The nitrogen fixing legumes and cover crops in general should be tilled in at first flowering. The legumes, if inoculated, will yield the highest number of nodules in soil that is not rich in the micorrhyzzae. For the soil bacteria to associate with the roots they need to be in the rhyzosphere of the root. Flowering is when nitrogen fixation peaks. After flowering energy is transferred to the seed and some of the benefit is lost. Green manures are a fast maturing crop and by their nature they can be weedy. All legumes can fix nitrogen, but some are just better at it than others. Red clover, cowpea, hairy vetch are the most common ones. Other green manures like buckwheat, oats, winter rye, winter wheat add biomass. Some cover crops and plants are good for breaking through hard soil like daikon and fodder radish.
All cover crops can protect soil from erosion and supress weeds.
Some of the scavenger crops like mustard cabbage scavenge left over nutrients and sun hemp, India Mustard, and certain types of marigolds can reduce nematodes.
https://extension.umd.edu/sites/default ... rCrops.pdf
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