Effects of long-term K application on crop yield and soil K content and forms in rice-wheat rotation were studied. The results showed that long-term K application markedly increased crop yield, the balance of the available K in purple soil was maintained and the ability of soil to supply K was raised. A joint application of rice straw ( 7 500 kg hm^-2a^-1) and chemical K fertilizer ( about K2O 140 kg hm^-2a^-1) is recommended to keep the balance of K in purple soil.