Green Manure
Part of Fertilizers & Soil Amendments
Cover crop incorporation as fertilizer — growing your own fertility.
Why This Matters
Green manure is the practice of growing a crop specifically to be chopped down and incorporated into the soil as fertilizer. It is agriculture’s most self-sufficient fertility strategy — you grow your own fertilizer using nothing but seeds, soil, sunlight, and water. No animal manure needed. No off-farm inputs. No trade goods spent. In a rebuilding scenario where livestock may be scarce and every resource is precious, green manure crops can maintain or build soil fertility entirely from within the farm system.
The concept is simple but the impact is profound. A thick stand of clover or vetch, grown over winter and turned into the soil in spring, can add 100-200 kg of nitrogen per hectare — equivalent to 10-20 tonnes of composted cow manure. It also adds organic matter, improves soil structure, prevents erosion, suppresses weeds, and feeds soil biology. No other single agricultural practice delivers this many benefits simultaneously.
Green manuring was standard practice in Roman agriculture, Chinese rice farming, and European medieval agriculture. It fell out of use when synthetic fertilizers made it seem unnecessary. In a post-industrial world, it becomes essential again.
How Green Manure Works
Nitrogen Fixation
The most valuable green manure crops are legumes — plants in the bean and pea family. Legumes form a symbiotic relationship with soil bacteria called Rhizobium. These bacteria colonize the plant’s roots, forming small nodules (bumps) visible on the root surface. Inside these nodules, the bacteria convert atmospheric nitrogen (N2) — which is 78% of the air but completely unavailable to plants — into ammonia (NH3), a form plants can use.
When the green manure crop is cut down and incorporated into the soil, all the nitrogen stored in the plant tissue (stems, leaves, roots) becomes available to the following crop as the plant material decomposes.
Nitrogen fixation rates (approximate, under good conditions):
| Green Manure Crop | N Fixed (kg/hectare) | Growing Season |
|---|---|---|
| Crimson clover | 100-180 | Fall-spring (winter annual) |
| Red clover | 80-150 | Spring-fall (biennial) |
| White clover | 60-130 | Year-round (perennial) |
| Hairy vetch | 100-200 | Fall-spring (winter annual) |
| Field peas | 50-120 | Spring-summer or fall |
| Fava beans (broad beans) | 80-150 | Fall-spring (cool season) |
| Lupins | 80-160 | Fall-spring |
| Alfalfa (lucerne) | 150-300 | Multi-year perennial |
| Soybeans | 60-120 | Summer annual |
| Cowpeas | 80-150 | Summer annual (warm climate) |
Organic Matter Addition
Beyond nitrogen, green manure crops add large quantities of organic matter to the soil. A thick stand of rye or clover can add 3-8 tonnes of dry matter per hectare. This organic matter:
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Feeds soil organisms: Earthworms, fungi, and bacteria thrive on incorporated plant material. These organisms are the engine of soil fertility — they cycle nutrients, create soil structure, and suppress disease.
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Improves soil structure: Decomposing plant material binds soil particles into aggregates, creating the crumb structure that allows roots to penetrate, water to drain, and air to circulate.
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Increases water retention: Organic matter acts as a sponge, holding 5-10 times its weight in water. Soils high in organic matter are dramatically more drought-resistant.
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Builds humus: Over time, decomposing organic matter transforms into humus — stable, long-lasting soil carbon that holds nutrients and water for decades.
Types of Green Manure Crops
Legumes (Nitrogen Fixers)
These are the primary green manure crops because they add nitrogen. Always include a legume component in your green manure mix.
Cool-season legumes (plant in fall or early spring):
- Crimson clover: Fast-growing, beautiful red flowers, excellent bee forage. Winter-kills in very cold climates (below -15°C).
- Hairy vetch: The workhorse of winter green manures. Extremely cold-hardy, vigorous growth, highest nitrogen fixation. Can become weedy if allowed to set seed.
- Field peas: Quick-growing, easy to incorporate. Less cold-hardy than vetch.
- Fava beans: Large plants producing edible beans — you can harvest some for food and turn the rest under as green manure.
Warm-season legumes (plant after last frost):
- Cowpeas: Heat-loving, drought-tolerant. Excellent for tropical and subtropical climates.
- Soybeans: Good nitrogen fixation, edible crop. Can be grown to partial maturity and turned under.
- Mung beans: Quick-growing, good for short gaps between main crops.
Grasses and Cereals (Carbon and Biomass)
Grasses add large amounts of organic matter (carbon) but do not fix nitrogen. They are excellent for improving soil structure and suppressing weeds.
- Winter rye: The most cold-hardy green manure. Grows vigorously in cool weather, produces massive root systems that break up compacted soil. Must be killed before it goes to seed or it becomes a weed.
