Abstract: An experiment on the long-term application of potassium (K) fertilizer and wheat straw in irrigated soils of Ningxia, northwestern China began in 1993. The objective of the experiment was to determine the effect of K fertilizer application and wheat straw return to soil on crop nutrient content and yield and the status of soil K in plowed soil layers. The results show that K fertilizer and wheat straw increase yield of wheat and maize. K fertilizer treatments increase wheat yield by 244 kg·hm^-2 and maize yield by 397 kg·hm^-2 annually. The order of all treatments effect on yield is as follows: NPK+straw > NPK > NP+straw > NP. K fertilizer has a significant effect on crop yield when it is continuously applied for 8~10 years. Significant response of yield to K application occurs earlier in maize than in wheat. Furthermore, K fertilizer and wheat straw return to soil increase potassium content in the kernel and straw of the crops. There is a higher potassium content in straw, while it in kernels accounts for only 13%~17% of the total potassium content of plant. Furthermore, K fertilizer enhances the elements contents of kernels and accelerates element absorption by kernels. However, K fertilizer reduces trace- and micro-elements in straw. The content of 0~20 cm soil available K and slowly available K in non-potassium (NP and NP+Straw) treatments for long-term application decrease. Total K content in all treatment declines at a rate of 0.8~1.2 g·kg^-1.