110 PRINCIPLES OF SOIL MICROBIOLOGY 



americanum, a facultative aerobic organism, isolated by Pringsheim, 30 

 Granulobacter pectinovorum of Beijerinck and Van Delden, and others. 



Distribution of anaerobic nitrogen-fixing bacteria in the soil. Wino- 

 gradsky has already demonstrated the wide occurrence of Clostridium, 

 which he isolated from every soil sample taken in St. Petersburg 

 and in Paris. A larger form was found in southern Russia. More 

 recent studies 31 demonstrated the presence of this organism in practically 

 all the Russian soils; the various strains found in different localities 

 vary greatly in their morphology. Similar results were obtained for 

 German soils. 32 An examination of 152 different soil samples taken in 

 different parts of the world indicated the presence of Bac. amylobacter 

 in 137 cases, including surface soils and subsoils, cultivated and virgin 

 soils, except in a few acid peat soils. 33 The occurrence of Bac. amylo- 

 bacter in Vesuvian soils has also been pointed out. 34 



The number of nitrogen-fixing Clostridia in the soil was found to be 

 over 100,000 per gram, or much more abundantly than Azotobacter. 35 

 This led various investigators to conclude that the Clostridium rather 

 than Azotobacter is the most important group of non-symbiotic nitro- 

 gen-fixing bacteria. Duggeli 36 found 100 to 1,000,000 anaerobic and 

 to 100,000 aerobic-nitrogen-fixing bacteria per gram of soil. Plots 

 receiving sodium nitrate as a source of nitrogen contained 10,600 to 

 12,000 Bac. amylobacter and 4,900 to 6,300 Azotobacter cells; plots re- 

 ceiving no nitrogen, but potassium and phosphorus-fertilizers, contained 

 1,120,000 Bac. amylobacter and 98,700 Azotobacter cells per gram of soil. 



Physiology of anaerobic nitrogen- fixing bacteria. Winogradsky found 

 CI. pastorianum to be an obligate anaerobic form which can develop 

 under aerobic conditions only in the presence of aerobic bacteria. How- 

 ever, the aerobic CI. americanum isolated by Pringsheim was very similar 



30 Pringsheim, H. tlber ein stickstoffassimilierendes Clostridium. Centrbl. 

 Bakt. II, 16: 795-800. 1906; 20: 248-256. 1907; 21: 673. 1908; 23: 300. 1909; 

 24:488-496. 1909;36:468-472. 1913;40:21-23. 1914. 



31 Omeliansky and Solounskoff, 1915 (p. 108). 



32 Freudenreich, E. v. Ueber stickstoffbindende Bakterien. Centrbl. Bakt. 

 10: 514-522. 1903. 



33 Haselhoff, E., and Bredernann, G. TJntersuchungen iiber anaerobe stick- 

 stoffsammelnde Bakterien. Landw. Jahrb. 35: 381^414. 1906. 



34 Riccardo, S. Primo contributo alia conoscenza dei Batteri fissatori di azoto 

 nei terreni vesuviani. Ann. R. Sc. Sup. Agr. Portici, 18: 1-50. 1923. 



36 Truffaut, G., and Bezssonoff, N. Augmentation du nombre des Clostridium 

 Pastorianum (Winogradsky) dans les terres partiellement sterilisees par le sulfure 

 de calcium. Compt. Rend. Acad. Sci. 172: 1319-1322. 1921. 



36 Duggeli, 1921 (p. 39). 



