560 PRINCIPLES OF SOIL MICROBIOLOGY 



gen for the same amount of sugar. However, more recent work 4 tends 

 to show that CI. pastorianum is more abundant and more universally 

 distributed in the soil ; also that it actually fixes far more nitrogen when 

 the C0 2 formed is considered as the real index of energy consumption/' 

 The activities of these two groups of organisms do not exclude, however, 

 one another; this has been brought out by Omeliansky, who demonstrated 

 that, by "symbiosis" or "commensalism" in reference to the oxygen 

 tension, even larger quantities of nitrogen are fixed. 



Gainey 6 compared the nitrogen fixing capacity of a large number of 

 soils in which Azotobacter was present or absent. A total of 418 

 soils were examined, of which 199 contained Azotobacter and 219 did 

 not. The presence of Azotobacter was noted by pellicle formation 

 and microscopic examination of a culture prepared by inoculating some 

 soil into 50 cc. of a medium consisting of: 



K2HPO4 0.5 gram CaCl 2 Trace 



MgS0 4 0.2 gram Mannite. 20 grams 



NaCl 0.2 gram Distilled water 1000 cc. 



FeCl 3 Trace 



This was neutralized to phenolphthalein by means of sodium hydroxide 

 solution. A small quantity of sterile CaC0 3 was added to each flask 

 before inoculation in order to insure sufficient base for the neutralization 

 of the organic acids formed. 



The average amount of nitrogen fixed, in 50 cc. mineral solution con- 

 taining 2 per cent mannite, in three weeks by 367 soils was 6.36 mgm. ; 

 174 soils containing Azotobacter fixed an average of 8.30 mgm. of nitro- 

 gen and 193 soils not containing Azotobacter fixed only 4.61 mgm. 

 Thus the quantity of nitrogen fixed by other microorganisms in the soil 

 is practically one-half of that fixed when Azotobacter is present. The 

 amounts of nitrogen fixed by pure cultures of the other bacteria is 

 usually not greater than 1 to 2 mgm. per 1 gram of sugar consumed, 

 although, in some cases, larger quantities were reported. These differ- 

 ences in amounts of nitrogen fixed are undoubtedly due to the efficiency 



* Pringsheim, 1908 (p. 564); Omeliansky, 1923 (p. 559); Truffaut, G., and Bezs- 

 sonofT, N. Sur la predominance de l'activite des fixateurs anaerobies d'azote 

 dans le sol. Compt. Rend. Acad. Sci., 181: 165-167. 1925. 



6 Bonazzi, A. The mineralization of atmospheric nitrogen by biological 

 means. IVth. Intern. Soil. Sci. Conf. IIIB, No. 8, Rome. 1924, 42 p. 



6 Gainey, P. L. Influence of the absolute reaction of a soil upon its Azotobacter 

 flora and nitrogen-fixing ability. Jour. Agr. Res., 24: 907-938. 1923. 



