Objectives Oil tree peony is an emerging woody oil crop in China, but unbalanced fertilization has limited the seed yield and quality of oil tree peony severely. It is important to investigate the effect of fertilization on the seed yield and oil production of oil tree peony and to seek the optimum nitrogen (N), phosphorus (P₂O₅) and potassium (K₂O) application rates for its high seed yield and oil production.

Methods In this study, five-year-old Paeonia ostii ‘Fengdan’ was used in a field experiment. The amount and proportion of N, P₂O₅ and K₂O fertilizers were designed by “3414” incomplete orthogonal regression. The conventional fertilizer rates were urea 750 kg/hm², triple superphosphate 270 kg/hm² and potassium sulfate 600 kg/hm², and the four levels of N, P₂O₅ and K₂O in the treatments included 0 (no fertilization), 1(half conventional), 2 (conventional) and 3 (1.5 folds of conventional). The seed yield and oil content were measured, and empirical models were established to achieve the optimum rate for fertilizer application.

Results 1) The application of N, P₂O₅ and K₂O fertilizers improved the agronomic traits including the pods per plant, single fruit weight, fruit diameter and 100-seed weight, and the overall seed yield. Respective application of N, P₂O₅ and K₂O fertilizer increased seed yield by 283.7, 276.8 and 150.6 kg/hm², or 55.5%, 50.3% and 23.5%. The corresponding increase of net income was 7310.4, 7494.3 and 2118.9 yuan/hm², respectively, and the agronomic efficiencies were 0.96, 2.76 and 0.59 kg/kg, respectively. 2) The effect of fertilizer on yield was N > P₂O₅ > K₂O, while fertilizer agronomic efficiency and its improvement followed the order: P₂O₅ > N > K₂O. Over-application of N, P₂O₅ and K₂O fertilizer resulted in lower yield and significantly reduced economic efficiency and fertilizer utilization efficiency. The recommended amount of fertilizer treatment was found to be optimal. The maximum agrong efficiency was observed at “1”. 3) Application of N, P₂O₅ and K₂O affected oil yield by affecting the seed number and seed oil content. The highe seed number was up to 65.0% and the seed oil content was up to 32.9%, which were respectively increased by 13.0% and 12.6% compared to the control group. The oil yield was 193.3 kg/hm² under the balanced fertilization. The effects of the three fertilizers on oil production was in the order of N > P₂O₅ > K₂O. 4) There were obvious interactions among the N, P₂O₅ and K₂O fertilizers. Combined application increased fertilizer efficiency, seed yield and the oil content of seed. Over application of any of the three fertilizers would reduce seed yield. 5) Among different models to fit yield versus the amounts of N, P₂O₅ and K₂O fertilization, we found the binary secondary fertilizer efficiency model was the optimal one. According to the model, the optimum fertilizer amounts of N, P₂O₅ and K₂O were 343.2, 109.7, and 248.4 kg/hm², respectively. The optimal ratio of N, P and K fertilization was 1∶0.32∶0.72.

Conclusions Optimal fertilizer ratio and amount are required to increase the production of ‘Fengdan’ efficiently. Appropriate application of N, P₂O₅ and K₂O fertilizers can significantly improve the agronomic traits, yield, seed number, and seed oil content. The optimal amounts of N, P₂O₅ and K₂O application are 343.2, 109.7, and 248.4 kg/hm² and the optimal ratio of N, P and K fertilization was 1∶0.32∶0.72.