Long-term potash application and wheat straw return reduced soil potassium fixation and affected crop yields in North China

In light of China’s mineral potassium (K) deficit, it is imperative to improve K fertilizer use efficiency and conserve soil K. This study explored the effects of 15 years (1993–2007) of continuous potash application and wheat straw return on the K fixation capacity of cultivated soils in North China. Soil was collected from a number of long-term test sites, with different rates of potash application and wheat straw return and under different cropping systems. The results showed that long-term continuous potash application and straw return significantly reduced soil K fixation capacity. In addition, soil K fixation increased with the K concentration of the treatment solution. Soil K fixation levels were higher under double cropping systems than for single cropping systems. Soil K fixation capacity showed a clear zonal distribution pattern across North China; the soil K fixation capacity gradually increased from west to east. When the K concentration of the treatment solution was 1000 mg L−1, the average K fixation of soils fertilized only with N and P at experimental sites from Northwest, North-central and Northeast China were 359, 443, and 469 mg kg−1, respectively. The pattern of soil K fixation capacity across the different regions of North China matched the pattern of the timing of significant yield responses to K applications in the different regions. This research suggests that there should be a requirement for straw return as well as K fertilization to optimise K supply to crops during production.