THE NITRIFICATION PROCESS IN SOILS AND BIOLOGICAL FILTERS

Abstract

SUMMARY. An extensive review of the literature on nitrification shows that the workers in this subject may be divided into three groups, viz. (a the Winogradsky or purely autotrophic group, (b those who include growth in organic media, (c those who exclude autotrophic growth. The process of nitrification in soils and filter beds is essentially the same and the organisms concerned exhibit the same cultural characteristics, viz. autotrophic growth in purely mineral solution containing carbonates and low thermal death‐point (56° C). In the absence of carbonates heterotrophic organisms can supply the necessary carbon, dioxide by the decomposition of organic matter, a process which accounts for nitrification in acid soils and filter beds. In the absence of a salifiable base the oxidation of ammonia is arrested by the formation of free acid and at a pH of 5.5 the nitrous acid is spontaneously oxidised to nitric acid. The inhibitive action of organic matter on nitrification can be accounted for by accumulation of carbon dioxide and ammonia and deficient aeration. Removal of any of these factors results in increased nitrification. Increased nitrifying power of soil after passage through earthworms is recorded and accounted for. The restrictive influence of Winogradsky's ideas on autotrophism ara discussed, and the possibility of nitrifying bacteria being a phase in the life cycle of heterotrophic organisms is suggested.