566 PRINCIPLES OF SOIL MICROBIOLOGY 



may last up to 21 to 24 days of growth and is then followed by a decrease 

 to 32 to 39 days. 22 In old cultures, the most active CO2 evolution may 

 be observed even later (on the thirty-sixth day of growth) . When aera- 

 tion is increased, the maximum evolution of C0 2 takes place at a much 

 earlier period, namely, between the sixth and ninth days. The curves 

 shown in figure39 were obtained by growing Az. chroococcum in a glucose 

 medium free from nitrogen. 



The differences in the products formed by the different nitrogen fixing 

 bacteria from the carbohydrate used as a source of energy account to a 

 large extent for the difference in the amounts of nitrogen fixed. One 

 gram of glucose liberates 3.7 Calories when oxidized completely to C0 2 

 and water; it liberates only 0.08 Calorie when changed into butyric 

 acid and 0.19 Calorie when changed to acetic acid under anaerobic con- 

 ditions. The energy made available to Azotobacter, from the same 

 amount of sugar decomposed, may be forty-six times as much as to 

 CI. pastorianum. The fixation of nitrogen consumes a large amount of 

 energy; the greater the amount of energy liberated the greater will be 

 the amount of nitrogen fixed. The organism that will be able to utilize 

 a larger amount of energy from the same amount of substrate will, 

 therefore, be able to fix larger quantities of nitrogen, even if it is not more 

 efficient. This accounts for the statements made 23 that CI. -paslorianum 

 is actually a more energetic nitrogen "fixing" organism than Azotobac- 

 ter; this holds true of course only when the energy made available 

 in the process is used as a basis for comparison. 



Respiration and nitrogen-fixation. The utilization of energy enables 

 the organism to obtain the nitrogen necessary for the synthesis of its 

 protoplasm from the atmosphere. The amount of nitrogen fixed and 

 the efficiency of the process depend on a series of physical, chemical, 

 and biological factors including temperature, composition and concen- 

 tration of medium, aeration, age and development of culture, and racial 

 peculiarities. 24 The nature and concentration of the energy source and 

 presence or absence of combined nitrogen are of special importance. 

 The actual quantity of cell substance formed per unit of sugar consumed 



22 Krainsky, A. B. Fixation of free nitrogen in the soil by Azotobacter chro- 

 jococcum, its physiological properties and activities in the soil (Russian). Zhur. 

 Opit. Agron., 9: 689, 749. 1908; Univ. Izv. Kiev, 52: 1-58, 59-131, 133-182. 

 1912. 



23 Bonazzi, 1924 (p. 560). 



24 Omeliansky, W. L. On the relation between nitrogen-fixation and utiliza- 

 tion of non-nitrogenous organic matter by nitrogen-fixing bacteria (Russian). 

 Arch. Sci. Biol. Petrograd, 18: No. 4. 1914. 



