564 PRINCIPLES OF SOIL MICROBIOLOGY 



pastorianum) produce largely organic acids. Different amounts of 

 energy are thus made available for the organisms (see p. 412). The 

 amount of nitrogen fixed should, therefore, be calculated on the basis of 

 energy utilized and not on the basis of sugar consumed. By comparing 

 two strains of Clostridia, one anaerobic and the other aerobic, Pring- 

 sheim 16 reported that the difference in the amount of nitrogen fixed 

 per unit of sugar consumed is due to the relative amounts of acids and 

 gases formed. The anaerobic organism produced 45 per cent acid and 

 55 per cent gas out of the sugar consumed. The aerobic strain produced 

 only 33 per cent acid and 67 per cent gas because of the more complete 

 decomposition of the organic matter; this strain fixed, therefore, more 

 nitrogen. The ratio between the sugar changed into gas and the nitro- 

 gen fixed was nearly the same for both organisms, namely, 23 and 21. 

 The differences in the consumption of sugar by the different organisms 

 cannot serve, therefore, as criteria. Azotobacter fixes as much as 3 to 

 20 mgm. of nitrogen per gram of sugar consumed, while CI. pastorianum 

 fixes a maximum of 2 to 3 mgm. nitrogen for the same amount of sugar. 

 However, when the actual energy liberated is compared, the latter 

 organism may be found to be more efficient. 



Chemistry and decomposition of carbohydrates. The anaerobic nitro- 

 gen-fixing bacteria decompose carbohydrates and their derivatives 

 with the formation of various acids, chiefly butyric and acetic, and 

 various gases. One liter of medium containing forty grams of 

 glucose inoculated with the anaerobic organism and placed in a 

 nitrogen atmosphere showed a gain of 53.6 mgm. of nitrogen in 20 

 days. All the sugar disappeared, giving rise to 3.714 grams acetic acid, 

 14.164 grams n-butyric acid, | cc. alcohol, chiefly iso-butyl, and traces 

 of lactic acid. The amount of acids as well as the relation of the gases 

 (C0 2 :H 2 ) were found to vary in the different experiments. 17 The 

 gases were found to make up 55 to 67 per cent of the sugar decomposed 

 and to consist of 49 per cent carbon dioxide and of 51 per cent hydrogen. 18 



Azotobacter decomposes carbohydrates, higher alcohols and organic 

 acids, without the formation of considerable amounts of intermediary 

 products, such as various organic acids; C0 2 is the only gas formed; the 

 reaction of the medium does not become more acid, but may even 



16 Pringsheim, H. Uber die Verwendbarkeit verschiedener Energiequellen 

 zur Assimilation des Luftstickstoffes und die Verbreitung stickstoffbindenden 

 Bakterien auf der Erde. Centrbl. Bakt. II, 20: 248-256. 1908. 



17 Winogradsky, 1893 (p. 107). 



18 Omeliansky, 1923 (p. 559). 



