178 PRINCIPLES OF SOIL MICROBIOLOGY 



Oxides of nitrogen are determined by combustion in the platinum 

 spiral before oxygen (or air) is admitted, in the presence of hydrogen. 

 The contraction in volume serves as an index of N 2 (N 2 + H 2 — * 

 H 2 + N 2 ). The oxides of nitrogen may be absorbed from 100 cc. 

 sample of gas in 200 cc. m/50 KOH solution, then oxidized to nitrate 

 by adding 5 cc. of 30 per cent hydrogen peroxide. The solution is evapo- 

 rated to dryness on a water bath and nitrates determined by the phe- 

 noldisulphonic acid method. 78 Volatile amines and mercaptans do not 

 occur in great abundance among the decomposition products in the soil, 

 but are found largely in the anaerobic decomposition of manure: 79,80 

 H 2 S gas can be determined by absorption with acetates of lead and cad 

 mium, or ammoniacal cadmium chloride solution, then titrating the 

 CaS with iodine in acid solutions. 81 



Among the gases formed by the proteolytic bacteria, like Bac. sporo- 

 genes, we find largely C0 2 and some hydrogen; the odoriferous gases 

 consist largely of H 2 S; some elementary nitrogen and N 2 are also 

 formed. The saccharolytic organisms, like Bac. welchii, produce a 

 large amount of hydrogen, often as much as 50 per cent of the gases. 81 

 The ratio between the C0 2 and hydrogen depends largely upon the 

 environmental conditions of growth. 



Anaerobic bacteria form various acids (acetic, butyric, lactic), alco- 

 hols (ethyl, butyl), and in some cases acetone. Often closely related or- 

 ganisms vary greatly in their metabolic products. For example, while 

 different members of the Bac. amylobacter group (Clostridia, Plectridia, 

 Granulobacter) produce butyric acid, the closely related Bac. felsineus 

 does not do so. 



Soil processes in which anaerobic bacteria take an active part. Atten- 

 tion has already been called to a number of important physiological 

 processes in the soil, in which anaerobic bacteria take an active part. 

 It is sufficient to indicate that such processes as decomposition of cellu- 

 loses, pectins and proteins, and the fixation of nitrogen non-symbioti- 

 cally are as active anaerobically as aerobically. Ammonia formation 



78 Allison, V. C, Parker, W. L., and Jones, J. W. Determination of oxides of 

 nitrogen. Tech. Paper No. 249, U. S. Bureau of Mines. 1921. 



79 Guggenheim, M. Die biogenen Amine. 1920. 



80 Hirsh, P. Die Einwirkung von Mikroorganismen auf die Eiweisskorper. 

 Borntraeger. Berlin. 1918. 



81 Anderson, B. G. Gaseous metabolism of some anaerobic bacteria. XIX. 

 Methods. Jour. Inf. Dis. 35: 213-243. 1924. 



