The experiments were carried out on the long-term positioning experiment platform of Dongping Agricultural Research Institute since 2010. The split-plot experimental design was used with different P2O5 input in the main plots and different K2O input in the sub-plot and different N input in the sub-sub plots. The P2O5 values were 0, 90, 120, and 150 kg hm-2, which were represented as P0, P1, P2, and P3, respectively. The K2O was 0, 180, 240, and 300 kg hm-2, expressed in terms of K0, K1, K2, and K3, respectively. The N was 0, 180, 240, and 300 kg hm-2, denoted as N0, N1, N2, and N3, respectively. From 2020 to 2021, Denghai 605 was used as the experimental material to deeply analyze the effect of nutrient application rate on dry matter and nitrogen concentration of summer maize leaves, aimed to build a critical nitrogen dilution curve for the nutrient growth stage of summer maize leaves, and explore the feasibility of diagnosing and evaluating nitrogen nutrition status of summer maize with nitrogen nutrition index model under different fertilizer application rates. The results showed that the dry matter and nitrogen concentration of summer maize leaves prior anthesis increased with the increment of nitrogen, phosphorus, and potassium fertilizer application. The nitrogen concentration in leaves decreased with the growth process and the accumulation of leaves dry matter, showing a dilution phenomenon. The variations of leaves dry matter and nitrogen concentration could be divided into two groups: nitrogen limitation and non-nitrogen limitation. Further, the curve model of critical nitrogen concentration at the vegetative growth stage of summer maize leaves under different phosphorus and potassium levels was constructed: NLC0 = 2.745 LDM-0.529, NLC1 = 3.245 LDM-0.334, NLC2 = 3.557 LDM-0.290, NLC3 = 3.639 LDM-0.286. Nitrogen nutrition index was calculated based on the critical nitrogen concentration dilution curve, which was highly significantly related to the relative leaves dry matter and the relative grain yield. Combining the linear plus platform relationship between nitrogen nutrition index and the relative leaves dry and the relative grain yield, the crop nitrogen status could be well evaluated under the conditions of nitrogen limitation and non-nitrogen limitation. Therefore, the critical nitrogen concentration and nitrogen index at vegetative growth stage of summer maize can effectively predict the critical nitrogen concentration and characterize the nitrogen nutrient status of summer maize.
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