VII.] FORMATION OF NITRATES IN SOIL 219 



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 europcea, was obtained from all the soils of 

 the old world he examined ; the other, ascribed to 

 the genus Nitroccccus, 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 

 with the nitrous or nitric acids produced, for nitrification 

 ceases as soon as the medium becomes at all acid. 

 While calcium carbonate is the substance which, 

 as a rule, is effective to this end, many organic salts will 

 also supply the necessary base. Ammonium salts of 



* Instruction sur la fabrication du nitre : Par les regisseurs 

 gt'ne'raux des poudres et salt petres, 1777, " Elles doivent l'etre 

 toujours avec une addition de terre calcaire qui puisse servir de 

 base a l'acide nitreux." 



