Nitrate contents in leaf vegetables were strongly influenced by types, supply and time of fertilizers, and especially, relationship between nitrogen application and nitrate accumulation was closely related. Therefore, the regulation of nitrogen supply and cultivation condition can highly reduce nitrate accumulation of leaf vegetables. This work was performed to investigate changes in yields and nitrate contents of lettuce and spinach by different fertilization. Nitrate accumulation in leaf was absolutely dependent upon chemical fertilizer (NPK) application. For no-fertilization and single compost supply, nitrate was detected below 1,500 mg per kg in lettuce and 1,000 mg per kg in spinach. Leafy production in two vegetables was determined by application amounts of compost rather than chemical fertilizer. When supplied over 4 MT of compost, yields of lettuce and spinach showed over 9,500 kg per 10a and 2,500 kg per 10a, respectively. However, an excess compost supply could result in an accumulation of salt and phosphorus in soil rather than an amendment of soil fertility. It was thought that the adequate application of chemical fertilizer and compost could bring about an increase of vegetable yields, a diminishment of nitrate accumulation and the maintenance of soil fertility.

Table 1. Yields and leaf-nitrate contents of lettuce and spinach by different fertilization Fertilization Lettuce Spinach Yield(kg/10a)Nitrate(mg/kg)Yield(kg/10a)Nitrate(mg/kg) No-fertilization 6,715 910 1,629 111 NPK 7,966 2,283 2,471 1,749 NPK+Comp. 1ton 8,186 2,055 2,497 1,426 NPK+Comp. 2ton 8,948 2,411 2,514 1,621 NPK+Comp. 4ton 10,091 2,818 2,664 2,356 Compos 1ton 10a-1 7,972 732 1,710 71 Compos 2ton 10a-1 8,158 827 1,747 301 Compos 4ton 10a-1 9,527 1,215 5,556 974 Compos 6ton 10a-1 9,756 1,487 2,539 952

Key words : Nitrate, yield, lettuce, spinach