Nitrogen response experiments were conducted on pairs of wheat Ðelds at ten farms in East Anglia, UK, in 1984È1985 and 1985È1986, such that the previous crop of one of the pair was sugar beet (with tops ploughed in) and the other was wheat. Topsoil organic matter contents ranged from 1É4 to 4É3% and were associated with signiÐcant di †erences in soil mineral N and (NH 4
-N NO 3 between sites. Average soil mineral nitrogen to 90 cm in October after N) (N min ) sugar beet was 34 kg N ha~1, signiÐcantly less (P \ 0É03) than the 58 kg N ha~1 after wheat. However, the net di †erence between in October and plus N min N min crop N in April was ]25 kg N ha~1 after beet, signiÐcantly more (P \ 0É002) than the ]2 kg ha~1 after wheat, suggesting some mineralisation from the tops over winter. There were no evident di †erences in net N mineralisation during spring and summer, and by harvest there was no signiÐcant di †erence due to previous crop in wheat yield with nil N or with ample N ; mean optimum yields were 8É2 t ha~1 after beet and 7É8 t ha~1 after wheat. The mean optimum amount of fertiliser N (determined from a Ðtted linear plus exponential function) was 188 kg ha~1 after beet, not signiÐcantly di †erent from the optimum of 197 kg N ha~1 after wheat. It is concluded that amounts of fertiliser N should be similar following sugar beet and following wheat.