Two soils, one consisting of 1:1 clay minerals at pH 4.5 and the other containing 2:1 clay minerals at pH 7.0, were used to estimate the conversion of added NH+4 under different C/N ratios (glucose as the C source) and the addition of potassium. Under lower C/N ratios (0:1 and 5:1), a large part of the added NH4+ in the acid soil was held in the forms of either exchangeable or water soluble NH4+ for a relatively long time and under higher C/N ratio (50:1), a large amount of the added NH4+ was directly immobilized by microorganisms. In the second soil containing appreciable 2:1 type clay minerals a large part of the added NH+4 at first quickly entered the interlayer of the minerals under both lower and higher C/N ratios. In second condition, however, owing to microbial assimilation stimulated by glucose the newly fixed NH4+c ould be completely released in further incubation because of a large concentration gradient between external NH4+ and fixed NH4+ in the mineral interlayer caused by heterotrophic microorganisms, which imply the fixed NH4+ to be available to plants. The results also showed that if a large amount of K+ with carbon source together was added to soil, the higher K+ concentration of soil solution could impede the release of fixed NH4+ , even if there was a lot of carbon source. Two soils, one consisting of 1: 1 clay minerals at pH 4.5 and the other containing 2: 1 clay minerals at pH 7.0, were used to estimate the conversion of added NH + 4 under different C / N ratios (glucose as the C source ) and the addition of potassium. Under lower C / N ratios (0: 1 and 5: 1), a large part of the added NH4 + in the acid soil was held in the forms of either exchangeable or water soluble NH4 + for a relatively long time and under higher C / N ratio (50: 1), a large amount of the added NH4 + was directly immobilized by microorganisms. In the second soil containing appreciable 2: 1 type clay minerals a large part of the added NH + 4 at first quickly entered the interlayer of the minerals under both lower and higher C / N ratios. However, due to microbial assimilation stimulated by glucose the newly fixed NH4 + c can be completely released in further incubation because of a large concentration gradient between external NH4 + and fixed NH4 + in the mineral interlayer caused by heterotrophic microorganisms, which imply the fixed NH4 + to be available to plants. The results also showed that if a large amount of K + with carbon source together was added to soil, the higher K + concentration of soil solution could impede the release of fixed NH4 +, even if there was a lot of carbon source.