Potassium assessment of grain producing soils in North China

Agricultural land around the world is often potassium (K) deficient, and decreasing soil K availability is a global challenge for grain producers. In China, negative K budgets are ubiquitous and K depletion is accelerating. Previous studies have mainly focused on the effect of insufficient K fertilizer on single soil types and cropping systems but have not evaluated the effects of balanced fertilizer application on K budgets. In this study, soil K budgets of grain producing soils and the effects of K fertilizer application on crops under different cropping systems and soil types were evaluated from 1992 to 2006. Two treatments each at nine locations in North China were replicated four times, four plots received nitrogen (N) and phosphorus (P) fertilizer application exclusively, and the other four plots received N, P, and K fertilizer application. Basic soil properties, available K, slowly available K, crop rotations, crop K absorption, and yield were monitored and evaluated and K budgets and K use efficiencies were calculated. Average annual K absorption by crops with K fertilizer application was 230.2 kg ha−1 K2O, and the average annual deficit was 46.8 kg ha−1 K2O. Long-term application of K fertilizer increased available soil K on all nine sites by an average of 35.7 mg kg−1. Annual K absorption by crops averaged 99.0 kg ha−1 in the northeast region, 181.2 kg ha−1 in the north-central region, and 220.9 kg ha−1 in the northwest region. Annual K fertilizer use efficiency in the northeast (39.9%) and north-central (38.8%) regions were higher than in the northwest (17.8%). Correlations between annual K budgets and K use efficiency with fertilizer application, crop rotation, and soil type were weak. High spatial variability and low correlation between variables is a result of the complex mechanisms of K cycling. Research directions are proposed which would contribute to a better understanding of these mechanisms and processes and how they affect agronomic and economic objectives.