Reducing Uneven Fruit Ripening and Improving the Quality of Durian (<i>Durio zibethinus</i> Murr.) Fruit Using Plastic Mulching Combined with Polyhalite Fertilizer

Uneven fruit ripening (UFR) is currently causing a decrease in the quality and value of “Ri 6” durian fruit. The soil moisture and nutrient (K, Ca, and Mg) levels present during the fruit development stage are the two main factors affecting UFR in durian fruit. However, measurements that can be used to determine the decrease in the UFR rate of durian remain unknown. Therefore, this study sought to evaluate the impact of plastic mulching (PM) and polyhalite fertilizer (PH) on improving the UFR rate and quality of durian fruit. A field study was conducted at three different durian orchards in the Vietnamese Mekong Delta (VMD) throughout two seasons (2022–2023 and 2023–2024). We used PM a month before fruit harvesting, combined with PH applied during the fruit development stage. Four treatments were used: (T1) control; (T2) PM, plastic mulching a month before durian fruit harvesting; (T3) PH, polyhalite fertilizer application (3 kg tree⁻¹ year⁻¹); and (T4) PM + PH, polyhalite fertilizer application (3 kg tree⁻¹ year⁻¹) and plastic mulching a month before durian fruit harvesting. The farmer’s fertilization practice (450 g N–450 g P–450 g K per tree⁻¹ during the fruit development period) was used in all treatments. Parameters such as soil physicochemical properties, fruit quality, and leaf mineral nutrient concentration were investigated at the harvesting stage. The results show that using PM + PH decreased soil moisture (>15%) but increased the concentrations of K, Mg, and Ca in both soil and durian leaves, thereby reducing the UFR rate (>80%) compared with the control. Additionally, applying PM + PH increased the aril proportion (>18%) and total soluble solids (approximately 5%) in durian fruit in comparison with the control. In conclusion, combining PM and PH improved the UFR rate and durian fruit quality. Therefore, we recommend that farmers apply these methods to their durian orchards to decrease physiological disorders and enhance fruit quality, thus contributing to achieving sustainable durian production in the VMD.