Nitrification in Soil 181 



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 ()xiDizix(i Organisms (from' Wilson) 



Al)iiii(laiic(' 

 j)\l ut' Organisms 



iK-i 1,000 



(i.l .'{,.500 



().(i (i.-iSO 



(i.S '2.5, ()()(» 



7.0 .T),!^^) 



chemical nature at tlie \ery surface sod layer. The evidence sub- 

 mitted to substantiate both these theories is still insufficient. On 

 further study, at least in tropical countries, negative results were 

 obtained; the results obtained by Rossi were explained by the spe- 

 cific effect of drying upon the removal of nitrate already present in 

 the soil. Nath concluded that sunlight and ultraviolet hght have no 

 effect on the oxidation of ammonia in soil; actually nitrates are de- 

 composed under these conditions. 



At best the quantities of nitrite and nitrate formed by chemical 

 agencies are insignificant and of httle importance in the soil. The 

 biological process of nitrification, as established by Schloesing and 

 Miintz, Warington, and Winogradsky, is by far the most important. 



Mechanism of Biological Oxidation of Ammonia. Various re- 

 actions have been suggested to explain the mechanism of oxidation 

 of ammonia to nitrite by the nitrite-forming bacteria. The follow- 

 ing two reactions are most probable: 



2XH3 + 3O2 -> ^2HX02 + ^2H20 



2NH3 + 3O2 -> X2O3 + 3H2O 



The free energy efficiency of Nitrosomonas is usually taken to be 

 about 6 per cent. Lees and Hofmann have recently calculated such 

 energy for different stages of growth of the organism. At the early 

 stages of growth, the \'alue was found to be about 50 per cent, falling 

 rapidly as nitrite accumulated; with a nitrite concentration of 1.5 mg 

 nitrogen per ml, it reached about 6 per cent. This rapid fall in effi- 

 ciency was beliex ed to be due to an increased respiration loss follow- 

 ing the maintenance of a low intracellular nitrite concentration with 

 an increasing nitrite content. Using paper chromatography, Hof- 

 mann found that the amino acid content of the protein of nitrifying 

 bacteria is similar to that of the proteins of other organisms. 



