Objectives This study analyzes the relationship between aboveground biomass and maize phosphorus (P) content under different P application rates to construct a critical P dilution curve for fast diagnosis of maize P nutrition.

Methods Field experiments were conducted in Guanzhong Plain, Shaanxi Province in 2019 and 2020, using the maize cultivars Zhengdan 958 (ZD958) and Yuyu 22 (YY22) as test materials. The four treatments of P₂O₅ application rates were 0, 60, 120, and 180 kg/hm². The aboveground parts of maize plants were sampled at jointing, tasseling, filling, and maturity stages to determine dry matter accumulation, P concentration, and yield. The data obtained in 2019 were used to construct the critical P dilution curve model and the corresponding P nutrition index (PNI). The data obtained in 2020 were used to verify the models' accuracy.

Results  Increased P application rate (P<0.05) increased yield, aboveground biomass, and P concentration of summer maize, but there was no significant difference (P>0.05) between the two varieties. With increasing P application rate, the yield of summer maize increased at first and later decreased, with P120 treatment recording the highest. The yield effect equation showed that the average theoretical maximum yield of the two varieties of summer maize in the years under study corresponds to a P application rate of 110.2 kg/hm². P application had no significant effect (P>0.05) on the number of ears per hectare but increased the kernel number per ear and 100-grain weight. However, the effect of the application on kernel number per ear was greater than that on 100-grain weight. The aboveground biomass was in the order P0<P60<P180<P120, with no significant difference (P>0.05) between P180 and P120 treatments. Maize P concentration increased with increasing P application rate; however, it decreased with the advancement of growth and the accumulation of aboveground dry matter by weight. Based on the aboveground dry matter weight and P concentration of maize plants in 2019, the critical P dilution curve model for summer maize was established as: P_c=8.11DM^−0.22 (R²=0.886). The RMSE and n-RMSE between the calculated and the measured plant P concentrations in 2020 were 1.146 and 18.23%, showing high accuracy. The PNI values calculated with the critical P concentration curves increased and decreased across the growth stages, and this was augmented by increased P application at each growth stage. PNI was also positively correlated with relative P uptake (RP_upt), relative aboveground biomass (RDW), and relative yield (RY).

Conclusions The constructed critical P dilution curve model and P nutrition index model (PNI) could help predict P nutrition conditions of summer maize at different growth periods. Therefore, it is useful for instant P nutrition diagnosis and optimal P fertilization in summer maize.