Nitrogen uptake in organically managed spring sown lupins and residual effects on leaching and yield of a following winter cereal

Three unfertilized spring sown species of lupins (Lupinus angustifolius) and peas (Pisum sativa) were compared in terms of N fixation and subsequent leaching under a following winter barley crop in an organically managed rotation. Fallow plots were included to assess the potential weed burden and the ability of the sown crops to compete with weeds when no herbicides were applied. Although peas out-yielded lupins (5.4 t compared with 3.5 t grain respectively), the yellow lupin (Wodjil) fixed more N than peas (180 compared with 120 kg N/ha) and all three lupins had higher protein contents (> 30%) than peas (22%). Winter leaching amounted to > 50 kg nitrate-N/ha from winter barley, regardless of whether it followed treatments which were previously fallow or cropped with legumes. There were no significant differences in leaching between the lupin species. Leachate in the first 350 mm of drainage under winter barley, following the different legume species, exceeded the European Union limit for nitrate in drinking water in all treatments. Yields of winter barley, grown without fertilizers or herbicides following legumes, were not significantly different (mean 4 t/ha), but there were higher levels of P and K in the grain compared with the amounts made available from the previously fallow soil. At this site in SW England, the crops grew well and our results suggest that lupins could provide a useful break crop in an organic arable cropping rotation and an alternative source of home-grown, high protein feed.