REDUCTION PROCESSES IN THE SOIL 555 



Attention must be called here to the fact that many of the studies on 

 denitrification were carried out in solution and not in soil. It is known 

 that in very wet soils and in liquids which do not have a ready access to 

 oxygen the bacteria utilize the oxygen from the nitrate molecule for 

 oxidation purposes, while this does not occur in the presence of sufficient 

 oxygen as in well aerated soils. 



The original suggestion of Winogradsky that the introduction of large 

 quantities of manure favors denitrification while preventing nitrification 

 has been proved erroneous. Both processes may take place side by side. 

 The losses of nitrogen in the manure compost were found 53 to be due 

 largely to the presence of nitrate forming bacteria. When these bacteria 

 are eliminated or conditions are made unfavorable for their action the 

 losses are considerably reduced. The presence of the bacteria resulted 

 in an increase in the loss of nitrogen from 6.28 to 23.75 per cent in the 

 case of cow manure and from 0.73 to 11.66 per cent in the case of horse 

 manure. 54 



It is often observed that the addition of large quantities of undecom- 

 posed organic matter to the soil particularly rich in carbohydrates and 

 poor in nitrogen injures crop growth. This is not due to denitrification, 

 to which it has often been ascribed, but to the fact that, in the presence 

 of an excess of available organic matter, the fungi, actinomyces, and vari- 

 ous heterotrophic bacteria synthesize an extensive protoplasm. For 

 this purpose, they assimilate the nitrates and ammonium compounds 

 present in the soil and thus compete with higher plants. 



The conclusion may be reached that the phenomenon of denitrifica- 

 tion is of no economic significance in well aerated, not too moist soils, 

 in the presence of moderate amounts of organic matter or nitrate. How- 

 ever, in the case of soils kept under water for some time, as rice soils, 

 the addition of nitrates may even prove injurious due to the formation 

 of toxic nitrite. 55 The injury by denitrification in peat soils has been 

 mentioned above. 



Reduction of other oxygen-rich compounds in the soil. Among the 

 various inorganic, oxygen-rich compounds which can be readily reduced 



63 Niklewski, 1923 (p. 499); Smirnow, V. G. Role of nitrifying bacteria in 

 the process of decomposition of manure. Zhur. Opit. Agron., 16: 329-386, 1915. 



64 A detailed review of the subject is given by Ldhnis, 1910 (p. xiii); see also 

 Russell and Richards. Jour. Agr. Sci., 8: 495-563, 1917. 



65 Nagaoka, M. Effect of nitrate of soda on paddy soils. Bui. Coll. Agr. 

 Tokyo, 6: No. 3. 1904; Kelley, W. P. The assimilation of nitrogen by rice. 

 Hawaii Agr. Exp. Sta. Bui. 24. 1911. 



