This study aims to reveal the effects of different nitrogen-phosphorus-potassium fertilization treatments on the fine root characteristics, soil nutrients, and fruit yield of Sapindus mukorossi ‘Yuanhua’. It further analyzes the relationship between NPK ratios and these factors, providing scientific basis for establishing a precision fertilization management system for new Sapindus varieties.

Using a new Sapindus mukorossi ‘Yuanhua’ plantation as the experimental subject, a “3414” fertilization experiment design was employed. Fine root traits, soil nutrients, and fruit yield were measured to investigate responses to different NPK ratios.

Fine roots exhibited a surface aggregation pattern, predominantly distributed in the topsoil layer (0-20 cm). With increasing soil depth, fine root biomass, root length density, and average diameter all decreased progressively. Within the 0-20 cm soil layer, compared to the control, all fertilization treatments significantly increased sapindus fine root biomass, root length density, specific root length, and average diameter. Except for pH, soil organic carbon, total nitrogen, total phosphorus, total potassium, available phosphorus, available potassium, and alkali-hydrolyzable nitrogen content were higher in all fertilization treatments than in the control. Significant differences existed among fertilization treatments in their effects on Sapindus mukorossi fruit yield, with the N₂P₂K₂ treatment yielding the highest fruit production, representing a 258.67% increase compared to the control. RDA analysis indicated that soil available phosphorus content was the primary factor influencing fine root traits. Correlation analysis revealed significant positive correlations between soapberry fine root biomass and root length density with soil organic carbon, available nitrogen, and available phosphorus content. Soil total phosphorus and available phosphorus showed positive correlations with root diameter but negative correlations with specific root length. Soil pH showed a significant negative correlation with fine root biomass and negative correlations with specific root length and diameter. Structural equation modeling indicated that soil nutrient content was the primary direct driver of Sapindus mukorossi yield, while fine root traits showed no significant correlation with fruit yield.

In summary, the optimal fertilization ratio recommended for Sapindus mukorossi ‘Yuanhua’is 600 kg/hm² nitrogen, 500 kg/hm² phosphorus, and 400 kg/hm² potassium. Thus, rational application of NPK compound fertilizers can enhance soil nutrient content, alter fine root development traits, and increase Sapindus mukorossi fruit yield.