Sufficient and stable crop yields build the basis for feeding a growing world population. The future challenge in organic agriculture will be to improve crop yields without trade-offs between productivity and sustainability of agricultural management. In organic agriculture, nitrogen (N) supply is based on symbiotic N2-fixation (Nfix), mainly by cropping of legumes, which limits the yield potential of non-legumes. Including below-ground N, grain legumes fix 200 kg N ha-1 (60–420 kg ha-1 year-1). A grass-clover ley fixes 230 kg N ha-1 (90–470 kg N ha-1 year-1) if N transfer from clover to associated grass is taken into account and cover crops fix 80 kg N ha-1 (20–150 kg N ha-1 year-1). The direct contribution of a preceding one-year grass-clover ley to feed a succeeding non-legume crop is about 25 kg N ha-1 and the potential N contribution to the crop rotation is about 75 kg N ha-1. These values are largely variable depending on the realized Nfix on a specific site. A two-year grass-clover ley could contribute up to 50 kg N ha-1 or more if conditions are favourable. With a proportion of fodder legumes in the rotation of 33% and an optimistic Nfix of about 280 kg N ha-1 per year, the yield potential of organic cereals is 60% of conventional in high-yielding regions like central Europe.