CHAPTER XXI 



Reduction Processes in the Soil- — Nitrate Reduction 



Reduction processes in the soil. Just as aerobic conditions in the soil 

 favor oxidation processes, so do anaerobic conditions (exclusion of free 

 oxygen) favor processes of reduction. Either organic or inorganic 

 compounds may be formed, as a result of these processes, depending 

 upon the constituents of the medium. 1 It is not necessary for the soil 

 to be saturated with water for the conditions to be anaerobic. Wino- 

 gradsky 2 demonstrated, by the development of anaerobic nitrogen- 

 fixing bacteria (see p. 783), that when the soil contains water 

 equivalent to only about 40 per cent of its moisture-holding capacity, 

 anaerobic bacteria find conditions favorable for their development 

 up to the surface of the soil. 



A soil possessing a reducing power will form naphthol-blue from a 

 mixture of para-nitroso-dimethylaniline and a-naphthol but will not 

 readily oxidize easily oxidizable substances, such as aloin. 3 Other 

 indicators, like p-nitromalachite green which is reduced to p-amino 

 malachite green, 4 can also be used to determine the oxidation-reduction 

 potential of the soil. Reduction phenomena are also characterized by 

 the reduction of inorganic salts rich in oxygen, especially nitrates and 

 sulfates. In the absence of atmospheric oxygen, the organic matter of 

 the soil is broken down with the formation of hydrogen which, in statu 

 nascendi, brings about the reduction of the salts rich in oxygen. Forma- 

 tion of H 2 S is thus a secondary phenomenon. Under aerobic con- 

 ditions, however, the formation of H 2 S is primary since it results in the 

 decomposition of proteins. Bad. coli, for example, decomposes glucose 



1 Van Wolzogen Ktihr, C. A. Biochemical reduction processes in the soil. 

 Arch. Suikerind, Nederlandsch. Indie, 23: 501-511. 1915. 



2 Winogradsky, S. Sur l'etude de l'anaerobiose dans la terre arable. Compt. 

 Rend. Acad. Sci., 179: 861. 1924. 



3 Sullivan, M. X. Reduction processes in plant and soil. Science, 39: 95S. 

 1914. 



4 Felton, L. D. A new indicator for testing reducing power of bacteria. Jour. 

 Inf. Dis., 34: 414-419. 1924. 



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