Objective This study investigated the effects of different magnesium fertilizer application rates on the yield and fruit quality of Bingtang sweet orange, and determined the optimal application rate, aiming to provide a theoretical basis for improving the yield and quality of Bingtang sweet orange.

Methods A two-year field positioning experiment was conducted in Xinping County, Yuxi City, Yunnan Province, from 2022 to 2023, using 8-year-old Bingtang sweet orange trees. Five magnesium fertilizer application rates (MgO) were established: CK (0 kg/hm²), MgO60 (60 kg/hm²), MgO120 (120 kg/hm²), MgO180 (180 kg/hm² MgO), and MgO240 (240 kg/hm²). At harvest, soil samples from 0-20 cm and 20-40 cm depths were collected to determine soil physicochemical properties, and fruit samples were collected for fruit yield and quality indices. The entropy weight-TOPSIS method combined with hierarchical cluster analysis was employed to determine the optimal Mg fertilizer application rate.

Results In 2022, Mg fertilizaion had no significant effect on the yield of Bingtang sweet orange. In 2023, however, the yields under MgO180 and MgO240 treatments reached 50.8  t/hm²  and 53.4  t/hm², respectively, which were significantly higher than those of the CK. In 2022, the MgO240 treatment significantly increased fruit soluble solids content and the sugar–acid ratio compared with CK; in 2023, the MgO60 and MgO120 treatments showed significantly higher soluble solids content and sugar–acid ratio than CK. Linear regression analysis showed that Mg fertilizer application rate was significantly positively correlated with yield (except in 2022) and fruit soluble solids content (except in 2023). With increasing Mg fertilizer application rate, leaf Mg concentration of autumn shoots significantly increased in 2023. Mg fertilizer application rates had no significant effect on soil exchangeable Mg content. The entropy weight-TOPSIS comprehensive evaluation showed that the overall performance ranking of treatments in both years was MgO240 > MgO180 > MgO120 > MgO60 > CK. Hierarchical cluster analysis indicated that MgO240 and MgO180 were grouped into the same cluster. The output value, net income and output-input ratio of the MgO180 and MgO240 treatments were higher than those of the other treatments.

Conclusions Mg fertilizer application significantly increased leaf Mg concentration, yield, and fruit quality of Bingtang sweet orange. Based on the two-year experimental results combined with entropy weight-TOPSIS and hierarchical cluster analysis, the recommended Mg fertilizer application rate (MgO) for 8-year-old Bingtang sweet orange trees in the latosolic red soil region of Yuxi, Yunnan Province, is 180～240 kg/hm².