Potassium-fertilizer management in winter oilseed-rape production in China

Optimal potassium (K) fertilization is beneficial for oilseed-rape (Brassica napus L.) yield and quality. However, the discrepancy between the high K demand of winter oilseed rape and low soil fertility and insufficient potassium input has limited the sustainable development of oilseed-rape production. A series of on-farm experiments in the key winter oilseed-rape domains of China was conducted from 2004 to 2010 to evaluate K-fertilizer management for winter oilseed rape. Currently, the average NH4OAc-extractable K content in the 020cm soil layer is 89.1 mgkg1 indicative of slight deficiency. In addition, farmers in China usually fail to use sufficient K fertilizer in oilseed-rape production, the average mineral-potassium-fertilizer input in 2010 being only 35kg K ha1, far lower than the recommended rate of potassium for winter oilseed rape. Adequate potassium fertilization significantly raises seed yield. The average yield-increase rate for the major production regions due to K-fertilizer application was 18.5%, and the average K fertilizeruse efficiency 36.1%. Based on the negative correlation between yield response to potassium fertilization and available soil K content, a soil-K-test index was established for winter oilseed rape with a threshold value for NH4OAc-extractable soil K of 135 mgkg1. When available soil K-content is below this threshold value, more K fertilizer should be applied to achieve high seed yield and to increase soil fertility. The major challenge for K-fertilizer management in winter oilseed-rape production in China will be to guide farmers in the different regions in making reasonable use of K fertilizer through soil K-testing technology in order to maintain both seed yield and soil fertility.