Net changes in soil and crop nitrogen in relation to the performance of winter wheat given wide-ranging annual nitrogen applications at Ropsley, UK

The effects of eight rates of nitrogen (N) application (0-245 kg/ha) on the performance of winter wheat over five seasons (1991-1995) on a long-term held experiment (established 1978) at Ropsley (UK) are described. In each of the five seasons, N was withdrawn from replicate plots in order to study the residual effect of fertilizer. N applications in excess of 140 kg/ha left significant residues as soil mineral N (SMN) in the autumn which, despite some loss over-winter, had a significant effect on the yield and N offtake of the subsequent crop. The amount of N carried over was equivalent to 8-20% of the fertilizer N and was observed at N applications up to 40 kg/ha lower than the optimum rate (c. 200 kg/ha). Part of the unrecovered N was also considered to contribute to the long-term build-up of fertility at the site. The results suggest that restrictions on N use to below the optimum will reduce leachable N, but may have an impact on soil fertility and future crop productivity. In addition, the rate of N applied to preceding crops should be taken into account when formulating fertilizer advice on retentive soils.