RELATION BETWEEN SOIL MINERAL NITROGEN BEFORE SOWING AND OPTIMUM NITROGEN-FERTILIZATION IN ONIONS

To study the relationship between the amount of soil mineral nitrogen before sowing of onions (N-min) and the optimum amount of nitrogen fer tilizer (N-opt), 36 multi-level fertilizer nitrogen trials were conduc ted between 1975 and 1982. For 26 trials N-opt was within the studied range (0-200 kg N ha(-1)) and could be estimated using a quadratic res ponse function. A significant linear relationship between N-opt and N- min before sowing was only found when N-min in the layer 0-30 cm was c onsidered. The same 26 trials were analysed together using a quadratic and a linear exponential response function. However, with both method s the yield predicted from N-min did not prove to be superior to a fix ed nitrogen application rate of about 125 kg of nitrogen ha(-1). A ver ification pointed out that the relationship overestimated the optimum amount of fertilizer nitrogen found in 8 independent multi-revel ferti lizer nitrogen trials. A fixed rate of 100-125 kg of nitrogen ha(-1) y ielded better results. The possible reasons for the absence of a stron g relationship between the amount of soil mineral nitrogen before sowi ng and the optimum amount of nitrogen fertilizer are discussed.