Effects of nitrogen fertilization on nitrate content of spinach under the production safety standard

The field experiments were carried out to determine the effects of N application rates on spinach under two soil types, the grey sandy mud soil and coastal saline soil, and to analyze the dynamics changes of soil NO3--N and the relationship between the N rates, vegetable nitrate content and yield of spinach. The nonlinear planning optimization method with constraints was used to determine the appropriate amount of N with the highest yield of spinach under the condition which the NO3--N content of spinach should meet the safe production standard. The results show that the soil NO3--N contents are increased with the increase of N application rates, and the contents are decreased with time during the growing period of spinach, especially in the grey sandy mud soil. There are linear correlations between the contents of vegetable nitrate and the N application rates, while there are quadratic correlations between the yields of spinach and the N rates. Using the relationship of the N rates and vegetable nitrate as constraint condition, and the N rates and yields of spinach as objective function, the results of the nonlinear optimization method with constraints indicate that the nitrogen fertilizer application rates for the maximum vegetable yields along with vegetable nitrate fitted pollution-free vegetable production are 154.35 and 210.30 kg/ha in coastal saline soil and grey sandy mud soil respectively.