Optimal rate of nitrogen fertilizer application for organic onion farming in Hokkaido

Abstract

To increase the yield of organic onion (Allium cepa L.) crops in Hokkaido, we conducted experiments on application of organic nitrogen fertilizers. The crop was cultivated under conditions of varying range of soil nitrogen fertility under the Japanese Agricultural Standard of Organic Agricultural Products. The main results obtained are as follows: (1) The onion yield was significantly higher after a single nitrogen fertilizer application in spring than after a split application (2/3 applied in the previous autumn and 1/3 applied in the spring of the current year). (2) There was no difference in onion yield when organic fertilizers with nitrogen content of ≥40 g kg⁻¹ were used for the single fertilizer application in spring. However, the onion yield was lower after application of fertilizers with nitrogen content of approximately 20 g kg⁻¹ than after application of fertilizers with nitrogen content of ≥40 g kg⁻¹ . (3) We used the following formula to calculate the rate of nitrogen fertilizer application essential to obtain 44 Mg ha⁻¹, the target yield of organic onion: (optimum nitrogen uptake–nitrogen uptake without nitrogen)/nitrogen fertilizer utilization rate. Using this formula, the rate of nitrogen fertilizer application was calculated to be 140 kg ha⁻¹ when the soil nitrogen concentration was 50–70 mg kg⁻¹ (according to the soil nitrogen diagnosis standard determined using the hot-water extraction method). When the soil nitrogen fertility was lower or higher than this range, it could be adjusted by increasing or decreasing the rate of nitrogen fertilizer application by 40 kg ha⁻¹.