Ammonium fixation and adsorption experiments were conducted to study the effect of potassium on ammonium fixation into interlayer of vermiculite and ammonium adsorption at the surface of vermiculite both in the binary (NH-4～(+)-Ca～(2+)) and ternary (NH-4～(+)-K～(+)-Ca～(2+)) systems. In the ammonium fixation experiment, 5 mmol NH-4～(+) kg～(-1) was added alone, or after, before or simultaneously with 5 mmol K～(+) kg～(-1) to the vermiculite (vermiculite:solution = 1:1), and the incubation was conducted for 3 days under constant 20℃. In the adsorption experiment, after addition of 10 mmol Ca～(2+) L～(-1) as matrix ions, either NH-4～(+) alone with a concentration series from 0.1 to 5.0 mmol NH-4～(+) L～(-1) (binary exchange system of NH-4～(+) and Ca～(2+) or NH-4～(+) together with 5 mmol K～(+) L～(-1) (ternary exchange system of NH-4 ～(+), K～(+) and Ca～(2+) was added to vermiculite at a vermiculite to solution ratio of l:10 for 24-h equilibrium. The results of the fixation experiment showed that the presence of K～(+) increased NH-4～(+) fixation for all the treatments, regardless of the addition orders. For ammonium adsorption a linear relationship between activity ratios of NH-4～(+) to Ca～(2+) and Mg～(2+) in the equilibrium solutions (ARNH-4) and increases of exchangeable NH-4～(+) at the surface of vermiculite after 24- h equilibrium (△Ex-NH-4) was shown for almost the whole concentration ranges tested. Compared with the binary system, the slope of the curve of the ternary system was steeper, indicating a more intensive adsorption of NH-4～(+) in the presence of K～(+). It was demonstrated that K ～(+) did not reduce the ammounium fixation into the interlayer and ammonium adsororption at the surface of vermiculite, which indicated that, under the experimental conditions of this study, K～(+) did neither occupy the sites for NH-4～(+)-fixation in the interlayer nor the sites for NH-4～(+)-adsorption at the surface of vermiculite. Ammonium fixation and adsorption experiments were conducted to study the effect of potassium on ammonium fixation into the interlayer of vermiculite and ammonium adsorption at the surface of vermiculite both in the binary (NH-4 ~ (+) - Ca 2+) and ternary The ammonium fixation experiment, 5 mmol NH-4 ~ (+) kg ~ (-1) was added alone, or (NH4 +) - K + after or before with 5 mmol K ~ (+) kg -1 to vermiculite (vermiculite: solution = 1: 1), and the incubation was conducted for 3 days under constant 20 ° C. In the adsorption experiment, After addition of 10 mmol Ca 2+ L -1 as matrix ions, either NH 4 + alone with a concentration series from 0.1 to 5.0 mmol NH 4 + 1) (binary exchange system of NH-4 ~ (+) and Ca ~ (2+) or NH-4 ~ (+) together with 5 mmol K ~ (+) L ~ 4 ~ (+), K ~ (+) and Ca ~ (2+) was added to vermiculite at a vermiculite to solution ratio of l: 10 for 24-h equilibrium. The results of the fixation experiment showed that the presence of K ~ (+) increased NH-4 ~ (+) fixation for all the treatments, regardless of the addition orders. For ammonium adsorption a linear relationship between activity ratios of NH-4 ~ ( +) to Ca 2+ and Mg 2+ in the equilibrium solutions (ARNH-4) and increases of exchangeable NH-4 ~ (+) at the surface of vermiculite after 24- Compared with the binary system, the slope of the curve of the ternary system was steeper, indicating a more intensive adsorption of NH-4 ~ (+) in the presence of K ~ (+). It was demonstrated that K ~ (+) did not reduce the ammounium fixation into the interlayer and ammonium adsororption at the surface of vermiculite, which indicated that, under the experimental conditions of this study, K ~ (+) did neither occupy the sites for NH-4 ~ (+) - fixation in the interlayer nor the sites for NH-4 ~ (+) - adsorption at the surfa ce of vermiculite.