Optimization of fertigation scheduling for drip-irrigated watermelon based on its yield, quality and fertilizer and water use efficiency

Abstract: In order to improve yield, quality, irrigation water use efficiency and fertilizer partial productivity of watermelon, a precise management of water and fertilizer is necessary to enhance its production under greenhouse. In this experiment, drip irrigation method was used in loam soil of Guanzhong, China. The experiment was designed as a two-factor factorial with orthogonal rotation combination. The irrigation amount consisted of 0.49, 0.55, 0.70, 0.85, and 0.91 times the watermelon evapotranspiration, and the fertilizer application rate was 0.50, 0.65, 1.00, 1.35, and 1.50 times the fertilizer application rate calculated by target yield method. There were a total of 16 treatments and each replicated three times. The weight of each index was calculated by subjective analytic hierarchy process, objective entropy weight method and combined weighting method based on game theory. A comprehensive evaluation model was established by analyzing the effect of drip fertilization on the comprehensive evaluation value through approximate ideal method. The results showed that the largest fruit cross and longitudinal section diameter (17.80 and 18.50 cm), the lower sugar content and total vitamin C content (1.18% and 12.56 mg/g) and yield (48 901.04 kg/hm2) were observed in the treatment with irrigation level of 0.70 times the evapotranspiration of watermelon and the fertilization level of 1.00 times the target yield. The smallest fruit cross and longitudinal sections diameter (15.29 and 15.14 cm) were obtained in the treatment of irrigation with 0.85 times the evapotranspiration and fertilization with 0.65 times the target yield. The lowest yield (30 171.88 kg/hm2) was observed in the treatment of irrigation with 0.70 times evapotranspiration and fertilization with 0.50 times the target yield. Irrigation water use efficiency and fertilizer partial productivity showed a downward trend with the increase of irrigation and fertilizer levels. The highest irrigation water use efficiency was in the treatment of irrigation with 0.49 times evapotranspiration and fertilization with 1.00 times the target yield. The highest fertilizer partial productivity was in the treatment of irrigation with 0.55 times evapotranspiration and fertilization with 0.65 times the target yield. The comprehensive evaluation indexes of watermelon revealed a downward parabolic graph in relation to irrigation and fertilization levels. When the irrigation amount was constant, the comprehensive evaluation indexes increased and then decreased with the increase of fertilization level. Moreover, a similar trend was observed with changing irrigation level under constant fertilization level The highest comprehensive evaluation indexes of watermelon (0.74) were obtained in the irrigation level at 0.73 times the evapotranspiration of watermelon and the fertilization level at 1.03 times the target yield. In addition, the irrigation and fertilization levels synergistically affected the comprehensive evaluation indexes of watermelon. The comprehensive evaluation indexes were high for the treatment of higher irrigation with higher fertilization level and lower irrigation level with lower fertilization level. The effect of irrigation level on the comprehensive evaluation indexes of watermelon was larger than that of fertilization. Thus, irrigation level at 0.73 times the evapotranspiration of watermelon and fertilization at 1.03 times the target yield were recommend for the drip fertilization of watermelons in Guanzhong soil where the water and fertilizer resources are relatively abundant. In areas where water resources are relatively scarce, the fertilization level should be appropriately decreased.