192 THE LIVING ORGANISMS OP THE SOIL [chap. 



two organisms concerned in the complete nitrification 

 process. The further study of the organisms was for 

 a long time hindered by the fact that they could not be 

 got to grow upon the gelatinous media employed in the 

 ordinary methods of isolating specific bacteria; and 

 though P. F. Frankland, by a dilution method, succeeded 

 in isolating and describing a nitrifying bacterium, it was 

 not until 1890 that Winogradsky cleared up the problem. 

 He prepared a solid nutritive medium containing no 

 organic matter but with silica in its gelatinous form as a 

 basis, and thus was able to separate nitrifying bacteria 

 from the large number of other species simultaneously 

 present in the soil. Winogradsky was able to isolate 

 two species of bacteria capable of transforming ammonia 

 compounds into nitrites. One of these, termed Nitro- 

 somonas europcza, was obtained from all the soils of 

 the old world he examined ; the other, ascribed to 

 the genus Nitrococcus, was peculiar to the soils of 

 America and Australia. The former occurs both as a 

 single, free-swimming form, and clustered together in a 

 colony or zooglcea state. 



Finally, there appears to be one type of organism 

 only, included in the genus Nitrobacter, which oxi- 

 dises the nitrites to nitrates. Winogradsky and other 

 observers have worked out the conditions of life of 

 these nitrifying organisms — the limits of temperature 

 for their growth, 5 and 55° C, have already been given, 

 the optimum temperature is about 37 C. Their action 

 is much restrained by the presence of organic matter, 

 or any quantity of alkaline carbonates or chlorides ; at 

 the same time, some base * must be present to combine 



* Instruction sur la fabrication du nitre : — Par les rdgisseurs 

 gMraux des poudres et salt pHres> 1777, "Elles doivent F6tre 

 toujours avec une addition de terre calcaire qui puisse servir de 

 base a l'acide nitreux." 



