Nitrogen fertilizer requirements of cereals following grass

The effect of increasing rates of nitrogen (N) fertilizer on the yield response of 3 or 4 consecutive winter cereal crops after ploughing out grass was investigated at six field sites on commercial farms in England and Wales. Amounts of N required for an economically optimum yield (> 3 kg of grain for each kg of fertilizer N applied) ranged from 0 to 265 kg ha -1 and were dependent on soil N supply, but not on crop yield. Optimum N rates were large (mean 197 kg N ha -1 ) at three sites: two sites where cereals followed 2-year grass leys receiving low N inputs ( < 200 kg N ha -1 ), and at one site where a cut and grazed 4 -year ley had received c. 315 kg N ha -1 of fertilizer N annually. At the other three sites where 4 and 5-year grass leys had received large regular amounts of organic manures (20-30 t or m 3 ha -1 ) plus fertilizer N (c. 300 kg ha - 1 each year), optimum N rates were low (mean 93 kg N ha 1 ) and consistently over-estimated by the farmer by an average of 107 kg N ha - 1 . Optimum N rates generally increased in successive years after ploughing as the N supply from the soil declined. Determination of soil C:N ratio and mineral N (NO 3 N + NH 4 N) to 90 cm depth in autumn were helpful in assessing fertilizer N need. The results suggest there is scope to improve current fertilizer recommendations for cereals after grass by removing crop yield as a determinant and including an assessment of soil mineralizable N during the growing season.