62 PRINCIPLES OF SOIL MICROBIOLOGY 



2. Only few of the autotrophic bacteria are obligate, some are faculta- 

 tive autotrophic. The latter, as in the case of some sulfur, hydrogen 

 and methane bacteria, can exist both autotrophically and hetero- 

 trophically. 



Bacteria deriving their energy from nitrogen compounds. The bacteria 

 which are able to derive their energy from the oxidation of simple forms 

 of nitrogen (nitrifying bacteria) are divided into two groups; (1) those 

 that oxidize ammonium salts to nitrites, and (2) those that oxidize 

 nitrites to nitrates. The latter comprise rather limited groups of micro- 

 organisms, both in their morphology and physiology. Representatives 

 of both groups were isolated and described by Winogradsky 2 in 1891 and 

 1892. 



Various purely chemical theories were suggested at different times to 

 explain the process of nitrification in nature. Pasteur 3 was the first 

 to suggest that the oxidation of ammonia to nitrate is accomplished 

 by the agency of microorganisms. This view was definitely confirmed 

 (1877) by Schlosing and Mi'mtz, 4 who demonstrated that the heating of 

 soil, otherwise capable of rapidly transforming ammonia to nitrites, to 

 100°C. or treating it with antiseptics (chloroform) was sufficient to pre- 

 vent nitrification; when afresh portion of soil was added to the treated 

 soil, the power of transforming ammonia to nitrates was restored. Aera- 

 tion was found to be essential to nitrification. It was obtained either 

 by bubbling air through the medium, or by spreading the medium in a 

 thin layer over the bottom of the container. According to Schlosing, 5 

 oxygen is taken up during the process of nitrification and the quantity of 

 oxygen consumed bears a constant ratio to the amount of nitrogen nitri- 

 fied. A temperature of about 37°C. and the presence of calcium carbonate 

 or alkaline carbonates in low concentrations (0.2 to 0.5 per cent) were 

 also found to be favorable. These investigations proved that the condi- 

 tions commonly utilized in the saltpeter heaps were quite essential for the 

 activities of the organisms; namely, (1) the presence of nitrogenous 



2 Winogradsky, S. Recherches sur les organismes de la nitrification. Contri- 

 butions a la morphologie des organismes de la nitrification. Arch. Sci. Biol., 

 St. Petersbourg, 1: 87, 127. 1892. 



3 Pasteur, L. Etudes sur les mycodermes. Role de ces plantes dans la fer- 

 mentation acetique. Compt. Rend. Acad. Sci. 54: 265-270. 1862. 



4 Schlosing, Th. and Miintz, A. Sur la nitrification par les ferments organises. 

 Compt. Rend. Acad. Sci. 84: 401. 1877; 85: 1018-1020. 1877; 86: 892. 1878; 

 89: 891-4, 1047-7. 1879. 



* Schlosing, Th. Sur la nitrification de l'ammoniaque. Compt. Rend. Acad. 

 Sci. 109: 423^28. 1889. 



