Effects of Controlled-Release Fertilizers on Nitrate Content and Yield of Japanese Mustard Spinach.

Controlled-released fertilizers (CRFs) are classified into 3 different types; slow-released fertilizers, coated fertilizers, and fertilizers containing nitrification inhibitors. CRFs have been reported to show higher fertilizer effects than conventional fertilizers such as urea and ammonium sulfate. Also, CRF application can reduce nitrate accumulation in some vegetables, which may cause methemoglobinemia by excess ingestion of those vegetables. Therefore, effects of application of different CRFs on nitrate content and yield of Japanese mustard spinach (Brassica rapa) were evaluated through 2 different pot experiments.

              A Typic Dystrochrept (Tokyo, Japan) was used. For the first experiment, the same amount of N was applied based on recommended N rate for the spinach from different fertilizers. Five treatments were no fertilizers added (CR), urea (UR), slow-released fertilizer (SF), resin-coated fertilizer (CF), fertilizer containing nitrification inhibitor (NI). For the second experiment, the recommend and reduced rates of N was applied from different fertilizers. Ten treatments were the recommended rate (CR, SF, CF, and NI), 10%-reduced rate (SF10, CF10, and NI10), and 20%-reduced rate (SF20, CF20, and NI20). After harvest, dry weight, nitrate content, nitrate accumulation of plants were analyzed.

              In the first experiment, there were no significant differences in the dry weights among all treatments. However, SF and IN treatments had lower nitrate accumulation than other treatments. The CRFs used in this study may have equivalent fertilizer effects compared with readily soluble fertilizer. In the second experiment, the dry weights in SF and SF10 were comparable. Also, SF, SF10, and SF20 showed significantly lower nitrate accumulation than that in UR. It appeared that SF reacted differently from CF and NI in the soil because it was urea condensation product. It was found that SF could cause low nitrate accumulation in plants due to slow release capability hence gradual plant absorption, without negatively affecting the plant yield.