Practice

Green Manure

Also known as: green manuring, crop-as-fertilizer

Growing a crop specifically to be incorporated back into the soil (rather than harvested) as a fertility input — the cover crop chopped, mowed, or tilled in to release its nitrogen, carbon, and minerals to the next cash crop. The term emphasizes the *fertility* function; closely overlaps with [[cover-crop|cover-cropping]], which emphasizes the *soil-cover* function. Most cover crops can serve as green manures and vice versa.

Green manure is what a cover crop becomes when it dies and decomposes in place. The plant grew, captured solar energy, fixed nitrogen (if legume) or scavenged minerals (if not), built root mass and biomass — and now that captured energy and chemistry returns to the soil as next season’s fertility input.

The term is older than “cover crop”; English agricultural literature has used “green manure” since at least the 18th century. Modern usage tends toward “cover crop” for the function in the field and “green manure” when emphasizing the fertility effect.

What green manures contribute

Nitrogen — legume green manures (vetch, clover, field pea, fava bean) can release 50–200 lb N per acre to the following crop, depending on species and biomass
Carbon / organic matter — biomass decomposes into stable humus; multi-year green-manuring measurably builds [[soil-organic-matter|soil organic matter]] over time
Mineral cycling — deep-rooted green manures (rye, sunflower, comfrey) bring up nutrients from below the cash-crop root zone
Soil structure — root channels and decomposing biomass improve aggregate stability, water infiltration, and aeration

How it differs from compost

Compost is decomposition managed off-site (in a pile) and applied as a finished material
Green manure is decomposition managed in place; the plant grows where it will decompose

Both are valuable; they complement rather than substitute. Compost provides finished, stable nutrients; green manure provides fresh active biology and a flush of more-rapidly-available nitrogen.

Classic green-manure crops

Buckwheat — fast (30–45 days), summer slot, modest fertility contribution, excellent for pollinator support
Crimson clover — overwintered, substantial N, beautiful spring bloom
Hairy vetch — overwintered, very high N contribution (often 100+ lb/acre)
Field peas / fava beans — early-spring or late-fall, cold-tolerant, good N
Rye + vetch — the classic overwintering combination; the rye supplies carbon and mass, the vetch supplies N
Comfrey (perennial) — not a cover crop but functions as continuous green manure; harvested as a chop-and-drop mulch around fruit trees

Practical home-garden integration

The simplest green-manure rotation for a home garden:

Year-round bed-not-in-use cover: as soon as a bed finishes a crop, sow buckwheat (warm season) or rye+vetch (cold season).
Mow or chop before flowering (especially before seed set).
Tarp for 3 weeks to fully kill regrowth (no-till) or till in shallow (turn-based gardens).
Plant the next cash crop into the resulting bed.

The bed coming through this cycle is measurably better than one left bare or fallow.

Sources

Managing Cover Crops Profitably (SARE, 3rd ed. 2007)
Robert Parnes, Fertile Soil: A Grower’s Guide to Organic and Inorganic Fertilizers (agAccess, 1990)

Rooted in life.

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Cover Crop cover crops chopped or tarped into the soil become green manure; the two terms overlap heavily

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