Synergistic Effects of Water, Fertilizer and Oxygen Regulation Based on Fuzzy Evaluation in Custard Apple Cultivation

To explore the mechanisms by which water, fertilizers, and dissolved oxygen affect the physiological growth and yield quality of custard apple, this study aims to optimize water–fertilizer–oxygen coupling regulation schemes for custard apple in dry hot valley regions through a multi-level fuzzy evaluation method, thereby addressing issues such as soil compaction and reduced aeration caused by long-term water and fertilizer drip irrigation. The experiment was conducted on custard apple in a dry, hot valley area, employing orthogonal and quadratic regression-orthogonal designs. Three factors were set at multiple levels: irrigation amount (60–100% ETc), fertilization rate (1500–1900 kg·ha−1), and dissolved oxygen concentration (6–10 mg·L−1). Custard apple development, production, and attributes were assessed. The two-year trial from 2023 to 2024 demonstrated that the new shoots, leaf area, and net photosynthetic rate of plants treated with W3F2O1 (100% ETc, 1700 kg·ha−1 fertilization rate, and high oxygen 6 mg·L−1) and W3F3O2 (100% ETC, 1500 kg·ha−1 fertilization rate, and high oxygen 8 mg·L−1) were significantly superior to those of W1F1O1 (60% ETc, 1900 kg·ha−1 fertilization rate, and high oxygen 6 mg·L−1), with a single-plant yield of 10.31 kg, and increases in diameter and length of 31.6% and 27.6%, respectively (p < 0.05); quality indicators were also optimal under W3F3O2 (100% ETC, 1500 kg·ha−1 fertilization rate, and high oxygen 8 mg·L−1) treatment, with soluble sugar and vitamin C levels increasing by 17.3% and 29.9%, respectively, compared to the control. Using a multi-level fuzzy evaluation to comprehensively evaluate the water–fertilizer–oxygen coupling, the comprehensive productivity of custard apples was significantly improved by optimizing the root zone microenvironment. It is recommended that dry hot valleys adopt an optimized range of 82.5–100% ETc irrigation, 1650–1847.86 kg·ha−1 fertilization, and 7.4–9.25 mg·L−1 dissolved oxygen, providing a theoretical basis for precise irrigation and sustainable cultivation of tropical fruit trees.