Effects of Interposing Sugar Beet on the Nitrogen Response of the Following Wheat Crop

Nitrogen response experiments were conducted on pairs of wheat fields at ten farms in East Anglia, UK, in 1984–1985 and 1985–1986, 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 1·4 to 4·3% and were associated with significant differences in soil mineral N (NH₄-N and NO₃-N) between sites. Average soil mineral nitrogen (N_min) to 90 cm in October after sugar beet was 34 kg N ha^-1, significantly less (P=0·03) than the 58 kg N ha^-1 after wheat. However, the net difference between N_min in October and N_min plus crop N in April was +25 kg N ha^-1 after beet, significantly more (P=0·002) than the +2 kg ha^-1 after wheat, suggesting some mineralisation from the tops over winter. There were no evident differences in net N mineralisation during spring and summer, and by harvest there was no significant difference due to previous crop in wheat yield with nil N or with ample N; mean optimum yields were 8·2 t ha^-1 after beet and 7·8 t ha^-1 after wheat. The mean optimum amount of fertiliser N (determined from a fitted linear plus exponential function) was 188 kg ha^-1 after beet, not significantly different 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. © 1997 SCI