Reduced Chemical Fertilizer Combined with Vermicompost Application Affects the Growth, Yield, and Quality of Watermelon

Continuous application of chemical fertilizers in plant cultivation can lead to the deterioration of the soil environment, resulting in reduced crop yield and quality. Currently, organic fertilizers, such as vermicompost, can partially replace chemical fertilizers and maximize yields while maintaining soil fertility. However, the effects of chemical fertilizers combined with vermicompost on watermelon ( Citrullus lanatus ) yield and quality are unclear. A field experiment was carried out on the watermelon cultivar Lihua No. 6. Six treatments were applied: no fertilizer (CK, control, 0N–0P–0K), 100% chemical fertilizer CF, 5.4N–1P–5.4K (256, 47, and 255 kg·ha −1 ), 75% chemical fertilizer + 25% organic fertilizer A1, 5.4N–1P–5.4K (192, 35, and 191 kg·ha −1 ) + 2250 kg·ha −1 vermicompost, 50% chemical fertilizer + 50% organic fertilizer A2, 5.4N–1P–5.4K (128, 24, and 127 kg·ha −1 ) + 4500 kg·ha −1 vermicompost, 25% chemical fertilizer + 75% organic fertilizer A3, 5.4N–1P–5.4K (64, 12, and 64 kg·ha −1 ) + 6750 kg·ha −1 vermicompost, and 100% organic fertilizer (A4, 9000 kg·ha −1 vermicompost). Indices related to the growth, yield, and quality of watermelons were determined. Compared with CK, chemical fertilizer alone or in combination with organic fertilizers significantly increased growth parameters (plant height and leaf area) and chlorophyll content. The five fertilizer treatments enhanced the single fruit weight, yield, and biomass. In addition, the yield of reduced chemical fertilizer plus organic fertilizer was comparable to that of watermelons treated with CF. Compared with CF, the fertilizer treatments, especially the 1:1 mixture of chemical and organic fertilizer (A2) group, had elevated fruit-soluble solids and soluble sugar content, and reduced organic acid levels. Therefore, a combination of 50% chemical fertilizer and 50% organic fertilizer can effectively enhance the yield and quality of watermelons. These findings have important implications for guiding the management of watermelon fertilization and development of sustainable agriculture.