Effects of combined application of N, P and K on yield and quality of watermelon in gravel-mulched fields

【Objectives】Unbalanced fertilization is a limiting factor to yield and quality of watermelon in gravel-mulched fields. So it has important practical significance that studying effect of fertilization on the yield and quality of watermelon, verifying the optimum nitrogen, phosphorus and potassium application rates for higher yield and quality of watermelon and improving production efficiency of gravel-mulched fields. 【Methods】 A late mature watermelon cultivar ‘Longkang 9’ was used as material, and micro-spot trials were carried out in gravel-mulched fields under condition of full plastic film mulching by “311-B” D-saturation optimum design of three factors. 【Results】According to the data obtained, mathematical regression models were set up based on the yield and quality respectively by quadratic regression analysis, in which N, P and K were independent variables and the watermelon yield and quality parameters were dependent variables. The model analysis shows that N, P and K significantly influence the watermelon yield and quality parameters, the influenced order of different factors on the watermelon yield is PNK and on the watermelon quality is PKN, which indicates that P is the primary factor to the yield and quality of watermelon in gravel-mulched fields. When N, P and K reach to 231.61, 204.34 and 191.78 kg/hm2, the marginal yield is 0. When single fertilization is high or low, it is not conducive to the formation of watermelon production, and due to the interaction effect, the combined application of N, P and K has a stronger role in promoting of the yield. The interaction effects among these factors are positive on the watermelon yield with the influenced order as N and P P and K N and K, with the biggest interaction area being N at 115.0-341.8 kg/hm2, P2O5 at 86.5-298.2 kg/hm2 and K2O at 101.6-298.4 kg/hm2. The model was used to make decision, the fertilization scheme of yield of 50 t/hm2 is N at 193.19-238.18 kg/hm2, P2O5 at 162.41-206.72 kg/hm2 and K2O at 164.85-214.53 kg/hm2, and fertilization scheme of the quality grade larger than 85 is N at 195.96-239.44 kg/hm2, P2O5 at 167.86-207.52 kg/hm2 and K2O at 212.87-267.62 kg/hm2. 【Conclusions】 In this experiment, when the fertilizer rates of N at 195.96-238.18 kg/hm2, P2O5 at 167.86-206.72 kg/hm2 and K2O at 212.87-214.53 kg/hm2, the yield reaches to 50 t/hm2 and the quality grade larger than 85. The most suitable ratio of N∶P2O5∶K2O is 1∶0.86∶0.98.