Increasing K+ adsorption can be an effective alternative in building an available K pool in soils to optimize crop recovery and minimize losses into the environment. We hypothesized that long-term fertilization might change K+ adsorption because of changes in the chemical and mineralogical properties of a rice (Oryza sativa L.). The aims of this study were (i) to determine clay minerals in paddy soil clay size fractions using X-ray diffraction methods and a numerical diagram-decomposition method; (ii) to measure K+ adsorption isotherms before and after H 2 O 2 oxidation of organic matter, and (iii) to investigate whether K+ adsorption is correlated with changes in soil chemical and mineral properties. The 30-yr long-term fertilization treatments caused little change in soil organic C (SOC) but a large variation in soil mineral composition. The whole-clay fraction (<5 μm) corresponded more to the fertilization treatment than the fine-clay fraction (<1 μm) in terms of percentage of illite peak area. The total percentage of vermiculite-chlorite peak area was significantly negatively correlated with the total percentage of illite peak area in the <5 μm soil particles (R=-0.946, P<0.0006). Different fertilization treatments gave significantly different results in K+ adsorption. The SOC oxidation test showed positive effects of SOC on K+ adsorption at lower K+ concentration ( 120 mg L-1) and negative effects at higher K+ concentration (240 mg L-1). The K+ adsorption by soil clay minerals after SOC oxidization accounted for 60-158% of that by unoxidized soils, suggesting a more important role of soil minerals than SOC on K+ adsorption. The K+ adsorption potential was significantly correlated to the amount of poorly crystallized illite present (R=0.879, P=0.012). The availability of adsorbed K+ for plant growth needs further study. Increasing K + adsorption can be an effective alternative in building an available K pool in soils to optimize crop recovery and minimize losses into the environment. We hypothesized that long-term fertilization might change K + adsorption because of changes in the chemical and mineralogical properties of a rice (Oryza sativa L.). The aims of this study were (i) to determine clay minerals in paddy soil clay size fractions using X-ray diffraction methods and a numerical diagram-decomposition method; (ii) to measure K + adsorption isotherms before and after H 2 O 2 oxidation of organic matter, and (iii) to investigate whether K + adsorption is correlated with changes in soil chemical and mineral properties. The 30-yr long-term fertilization treatments caused little change in soil organic C (SOC) but a large variation in soil mineral composition. The whole-clay fraction (<5 μm) corresponded more to the fertilization treatment than the fine-clay fraction (<1 μm) in terms of percentage of ill The total percentage of vermiculite-chlorite peak area was significantly negatively correlated with the total percentage of illite peak area in the <5 μm soil particles (R = -0.946, P <0.0006). Different fertilization procedures gave significantly different results in K + adsorption. The SOC oxidation test showed positive effects of SOC on K + adsorption at lower K + concentration (120 mg L-1) and negative effects at higher K + concentration (240 mg L-1) SOC oxidization accounted for 60-158% of that by unoxidized soils, suggesting a more important role of soil minerals than SOC on K + adsorption. The K + adsorption potential was significantly correlated to the amount of poorly crystallized illite present (R = 0.879, P = 0.012). The availability of adsorbed K + for plant growth needs further study.