Green Manures and Soil Fertility

What is a green manure?

A green manure is a cover crop grown mainly to be cut and returned to the soil while still green, or left as residue on the surface. The term describes the purpose rather than the species. The same plant can be a cover crop in one setting and a green manure in another.

The idea is to add fresh organic material, build soil structure and, in some cases, supply nitrogen.

Green manures have been used for centuries in farming and gardening, and they remain useful on small farms that cannot easily buy in fertility.

How do legumes supply nitrogen?

Legumes such as clovers, vetches and peas live with bacteria in their roots that capture nitrogen from the air. When the plants decay, part of that nitrogen becomes available to later crops. The amount depends on the species, the growth, the soil and the weather, and it is not a guaranteed amount.

Seed of some legumes is sold with an inoculant, a bacterial product matched to the species. Ask your seed supplier or extension office whether inoculation applies to your choice.

A common mistake is assuming a legume always adds nitrogen. Poor growth, poor soil or a failed inoculation can produce little benefit, so watch how the plants look.

• Legumes: nitrogen supply
• Grasses: roots and residue
• Mixes: several benefits, harder to manage
• Check inoculation needs for legumes

What do grasses and other crops add?

Cereals and grasses grow deep, dense roots that build organic matter and hold soil, and they scavenge leftover nutrients after a crop. Brassicas and other broadleaf types may help loosen soil or suppress weeds. Mixes combine benefits but need care at termination.

Choose species by goal, season and your climate, and ask your extension office what performs well locally.

Deep roots of some species can reach nutrients lower in the soil, and when the plants decay those nutrients return near the surface for the next crop. Mixing a grass with a legume is a common pairing, since the grass holds soil and the legume supplies nitrogen, though the mix needs a careful cutting time.

When should I cut or turn it under?

Timing matters. Many green manures give the most benefit when cut at a stage before seed set, often around flowering for legumes, according to extension guidance for the species. Cutting too late leaves tough residue and risks weeds from seed; cutting too early gives less benefit.

After cutting, allow time before planting, because fresh residue breaks down and can affect seedlings. Our guide on cover crop termination covers methods in more detail.

Cut with a mower or shears and leave the material to wilt before working it in or leaving it as mulch. Watch the weather so the material does not form a wet mat.

Where do green manures fit in a rotation?

Plan them into gaps: after an early crop, before a late crop or in a full fallow season in a bed that needs rest. Keep the plant families in mind so that the green manure does not host problems for the next crop.

On small farms, green manure often shares space with market crops, so map each bed's schedule for the year and note what follows what. Our crop rotation guide explains the planning.

Write the plan on paper before the season starts, and update it when something changes, so the rotation stays clear from year to year.

• After early crops
• Before late crops
• Full season in beds that need rest
• Avoid sharing a family with the next crop

What are the costs and limits?

Costs include seed, tools, any inoculant, water and the time the bed is not producing for sale or the table. Compare them with what you would spend on fertilizer and compost, bearing in mind that benefits build over seasons and vary by soil.

Green manure is not a replacement for a soil test. It does not fix a pH problem or supply every nutrient, and it can use soil water, which matters in dry regions.

Think of green manure as one tool among several, alongside compost, mulch and rotation, rather than as the only way to feed soil.

How do I plan and record it?

Start with one bed. Choose a species, sow by local frost dates, cut at the recommended stage and note the results on the next crop. Keep records of dates, species and observations so you can adjust.

Ask your extension office or conservation district for advice and local trial results, which are often more useful than general claims.

If a trial bed does well, expand it next season. If it does poorly, adjust species, timing or seeding before giving up.

Last reviewed: November 12, 2026. Written by the Homestead & Small Farm Editorial Team. See our editorial policy.