Maintaining Yield While Enhancing Fruit Quality and Economic Returns Through Deficit Irrigation and Potassium Optimization in Jujube (Ziziphus jujuba Mill.)

Under conditions of limited irrigation and excessive fertilizer application in the arid regions of Xinjiang, it is essential to adopt well-coordinated strategies to improve yield and crop water productivity (WP). In this study, a comparative experiment was conducted with three irrigation levels, T1 (100% crop evapotranspiration, ETc), T2 (75% ETc), and T3 (50% ETc), combined with three potassium application rates, K1 (540 kg ha−1), K2 (360 kg ha−1), and K3 (180 kg ha−1). The objective was to investigate their effects on the yield, quality, and economic benefits of jujube trees. Limited irrigation amounts significantly affected the photosynthetic characteristics, growth parameters, and ETc of jujube trees, whereas potassium fertilizer levels primarily regulated fruit development and yield formation. Compared with full irrigation, mild deficit irrigation caused a moderate yield reduction but significantly enhanced economic returns due to the improved water productivity and fruit quality. In contrast, severe water deficit led to substantial decreases in growth parameters and economic benefits by 12.87–45.70% and 81.69%, respectively. Potassium application demonstrated a significant threshold effect, with the K2 treatment showing greater improvements in fruit quality indices, including reducing sugars, vitamin C, and other key quality parameters, compared to the K3 treatment. Based on hierarchical–grey relational analysis, the combination of 75% ETc and 300 kg K ha−1 was identified as the optimal water–potassium management strategy. The net profit was 29,199 CNY. The benefit–cost ratio increased to 3.63, and the WP improved by 16.17% compared to full irrigation. Thus, this study provides an important theoretical basis and technical support for water-saving and quality-improving cultivation of jujube trees in arid regions.