Combining application of chemical fertilizer with manure significantly increased potassium availability in an alkaline soil

China has an extreme shortage of potassium (K) resources, despite the overuse of fertilizers that has been commonly reported in Chinese intensive cropping systems and has resulted in nutrient imbalance and adverse effects on the environment. Application of fertilizers with manure may increase nutrient recycling and use efficiency, but little is known about the potential mechanism of it on soil K fixation and supply capacity, especially in an alkaline vegetable cultivated soil. Here, we report the results of a six-year field experiment with treatments of chemical fertilizer (F), manure (M) and combination of chemical fertilizer with manure (60% F + 40% M). The FM treatment lowered the K surplus by 58.6 and 69.4 kg K ha−1 compared with the F and M treatments, respectively, hence maintaining an optimal K input–output balance. Compared to the control (CK), the fertilization practices (F, M and FM) significantly increased soil exchangeable K, slowly exchangeable K, and K supply capacity, while decreasing soil K fixation. The F, FM and M treatments significantly increased the potential buffering capacity of K by 65%, 77%, and 74%, thus promoting a higher soil K supply capacity by 88%, 122%, and 92%, respectively. The FM treatment had the highest K supply capacity. This response suggests that combining chemical fertilizers with manure significantly increased soil K availability by both supplementing K directly and changing the soil K-bearing minerals and K fixation and then promoting soil K supply capacity, which may ensure sustainable use of K and other nutrient resources in the long run.