazil have been 

 studied by Winogradsky and found possessed of exceptional vigour as 

 producers of nitrite from ammonia. As the name implies, these or- 

 ganisms are spherical in shape. Their size is large as compared with 

 many other bacteria and reaches ^^,075^ of an inch in diameter in some 

 cases 



Stage II. — Conversion of Nitrite into Nitrate. 



No organism of the class we have just considered has the power 

 of extending the oxidation of ammonia beyond that of the nitrite, 

 and a second distinct order of living workers is responsible for the 

 final stage of the process of nitrification. 



Nitrobacter is the term generally employed to describe the nitrate- 

 producing bacteria which differ so markedly from the ' nitroso' or 



