Desorbable quantity and desorption intensity of soil K and their buffer power are important indices in evaluation of soil K supply capacity. An indoor soil K desorption experiment was carried out to explore soil K desorption characteristics and their eigenvalues,and pot and field experiments as well to study relationships of the eigenvalues with crop response in yield. A desorption model for soil cadmium was used to fit 7 types of soils in 32 K desorption experiments. Results show that the model fitted well with correlation coefficients reaching the level of significance. In the pot and field experiments with water spinach(Iponoea aqatica)and flowering Chinese cabbage(Brassica parachinensis),desorption eigenvalues,like quantity of desorbable soil K(Q),initial K concentration in the soil solution(C_li)and concentration/buffer power ratio(C₁/∂),were related to total K uptake or yield(fresh weight)of the vegetables,showing apparent parabola curves. Furthermore,the pot experiment with water spinach indicated that the parabola relationships fitted all the soils regardless of soil type,suggesting that Q,C_li and C₁/∂ indexes indeed reflect K supply capacity of soils. Based on the parabola relationships and 90% of the maximum yield of the investigated vegetables under field conditions,critical indices of Q and C_li were worked out,to be 104 mg kg^-1 and 31 mg L^-1 respectively for flowering Chinese cabbage,and 207 mg kg^-1 and 82 mg L^-1 respectively for water spinach. If soils have Q or CC_li determined well beyond the critical index,they do not need Kfertilization. The findings indicate that the proposed model can be cited as a new method to evaluate availability and critical index of soil K.