On-farm trials of optimal fertilizer recommendations for the maintenance of high seed yields in winter oilseed rape (Brassica napus L.) production

Mineral fertilizer plays an important role in the maintenance of high rapeseed yields during oilseed rape (Brassica napus L.) production. On-farm experiments at 60 sites throughout the Yangtze River Basin in China were conducted for 2years to evaluate the effects of the recommended fertilization rates (180kgNha(-1), 90kg P(2)O(5)ha(-1), and 120kg K(2)Oha(-1)) on the rapeseed yield, and three indices of soil nutrient supply were adopted to estimate the yield responses to fertilizer recommendation. The results showed that the average contributions of nitrogen (N), phosphorus (P) and potassium (K) fertilizer to seed yield were 40.4, 23.1 and 11.5%, respectively. Compared with the farmers' fertilization practices (FFP), the recommended fertilization significantly enhanced the seed yield by 29.1% on average. The relationships between plant-available soil nutrient supply, seed yields of the nutrient omission treatments and the yield responses to fertilization were significant, rather than the results of soil testing. With plant-available soil nutrient supplies and seed yields increasing, the yield responses and fertilizer use efficiencies decreased. When the seed yields of no-N and no-P treatments were less than 2000kgha(-1) and 1500kgha(-1), the recommended fertilizer rate was beneficial for achieving high seed yield and high efficiency. As defined in our study, the recommended fertilization rates could be used as the regional mean optimal rates (RMOR) of fertilization for winter oilseed rape in the Yangtze River Basin. Partial adjustments according to crop nutrient uptakes and seed yields of the nutrient omission treatments will be feasible for the sites where they were nonresponsive to RMOR.