Effects of applying magnesium sulfate and calcium nitrate on soil properties and Dien pummelo quality in Vietnam

【Objective】 The aim of the study was to investigate the effects of magnesium sulfate （MgSO₄） and calcium nitrate Ca（NO₃）₂ on soil properties and Dien pummelo quality in Vietnam， which could provide reference for scree-ning optimized fertilization scheme and fruit quality improvement in pummelo cultivation in Vietnam.【Method】 The experiment was conducted at Vietnam National University of Agriculture during 2023 and 2024. With 2.5 kg ammonium sulfate + 1.5 kg phosphate + 2.0 kg potassium as base fertilizer， 5 treatments were set as base fertilizer + 2.0 kg MgSO₄ + 0.5 kg Ca（NO₃）₂ （T1）， base fertilizer + 3.0 kg MgSO₄ + 1.0 kg Ca（NO₃）₂ （T2）， base fertilizer + 3.5 kg MgSO₄ + 1.5 kg Ca（NO₃）₂ （T3）， base fertilizer + 4.0 kg MgSO₄ + 2.0 kg Ca（NO₃）₂ （T4）， and only base fertilizer （CK） as control. After application， soil property indexes （pH， available phosphorus， exchangeable potassium， exchangeable calcium， exchangeable magnesium） and fruit quality indexes of Dien pummelo （titratable acidity content， flavonoids content， polyphenol content， total soluble solids content， vitamin C content， single fruit weight， edible portion， and juice content） were determined. Quadratic regression and correlation analysis were performed to identify the optimal application rates of MgSO₄ and Ca（NO₃）₂.【Result】 Compared to CK， T1-T4 significantly （P<0.05， the same below） increased soil exchangeable Mg and exchangeable Ca contents （except for exchangeable Ca content of T1 in 2024）. Tritratable acidity content of Dien pummelo in T1-T4 significantly reduced compared with CK in both years. However， CK maintained the highest vitamin C contents of Dien pummelo in both years （35.9 and 35.8 mg/100 g respectively）， indicating that excessive Mg and Ca might slightly inhibit vitamin C accumulation. Regression analysis showed that， the edible portion and juice content of Dien pummelo reached their peaks at 3.0-3.5 kg/plant MgSO₄. For Ca（NO₃）₂， the optimal range for total soluble solids content and juice content of Dien pummelo was 1.0-1.5 kg/plant， while the edible portion peaked at a lower range of 0.5-1.0 kg/plant. Total soluble solids content of Dien pummelo showed positive correlation with both MgSO₄ and Ca（NO₃）₂ application rates， whereas tritratable acidity content was strongly negatively correlated with MgSO₄ and Ca（NO₃）₂ application rates.【Conclusion】 The combined application of MgSO₄ and Ca（NO₃）₂ greatly enhances the soil properties and fruit quality of Dien pummelo in Vietnam. The recommended application rates are 3.0-3.5 kg/plant for MgSO₄ and 1.0-1.5 kg/plant for Ca（NO₃）₂. Monitoring is required during fertilization to avoid nutrient element imbalance in soil and vitamin C content reduction in Dien pummelo at excessive application rates.