To increase the use efficiency of potassium (K) fertilizer, special attention was paid to the dynamics of soil K in the root zone and non-root zone. Difference in K dynamics between yellowish red soil and yellow cinnamon soil under rapeseed (Brassica napus L.)rice (Oryza sativa L.) rotation was studied using a rhizobox system. Results showed that soil water soluble K (Sol-K) and exchangeable K (Ex-K) in the root zone of both soils were reduced in the early stage of rapeseed growth. Along with plant growth and K uptake, soil Sol-K in the inner (0-20 mm to root zone), middle (20-40 mm) and outer (40-60 mm) compartments of the non-root zone of yellowish red soil migrated towards the root zone. As a result, soil Ex-K was transformed into Sol-K. The changes in soil Sol-K and Ex-K in the non-root zone of yellow cinnamon soil were similar to yellowish red soil, and soil non-exchangeable K (Nonex-K) in the root zone also decreased significantly. In the early stage of rice growth, waterlogging promoted diffusion of soil Sol-K from non-root zone to root zone and transformation of Ex-K into Sol-K. Along with the growth of rice and K uptake, soil Ex-K in each compartment of yellowish red soil decreased significantly. Soil Sol-K and Ex-K in the yellow cinnamon soil declined to a certain extent, and then remained unchanged, while soil Nonex-K kept on decreasing. It revealed that the plants first absorbed K in the root zone, of which K reserve was replenished by a gradual diffusion of K from the non-root zone. The closer to the root zone, the greater the contribution to K uptake by plants. Within one rotation cycle, Ex-K and Sol-K in yellowish red soil were the main forms of K available to the plants, and little Nonex-K could be absorbed. However, in the yellow cinnamon soil, Nonex-K was the main form of K available to the plants, followed by Ex-K and Sol-K.