A field trial was carried out to compare the performance of three species of overwintering leguminous green manure crops with a fallow. White clover produced the greatest amounts of nitrogen (45–60 kg/ha). Subterranean clover produced the greatest amount of dry matter (1600 kg/ha). Trefoil had the deepest root system but did not nodulate. The concentrations of soil nitrate were significantly higher under the white clover sward than under the other legume and fallow swards.