Abstract: The potassium characteristic of rendzina soil in the sub-tropic Karst of Guangxi was studied.The results show that the soil available potassium content and available potassium release amount can reflect the soil supply potassium capacity for a season crop in the agro-ecological system of sub-tropic Karst.When the soil availble potassium content is 30～32mg/kg，the crop is very difficult to absorb potassium and the growth is limited serious ly.The main nutrient-limited factor of soil is potassium.Phosphorus，surphur，boron and zinc are not enough for crop growth. Slow pot assium content and slow release potassium content can not show the soil supply potassium capacity，but they can show the soil sustainable supply potassium capacity.Applying potassium fertilizer is good for the balance of potassium in soil and can increase the crop yield.