Objectives The relationship between phosphorus use efficiency and soil phosphorus fertility is of great importance to quantify the optimum soil phosphorus level for the best management strategy of phosphorus resources. The judicious phosphorus management can help to achieve high crop production and meanwhile to minimize the potential risk of P loss to environment.

Methods Using wheat ‘Xiaoyan 22’ and maize ‘Zhengdan 958’ as tested crops, and use the loess soils subjected to diverse fertilizer treatments from a 25 years long-term experiment, we conducted a pot experiment with 5 sets of soils containing ascending Olsen-P values of 3.90, 15.00, 23.60, 35.70 and 50.00 mg/kg (labeled as F1, F2, F3, F4 and F5 in turn) . Five P₂O₅ rates of 0, 30, 60, 90 and 120 kg/hm² were applied for each soil. After harvest, the crop grain yield and above ground biomass were weighed, plant samples were pulverized for P content and accumulation analysis; soil samples of 50 g/pot were collected and the total P and Olsen-P contents in soil were analyzed; the responses of PUE to soil Olsen-P levels under winter wheat−summer maize cropping system were estimated.

Results Compared with P0, the application of phosphorus significantly increased the grain yield of winter wheat by 52.2%−119.7% and summer maize by 94.7%−212.7% in F1 soil, but showed no significant effects in the other soils. After two crops harvesting, no significant increases in soil total P content were observed in all the treatments except for at P₂O₅ rates of 60 kg/hm², 120 kg/hm² in F2 and 120 kg/hm² in F5. However, soil Olsen-P contents were increased by −4.08%−434.69%, 26.49%−112.77%, 6.74%−48.24%, 4.07%−43.65% and −4.84%−28.29%, respectively, on soils of F1, F2, F3, F4 and F5 over the corresponding P0 treatments. At the P application rates of 30, 60, 90 and 120 kg/hm², the PUE of wheat were significantly increased in linear with the increase of soil initial Olsen-P level (P < 0.05) , the coefficients of determination were 0.996, 0.899, 0.760 and 0.820, respectively. At P30, the PUE of maize was increased followed a pattern of parabola with the soil increase of initial Olsen-P, and the PUE reached 100% at the soil Olsen-P content of 12.32 mg/kg and its maximum of 155.24% appeared at soil Olsen-P of 33.63 mg/kg; at P60, P90 and P120, the PUE of maize increased significantly and linearly with the soil Olsen-P contents and reached its maximum value at Olsen-P of 12.22 mg/kg, 16.64 mg/kg and 14.39 mg/kg, respectively, and then kept a constant value. The soil Olsen-P dependent PUE for the entire winter wheat−summer maize cropping system was much like that of summer maize. The PUE reached 100% at the soil Olsen-P of 17.97 mg/kg at annual P application rate of 60 kg/hm² (wheat 30 kg/hm² and maize 30 kg/hm²) and the maximum value of PUE reached 131.51% at Olsen-P of 40.11 mg/kg. Under the same soil phosphorus level, the PUE of wheat and maize decreased significantly with the increase of P application rate, especially when the P application rate was higher than P₂O₅ 60 kg/hm².

Conclusions The optimum soil Olsen-P contents are from 17 mg/kg to 40 mg/kg for winter wheat, and from 13 mg/kg to 34 mg/kg for summer maize in the loess soil of Guanzhong area. From the viewpoint of the whole winter wheat−summer maize cropping system, the soil Olsen-P content should be maintained at the range of 17−40 mg/kg, and annual phosphorus application rate of P₂O₅ 60−120 kg/hm² was recommended.