Nitrogen fertilizer requirements of cereals following grass

The effect of increasing rates of nitrogen (N) fertilizer on the yield resp onse of 3 or 4 consecutive winter cereal crops after ploughing out grass wa s 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 wer e dependent on soil N supply, but not on crop yield. Optimum N rates were l arge (mean 197 kg N ha(-1) at three sites: two sites where cereals followed 2-year grass levels receiving low N inputs (< 200 kgN ha(-1)), and at one site where a cut and grazed il-year ley had received c. 315 kgN ha of ferti lizer N annually At the other three sites where 4 and 5-year grass ley 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 (m ean 93 kg N ha(-1)) and consistently over-estimated by the farmer by an ave rage of 107 kg N ha(-1). Optimum N rates generally increased in successive years after ploughing as the hi supply from the soil declined. Determinatio n of soil C:N ratio and mineral N (NO3N + NH4N) to 90 cm depth in autumn we re helpful in assessing fertilizer N need. The results suggest there is sco pe to improve current fertilizer recommendations for cereals after grass by removing crop yield as a determinant and including an assessment of soil m ineralizable N during the growing season.