A pot experiment was carried out using different types of soils to study their potassium (K) supply capacities and K dynamics under a ryegrass-rice rotation system, with a view to providing scientific basis for the research on mechanism of soil supplying K and reasonable K control. Results show that both aboveground biomass and K uptake of the crops in the group without K treatment (NP) were the highest in fluvo-aquic soil (FS), which was followed by yellow cinnamon soil (YCS), then red soil (RS), while in the group with K, fertilization, no significant difference (p>0.05) between the soils was observed in aboveground biomass and a decreasing order of YCS > FS > RS was in terms of K uptake. Treatment NPK was 55.6%, 45.2% and 23.2% higher than treatment NP for red soil, yellow cinnamon soil and fluvo-aquic soil, respectively, in biomass and 368.8%, 166.8% and 74.5%, higher, respectively in K uptake. In the ryegrass growth season, the concentrations of water soluble K and exchangeable K in treatment NP decreased in all the soils. The concentration of non-exchangeable K decreased significantly in fluvo-aquic, but remained almost unchanged in the early season, rose in the middle season and dropped in the late season in yellow cinnamon soil and red soil. Soil K was much higher in treatment NPK than in treatment NP, but varied in a similar pattern in all the three soils, regardless of treatments and K forms. During the rice growing period, in treatment NP, water soluble K in all the soils did not change much soil exchangeable K declined first and then rose, but non-exchangeable K showed a reverse trend, while in treatment NPK, soil exchangeable K rose significantly in the early, and declined in the middle and then turned slightly back again in the late period, whereas soil water soluble K and non-exchangeable K showed a rising and then falling trend. To sum up, In treatment NP, K consumption was higher in all the soils and in all the periods of the rotation, thus decreasing both soil water soluble K and exchangeable K and in turn triggering release of non-exchangeable K. Application of K fertilizer increases the concentrations of water soluble K and exchangeable K and the ratio of K transformed into non-exchangeable K, thus effectively improving K supply capacity of the soil, and eventually increasing the yield of ryegrass and rice significantly.