Economically optimal nitrogen rate and soil nitrate after canola harvest.
Spring canola has high nitrogen (N) requirements to reach maximum grain yields, but excessive N applications can result in an environmental degradation. The objectives of this study were to determine the effect of N fertilization on canola grain yields, to quantify the effect of N fertilization on residual soil nitrate content and to establish relationship between residual soil nitrate and economically optimum N fertilizer application. Canola plots were established on two sites during 2 yr. The treatments were five rates of mineral nitrogen (0, 40, 80, 120 and 160 kg N ha-1) applied before sowing. Canola grain yields significantly increased with increasing N fertilizer rates. Grain increments of 554 to 1096 kg ha-1 were measured. The economically optimum N fertilizer application derived from regression equations was 88 kg ha-1 except for one site where the value was 71 kg ha-1. Residual soil nitrate content (0-40 cm) measured at harvest significantly increased with the highest N fertilizer rates. In this study, the amount of residual soil nitrate after canola harvest have been maintained at a level lower than 40 kg ha-1 when the N fertilization was based on the economically optimum N fertilizer application. Thus, an application of N fertilizer according to canola requirements has reduced the accumulation of residual soil nitrate and reduced the risk of leaching.