Legume species and management for stockless organic farming

In systems of organic farming without livestock, with no nutrients imported in feed and bedding, or manure to transfer nutrients within the farm, supplying adequate nutrients for crop growth, particularly nitrogen (N), is a major challenge. Two identical experiments compared four legume species (red clover (Trifolium pratense L.), white clover (Trifolium repens L.), lucerne (Medicago sativa L.) and sainfoin (Onobrychis viciifolia Scop.)) as N providers over a two-year conversion period for a stockless organic rotation. After ploughing-in the legumes, two successive wheat crops were grown to measure N supply and yield response. The two experiments (1992 to 1995; 1993 to 1996) were on a silty clay loam soil in eastern England. Mean accumulated above-ground N over the two years, in cut and mulched foliage, was greatest in red clover (mean 502 kg ha(-1)) and least in sainfoin (201 kg ha-1), with lucerne (497 kg ha-1) and white clover (408 kg ha(-1)) lying in between. Differences in wheat grain yield were poorly related to these figures, and neither legume species nor management had a consistent effect on wheat grain yield. Grain N concentration was consistently greatest following white clover (by between 0.06 and 0.38%). It is suggested that this was because the pattern of mineralization of the white clover residues favoured later uptake by the wheat, which influenced grain N concentration more than yield. Low grain N content is a major constraint to the greater use of U.K.-grown organic wheat for breadmaking. Growing white clover rather than the preferred red clover or lucerne fertility-building crops in stockless organic rotations could make a significant contribution to improving grain quality in winter wheat.