Abstract
Laboratory studies were made on the nitrification of ammonium in the several soils which have various ammonium fixing power. In Nyuzen soil having little fixing power, 100 per cent of ammonium added was converted into nitrate during 4 weeks, but in every one of other soils which have more or less fixing power, amounts of nitrified nitrogen after incubation for 11 weeks were only about 40 to 75 per cent. And it was generally shown that the low values are due mostly to the fixation of ammonium by the soil minerals. Results obtained in the experiments on the relations between the forms of ammonium in the soils and rate of nitration, had shown that replaceable ammonium is easily converted into nitrate, and nonreplaceable or fixed ammonium is difficult in most cases. But in some soils, for example, in micaceous Akizuki soil not only fixed ammonium but also a part of nonreplaceable one was nitrified by soil microorganisms. It was suggested that such nitrification of fixed or nonreplaceable ammonium is probably started from replacement with hydrogen ions produced by these organisms.
