The Physiology of Nitrification in Kenya Highland Soil

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Millbank ]. W.
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1. The organisms responsible for the conversion of ammonia to nitrate in a soil widely distributed in the high altitude (over 6000') areas of Kenya have been investigated, both morphologically and physiologically, using the soil perfusion technique. 2. Morphologically, apart from motility, they closely resemble the classical genera Nitrosomonas and Nitrobacter. They are short rods, gram negative, non-motile, with round, compact and glassy colonies. 3. Physiologically, they also show close resemblance to the temperate forms. Nitrification is carried on only at the soil surface; the surface can be saturated with bacteria, and the bacteria adhere strongly to the Soil particles. 4. A soil which has been enriched and saturated with orgasms oxidises nitrite and ammonia at a constant rate, with no initial lag phase. The pH is not critical over a wide range. 5. The build-up of a population on additions of ammonium sulphate to untreated soil is slow, due to the onset of acid conditions created by the oxidation of ammonia to nitrite. Addition of alkaline substances to combat this gives rise to transitory accumulations of Nitrite, caused by differential pH optima for the proliferation process of the two organisms involved. 6. Chlorate and allyl thiourea are powerful inhibitors of nitrification m the soils studied. Methionine is broken down to mercaptans, which are inhibitory; Histidine is eliminated by the soil heterotrophs and so is not inhibitory.
East African Agricultural And Forestry Journal, 4, p. 293-311