Which green manure for which problem?

A green manure is not a fertiliser (except legumes): it feeds soil life and improves structure, so pick it by the problem. Wireworm: mustard, which it avoids, but mustard draws flea beetles, so not next to cabbage or radish. Compacted clay: oilseed radish or daikon 5–6 weeks before snow, roots left in the ground for the frost to split the soil. Short of nitrogen: legumes (vetch, peas, lupin), the only ones that take nitrogen from the air; cut at the bud stage, sow right after the snow before tomatoes and potatoes. Weeds: rye (sow the next crop 2 weeks after it). Diseases after tomatoes or onions: oats, best vetch with oats. Nematodes: phacelia, which fits before any crop. Short of phosphorus: buckwheat.