Objectives This study aimed to investigate the optimum combination of nitrogen (N), phosphorus (P), and potassium (K) fertilizers for the growth and effective component accumulation of Astragalus membranaceus var. mongholicus in northern Shaanxi.

Methods A field experiment was conducted in Zizhou County, Yulin City, Shaanxi Province, using two-year-old A. membranaceus var.mongholicus. A three-factor quadratic saturation D-optimal design was employed, and 10 NPK combination treatments were used in the experiment. The seedlings were transplanted in August 2021, and plant samples of A. membranaceus var.mongholicus were collected on July 25, August 14, September 3, September 23, and October 13, 2023 for the investigation of plant height, root length, root diameter, aboveground and underground biomass, and the content of the effective component astragaloside IV. The effects of fertilization treatments were evaluated using the membership function method and frequency analysis.

Results Significant differences were observed in the plant height, root length, root diameter, and above-ground and below-ground biomass of Astragalus membranaceus (Huangqi) across 10 fertilization treatments. The highest values for all final growth indices of Astragalus membranaceus (measured on October 13th) were achieved under the N4P4K2 treatment. Compared with the N1P1K1 treatment(no N, P, and K fertilizers) , N₄P₄K₂ (with application rates of N 150 kg/hm², P₂O₅ 225 kg/hm², and K₂O 79.41 kg/hm²) led to notable increases of 51%, 69%, and 52% in plant height, root diameter, and root length, respectively. The accumulation of dry matter in the above-ground and under-ground parts increased by 117% and 120%, respectively, and the root thickening was also significantly greater than that under all treatments except N₄P₂K₄ (with application rates of N 150 kg/hm², P₂O₅ 79.41 kg/hm², and K₂O 225 kg/hm²). The overall promoting effects of fertilizer combinations on the growth indices and the astragaloside IV content at the five harvest stages of Huangqi followed the order: combined application of N, P, and K fertilizers > combined application of any two of these fertilizers > single application of one fertilizer. The astragaloside IV content in the roots of Huangqi increased significantly with growth duration across all treatments. By integrating four indicators—root length, root diameter, below-ground biomass, and astragaloside IV content—with weights of 24%, 26%, 20%, and 30% respectively to calculate the comprehensive membership function values for each treatment, the N₄P₄K₂ treatment emerged with the highest score.

Conclusions The optimal fertilizer ratio for the rapid growth and high astragaloside IV content in Huangqi is N: 103.4−128.3 kg/hm², P₂O5 143.9−172.8 kg/hm², and K₂O 149.2−164.1 kg/hm², with a ratio of N﹕P₂O₅﹕K₂O as 1﹕1.35−1.39﹕1.28−1.44.