Effects of split application of nitrogen on yield and nitrogen recovery of spring-sown onions and on residual nitrogen

The recovery of nitrogen (N) by a crop and its subsequent use for production can potentially be increased by the split application of N. The significance of this strategy for onions was tested in seven field experiments on Dutch clay soils from 1991 to 1994. An experimental recommendation system consisting of split N application accounting for soil mineral N by replenishing the soil mineral N with fertilizer N up to a crop stage-dependent threshold value, was compared with the pre-sowing fixed N rate of 100 kg ha(-1) which is the current recommendation to farmers in the Netherlands. The experimental recommendation system resulted in an average application rate of 72-110 kg N ha(-1) for a three-way respectively two-way N splitting. A loss of yield was not observed. Reduction of the threshold values used in the experimental recommendation system could further reduce the total N rate without yield loss. As onion growth until the 3-4 leaf stage did not appear to be influenced by the N rate before sowing, a soil sampling before sowing can be omitted from the sampling scheme of the experimental recommendation system. N splitting did not affect the estimated N loss during the growing season, the residual N in soil or the apparent recovery rate of the N applied. However, these variables were significantly influenced by the total N rate, which indicated that N loss to the environment could only be decreased by either lowering the fixed N rate before sowing or lowering the threshold values of the experimental recommendation system. However, the latter can be advantageous because the soil mineralization in spring is taken into account thereby adjusting for possible spring N losses.