Legume-based green manures (LGMs) are crops that are grown with the specific purpose of improving soil quality and consequently the long-term productivity of crops. Although the traditional focus has been on the supply of nitrogen (N) to the system, they have a wide range of potential benefits that include improving soil quality, reducing soil erosion and increasing the biodiversity of farmland. LGMs are a key component of organic farming systems where the use of synthetic N fertilizers is not permitted. However, increases in the cost of inputs, concerns about environmental impacts of intensive use of agrochemicals, and the recently announced measures for the ‘greeningã of the European Common Agricultural Policy have led to renewed interest in the use of LGMs more widely. In Europe, the legumes in LGMs may be annual or perennial plants, grown on their own or more often as part of crop mixtures with a range of other crop types such as grasses or brassicas. The legumes most commonly grown are the clovers (Trifolium spp.), particularly red and white clover. Other legumes that may be grown to suit particular local goals or constraints include Medicago spp. (lucerne (alfalfa) and black medic), trefoils (Lotus spp.), vetches (Vicia spp.), lupins (Lupinus spp.), other minor forage legumes and grain legumes. To maximize fertility building in organic farming systems, LGMs are grown in place of cash crops for some of the crop rotation. In more intensive systems, LGMs may be grown for short periods between phases of regular crop production. This chapter reviews the use of LGMs in Europe and considers factors that affect N fixation in them and the transfer of fixed N to following crops. It examines how they can be integrated into practical rotational cropping systems and whether the economics of this makes the use of LGMs profitable. However, LGMs will not be agronomically or economically viable in all systems, and in these cases other types of green manures may be more appropriate. As demand for multifunctional agricultural systems grows, and is increasingly required by European agricultural policies, so does the potential for greater use of LGMs.
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