What Are Green Manures and How Do They Work?

A green manure is any plant grown with the specific intention of cutting it down and digging it into the soil while still green. Rather than composting the material off-site and returning it later, you are composting it in place, feeding soil organisms directly and returning nutrients to exactly the root zone where they will be needed.

The practice transforms what would be a season of lost productivity into active soil building. Every week a green manure grows, it is manufacturing organic matter, capturing sunlight energy, and — if it is a legume — pulling free nitrogen from the air.

How Green Manures Improve Soil

When you cut and incorporate a green manure, soil bacteria and fungi begin breaking down the plant tissue. This decomposition process releases nutrients gradually in plant-available forms. It also produces compounds that bind soil particles into stable aggregates — the crumbly structure that good vegetable soil is known for. The process takes three to six weeks depending on temperature, which is why incorporation timing matters.

Nitrogen-Fixing Green Manures

Legume green manures have Rhizobium bacteria in nodules on their roots. These bacteria fix atmospheric nitrogen, which the plant stores in its leaves and stems. When the material is incorporated, the nitrogen is mineralised by soil bacteria and becomes available to the next crop. Field beans, winter tares, clovers, and trefoil are all legume green manures with this ability.

Non-Legume Green Manures

Plants like phacelia, mustard, buckwheat, and winter rye do not fix nitrogen but contribute large volumes of organic matter. Their value is in improving soil physical structure, suppressing weeds, and feeding the soil biology that drives long-term fertility. In a no-dig system, these can be cut and left on the surface rather than incorporated, acting as a mulch that breaks down over the growing season.

Carbon-to-Nitrogen Ratio

Young, leafy green manures have a low carbon-to-nitrogen ratio and break down quickly, releasing nitrogen fast — often within two to three weeks of incorporation. Older, more stemmy material has a higher ratio and breaks down more slowly. For beds being planted with seedlings in four weeks, incorporate young growth. For beds with a longer wait, older material is fine and may provide better physical structure.

Fitting Green Manures into a Rotation

Green manures do not replace compost additions, but they reduce the quantity you need. In a standard four-bed rotation, the bed resting over winter under a legume green manure will need noticeably less feeding when it becomes the brassica or leek bed the following year. The bed coming out of an overwinter rye is physically looser and more workable than one left bare through the same period.

When Green Manures Are Not Ideal

Avoid green manures on beds needed urgently in spring, as incorporation requires a waiting period. Do not use mustard before brassicas. Be cautious with any green manure that produces abundant seed if you leave it too late to cut — phacelia and mustard can self-seed prolifically in warm conditions.