942 SOIL PHYSICS, CHKMISTRY AND MlCRO]'.I()I.OOV 



but the figures were very close (i to 3 nigms. of nitrogen per gram of sugar 

 decomposed). 



II. Fixation of Atmospheric Nitrogen by Mixed Cultures. — The work 

 of different investigators has demonstrated that in mixed cultures nitro- 

 gen-fixing is more considerable than in the case of pure cultures. On the 

 basis of this fact and assuming that mixed cultures approximate more 

 closelj^tothe natural life conditions of nitrogen-fixing bacteria, M. Omelian- 

 SKij studied the fixation of nitrogen in cultures of a large number of races 

 of Azotohacter and Clostridium Pasteurianum isolated from different Rus- 

 sian soils, associating with them many other micro-organisms usually ac- 

 companying them in the soil. From his man)^ experiments he concludes 

 that : 



(i) The study of the biochemical reactions by means of which the 

 bacteria in mixed cultures fix atmospheric nitrogen is interesting in several 

 respects, as it brings out clearly the various aspects of the natural process 

 occurring under conditions of combined action of the different organisms. 



(2) The organisms acting in combination with the nitre gen-fixing 

 bacteria in upper soil strata are ver}- numerous and they pla}' an extremely 

 important part in the life of the soil. 



(3) The synergetic activity of nitrogen-fixing and accompanying 

 microbes is, both in laboratory experiments and under natural conditions 

 (cultivable stratum of the soil), of a dift"erent character according to the 

 properties of the species taking part in the process and their environment. 

 In other cases the function of the satellite organism seems to consist in fix- 

 ing the oxygen of the air and in creating the anaerobic environment (for 

 Clostridium Pasteurianum). The species added to the cultures of nitrogen- 

 fixing microbes sometimes supph' the compounds of carbon needed for the 

 process of fixing nitrogen as energetic substance. In the case of the combi- 

 nation : Azotohacter + Clostriditim Pasteurianum, the function of the former 

 is not confined to fixing the oxygen of the air only, and consequently to 

 creating an anaerobic envoronment for the Clostridium, but this combina- 

 tion is also useful inasmuch as it destroys the injurious products of disas- 

 similation created by the second (chiefly butyric acid) and maintains the 

 action of the environment {Azotohacter is alkaligenic and the Clostridium 

 acidogenic) . 



(4) The satellite species may also unfavourably affect the nitrogen- 

 fixing microbe, either through products of assimilation or by consumption 

 of the carbon compounds needed by this microbe for nitrogen-fixing. The 

 energetic fixation of oxygen by the satellite aerobic species creates condi- 

 tions favourable to the development of Clostridium Pasteurianum, but at 

 the same time hinders the growi:h of the Azotohacter, which is necessarily 

 aerobic. 



(5) The form endowed with the maximum vitalit}'^ and at the same 

 time the most common form in which combination of the nitrogen-fixing 

 organisms takes place in the upper soil strata is that of symbiosis between 

 the aerobic and anaerobic nitrogen fixers, principally between Azotohacter 

 and Clostridium Pasteurianum. In spite of the opposite properties of the 



