104 PRINCIPLES OF SOIL MICROBIOLOGY 



grown in pure culture. 4 But, in addition to these bacteria, the soil 

 harbors other organisms, always living non-symbiotically, which are 

 capable of fixing gaseous nitrogen, in the presence of a proper source of 

 energy. Berthelot 5 suggested, on the basis of numerous investigations, 

 that the fixation of atmospheric nitrogen is well distributed among soil 

 microorganisms. He isolated a number of bacteria from the soil and 

 found that some of them were able to increase the amount of combined 

 nitrogen in the medium. Winogradsky 6 demonstrated in 1893 that the 

 property of nitrogen-fixation is limited to certain specific soil organisms; 

 the mere growth of an organism on nitrogen-free media was still no 

 indication that it was capable of obtaining its nitrogen from the gaseous 

 form of the atmosphere. An organism is considered as unable to fix 

 nitrogen, unless an actual increase in combined nitrogen has been demon- 

 strated by chemical analysis. However, Beijerinck 7 found that the 

 number of bacteria in the soil capable of fixing nitrogen is much larger 

 than suspected by Winogradsky; he designated as oligonitrophilic those 

 bacteria that were capable of developing in media containing only traces 

 of combined nitrogen and considered them as nitrogen-fixing forms. 



The first non-symbiotic nitrogen-fixing organism was isolated by 

 Winogradsky in 1893. Clostridium pastorianum (Bac. amylobacter 

 A.M. et Bred), an anaerobic organism, was foimd capable of bringing 

 about an increase in the amount of combined nitrogen in the medium, 

 in the presence of an available source of energy. Caron 8 soon (1895) 

 isolated a spore-forming organism, Bac. ellenbachensis a, closely related 

 to Bac. mycoides and Bac. megatherium, to which he ascribed the prop- 

 erty of fixing nitrogen. This claim was confirmed by Stoklasa, 9 who 



4 Negative results have recently been reported by Barthel, Chr. Meddel. 

 Centralanst. forsoksv. Jordbr. Bakt. Avd. 43, 1926. 



5 Berthelot, M. Fixation de l'azote atmospherique sur la terre vegetale. 

 Ann. chim. Phys. 13: 5-14, 15-73, 74-78, 78-92, 93-119. 1888. Nouvelles recher- 

 ches sur les microorganismes du sol fixateurs de l'azote. Bull. Soc. Chim. Ill, 

 11: 781-783. 1894. 



6 Winogradsky, 1893 (p. 107). 



7 Beijerinck, M. W. liber oligonitrophile Mikroben. Centrbl. Bakt. II, 7: 

 561-582. 1901. 



8 Caron, A. Landwirtschaftlich-bakteriologische Probleme. Landw. Vers. 

 Sta. 45: 401-418. 1895. 



9 Stoklasa, J. Studien uber die Assimilation elementaren Stickstoffs durch 

 die Pflanzen. Landw. Jahrb. 24: 827-863. 1893; Biologische Studien uber 

 "Alinit." Centrbl. Bakt., II, 4: 39, 78, 119, 284, 507, 535. 1898; 5: 350-359. 

 1899; 7: 257-270. 1901. 



