Effects of combined application of P and K on the yield，quality and nutrient utilization of watermelon in gravel-mulched field

On the base of applying nitrogen 200 kg/ha, a field experiment with three potassium rates (0, 130, 260 kg/ha) and four phosphorus rates (0, 90, 130, 170 kg/ha) was conducted to study the effects of combined application of P and K on the yield, quality and nutrient utilization efficiency of watermelon in Gravelmulched field. A late mature watermelon cultivar, Longkang 9, was used as material. The results show that the yield and quality of watermelon increase with the increase of P and K application rates. With P170 treatment, the single fruit weight and yield of watermelon increase by 13.75% and 43.79% respectively, and the average sugar, Vc and protein contents by 13.75%, 9.48% and 11.90% compared with P0. With the K260 treatment, the single fruit weight, yield of watermelon increase by 9.22% and 7.54%, and the average sugar, nitrate contents increase by 5.86% and 11.24% compared with K0. Significant positive interactive effect of P and K exists in the photosynthesis, yield and nutrient utilization efficiencies. Under low potassium rates, all the tested parameters keep increasing within a certain range of P application rate, while under high potassium levels, with the increase of phosphorus application rate, better effects are obtained with individual P application than individual K application. The highest photosynthetic rate, yield and nutrient utilization efficiency were achieved with K260P170 treatment. Therefore, the optimum fertilizer application rates were P2O5 170 kg/ha and K2O 260 kg/ha at the base of applying N 200 kg/ha, hence, the most suitable ratio of N∶P2O5∶K2O is 1∶0.85∶1.3 for good yield and quality of watermelon in this soil condition.