Nitrogen effects of green manuring on organic cabbage crops

In organic vegetable growing, nitrogen fixing green manure (GrM) play a key role in nitrogen supply for the crops. We investigated in two field experiments the influence of cutting and soil incorporation of GrM on nitrogen delivery for the proceeding crop cabbage. As a standard procedure, the GrM was mulched just before soil incorporation. In the first experiment the cutting regimes during growth of a grass-clover GrM (cultivated from Sept. to May; twice mulched or mown with removing oft the biomass from the field), had no effect on cabbage yield. However cabbage yield was 24% lower than the mulched or mown plots, when no cutting was performed during GrM growth. In a second experiment using a not frost resistant Phacelia-GrM or an over-wintering grass-ley (growing period Aug. to May), 45 kg soil mineral nitrogen was prevented from leaching. Phacelia augmented cabbage yield by 13% as compared to the yield from unfertilized, bare field plots. In contrast, cabbage yields were reduced by up to 10% after grass-GrM. At crop harvest, 15% of the nitrogen contained in the Phacelia biomass was recovered in the cabbage plants. Mulching and subsequent soil incorporation of the GrM within one to two days lead to similar or even better nitrogen efficiency as compared to more labour intensive practices.