Impacts of long-term lack of potassium fertilization on different forms of soil potassium and crop yields on the North China Plains

This study assessed changes in soil potassium (K) pools, soil K-bearing minerals, and crop yields without K-fertilizer application for 25 years on the North China Plains. Two long-term field experiments (over 25 years) in a wheat-maize rotation system were conducted: Experiment 1 was a randomized complete block experiment of three rates of fertilizer nitrogen (N: 0, 270, 540 kg ha(-1) year(-1)) and phosphorus (P: 0, 36, 72 kg ha(-1) year(-1)). Experiment 2 had an orthogonal treatment structure with three rates of N and P fertilizers and straw (0, 112.5, 187.5 kg N ha(-1) year(-1); 0, 40, 80 kg P ha(-1) year(-1); and 0, 2250, 4500 kg straw ha(-1) year(-1)). The three different tillage methods were as follows: T1, conventional tillage with straw incorporation; T2, conventional tillage with straw cover on soil surface; and M, minimum tillage. Three replicates were designed for each treatment in both experiments. Different forms of K, including total K, mineral K (MK), slowly available K (SK), and rapidly available K (RK), were quantified by standard methods. Crop yield was calculated based on random quadrants' data. The results from both experiments indicated that the depletion of RK promoted the conversion of MK to SK by K release from hydromica. Crop yields in all the treatments with P fertilizer were significantly higher than those in N alone and CK treatments in both long-term experiments, suggesting that P fertilizer had a greater contribution in improving crop yield than N fertilizer after long-term lack of K fertilization. The average values of different forms of K and crop yield in exp. 2 were clearly higher than those in exp. 1. After 11 years of high rates of straw without chemical K fertilization, the average soil RK peaked. However, 5 years later, it showed a negative K budget and crop yield began to decline. For over 25 years, fertilization and tillage managements had little impact on soil TK and MK contents because of the high feldspar and hydromica contents in the soil. The weathering of K-bearing minerals and fertilization significantly changed soil SK and RK contents, and hydromica was the main source of SK. P fertilizer was more important in increasing crop yield than N fertilizer and straw with long-term zero K inputs. To maintain high crop yield (12.02-13.82 t ha(-1) year(-1), the total yield of summer maize and winter wheat) and improve the fertilizer utilization efficiency, 150-228 kg K ha(-1) should be applied alongside 187.5 kg N ha(-1) year(-1), 80 kg P ha(-1) year(-1), and 4500 kg straw ha(-1) year(-1) every 16 years in this production system.