What is a green manure and should I use one?
A green manure (or cover crop) is a plant grown not to harvest but to be cut down and returned to the soil, or left as a living cover over winter.
What it does:
Prevents nutrient leaching. Bare soil over winter loses soluble nutrients — especially nitrogen — washed down beyond root depth by rain. A cover crop takes those nutrients up and holds them in living tissue, releasing them when it breaks down. This is often the largest single benefit.
Protects soil structure. Bare soil is battered by rain, which causes capping, erosion and compaction. Cover protects the surface and the roots maintain channels.
Adds organic matter when incorporated or left as a mulch.
Fixes nitrogen, if you choose a legume. Clover, field beans, vetch and lupins host rhizobia bacteria in root nodules that convert atmospheric nitrogen into a plant-available form — genuinely adding nitrogen rather than just recycling it.
Suppresses weeds by occupying the ground.
Breaks up compaction. Deep-rooting types such as fodder radish and rye penetrate compacted layers.
Choosing one:
Overwinter nutrient holding — grazing rye, field beans, winter tares.
Nitrogen fixing — clovers, vetch, field beans. Note that legumes fix most nitrogen when allowed to grow on before flowering.
Fast summer gap-filling — buckwheat, phacelia. Phacelia is also excellent for pollinators if allowed to flower.
Important practical points:
Cut before it sets seed, or you have created a weed problem.
Avoid brassica green manures (mustard, fodder radish) if you grow brassicas, as they share club root.
Allow time before sowing. Incorporated green material temporarily locks up nitrogen as it decomposes, so leave two to four weeks before sowing a crop.
No-dig gardeners cut and leave the residue on the surface instead of digging it in.