SOIL ORGANISMS 431 



definitely shown that certain of these organisms have the 

 power of utilizing atmospheric nitrogen, which later becomes 

 a part of the nitrogenous matter of the soil. Boussingault * 

 in 1858 suggested the possibility of such a phenomenon, but 

 it was not until 1883 that Berthelot 2 began experiments by 

 which he demonstrated that bare soils appreciably increase 

 in nitrogen on exposure under such conditions. Winograd- 

 ski 3 in 1894 was the first, however, to isolate an organism 

 capable of affecting such a transformation. This bacterium 

 was an anaerobic, rod-shaped organism producing spores 

 and a boat-shaped mass (Clostridium) ; hence the name, Clos- 

 tridium pastorianum. It is very widely distributed in soils. 



The most important organism fixing nitrogen independently 

 in the soil was discovered by Beijerinck 4 in 1901. This or- 

 ganism was an aerobic bacillus to which he gave the name 

 Azotobacter. It was at first thought that this bacillus could 

 not fix nitrogen unless certain other organisms, such as Granu- 

 lobacter, Radiobacter and Aerobacter, were also present. Lip- 

 man 5 has shown this idea to be erroneous, although the effi- 

 ciency of Azotobacter is much higher in mixed than in pure 

 cultures. A number of different species of Azotobacter have 

 been studied, the A. chroococcum apparently being the most 

 widespread. 



Clostridium pastorianum and Azotobacter are by no means 

 the only soil organisms capable of fixing nitrogen. Among 



1 See Voorhees, E. B., and Lipman, J. G., A Review of Investigations 

 in Soil Bacteriology ; U. S. Dept. Agr., Office of Exp. Sta., Bui. 194, 

 1907. 



2 Berthelot, M., Becherches nouvelles sur les microorganisms fixateurs 

 de l' azote; Compt. Rend. Acad. Sci. Paris, Tome 115, pp. 569-574 and 

 842-849, 1892-93. 



3 Winogradsky, S., Sur V assimilation de V azote gazeux de I' atmosphere 

 par les microbes; Compt. Rend. Acad. Sci. Paris, Tome 118, pp. 353-355, 

 1894. 



4 Beijerinck, M. W., Tiber Oligonitrophile Mikroben; Centrbl. Bakt., 

 II, Bd. 7, S. 561-582, 1901. 



5 Lipman, J. G., Experiments on the Transformation and Fixation of 

 Nitrogen by Bacteria; N. J. Agr. Exp. Sta., 24th Ann. Rep., pp. 217-285, 

 1903. 