- Oats: Quick-growing, winter-kills in cold climates (convenient — no need to actively terminate). Good for fall planting where winters are cold.
- Barley: Fast establishment, good weed suppression. More drought-tolerant than oats.
- Buckwheat: Not a grass, but grows extremely fast (flowers in 30 days, mature in 70-90 days). Excellent for quick gaps between crops. Mines phosphorus from deep soil and makes it available. Superb bee forage.
Brassicas (Biofumigation)
Brassica green manures (mustard, radish, rapeseed) release compounds called glucosinolates when incorporated into soil. These break down into isothiocyanates — natural fumigants that suppress soil-borne diseases, nematodes, and some weed seeds.
- Mustard: Fast-growing, good biomass. The biofumigation effect is strongest when plants are chopped and immediately incorporated while green.
- Daikon/tillage radish: Deep taproot (30-60 cm) that breaks compacted soil layers. The root decomposes over winter, leaving channels for water and future crop roots.
Planting and Management
When to Plant
Green manure crops fill the gaps in your crop rotation — they grow when nothing else is occupying the bed:
| Timing | Strategy | Best Crops |
|---|---|---|
| After summer harvest (fall planting) | Winter cover | Rye, vetch, crimson clover, field peas |
| After fall harvest (late fall) | Quick cover before winter | Oats (winter-kills), rye |
| Early spring (before main crop) | Quick biomass | Field peas, mustard, buckwheat |
| Mid-summer (between crops) | Short-season cover | Buckwheat, cowpeas, mung beans |
| Full-season fallow | Maximum fertility building | Clover, alfalfa, vetch (full growing season) |
Seeding
- Prepare the soil: Rake smooth, remove large debris. Green manure seeds are usually small and need good seed-to-soil contact.
- Broadcast seed: Scatter evenly by hand. Walk in parallel lines across the bed.
- Rake in: Lightly rake to cover seeds with 1-2 cm of soil. Alternatively, walk on the seeded area to press seeds into the soil (this is surprisingly effective).
- Water if dry: Seeds need moisture to germinate. If rain is not expected within 2-3 days, water lightly.
Seeding rates (grams per square meter):
| Crop | Seeding Rate | Notes |
|---|---|---|
| Crimson clover | 2-3 g/m² | Small seed, broadcast evenly |
| Hairy vetch | 3-5 g/m² | Medium seed |
| Field peas | 15-20 g/m² | Large seed, can be drilled |
| Fava beans | 20-30 g/m² | Very large seed, plant 10-15 cm apart |
| Winter rye | 10-15 g/m² | Medium seed |
| Oats | 10-15 g/m² | Medium seed |
| Buckwheat | 5-8 g/m² | Medium seed |
| Mustard | 1-2 g/m² | Very small seed, broadcast thinly |
Mix It Up
The best green manure is usually a mixture of a legume and a grass. The classic combination is rye + vetch or oats + peas. The grass provides structure (physical support for the vining legume), weed suppression (dense growth), and carbon. The legume provides nitrogen. Together they deliver better results than either alone.
Termination (Killing the Cover Crop)
The green manure crop must be killed and incorporated before planting the next food crop. Timing is critical:
When to terminate:
- Legumes: At peak flowering (maximum nitrogen content)
- Grasses: Before seed heads form (after heading, nutrients move from leaves to seeds, reducing green manure value)
- At least 2-3 weeks before planting the next crop (allows initial decomposition to release nutrients and prevent nitrogen tie-up)
How to terminate:
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Mow or scythe: Cut the crop at ground level. Leave the residue on the surface for a few days to wilt, then incorporate.
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Chop and dig: For small areas, chop the plants with a machete or hoe, then dig them into the top 15 cm of soil with a spade or fork. This is the most thorough method.
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Crimp/roll: Flatten the crop by rolling a heavy log or barrel over it. The broken stems die without being incorporated. Works well for no-till systems — plant directly through the mulch.
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Winter-kill: Choose crops that die naturally in winter (oats, buckwheat in cold climates). No active termination needed — the dead residue is simply incorporated in spring.
Nitrogen Tie-Up
When you incorporate a high-carbon green manure (mature grass, dry straw) into soil, soil microorganisms consume available nitrogen as they decompose the carbon-rich material. This temporarily reduces nitrogen available to your crops — called “nitrogen tie-up” or “nitrogen immobilization.” It lasts 2-4 weeks. Avoid this by: (1) incorporating green, immature material (high nitrogen, low carbon), (2) waiting 3-4 weeks between incorporation and planting, or (3) using a legume-heavy mix that adds enough nitrogen to offset tie-up.
Green Manure in Crop Rotation
Integration Examples
Simple rotation: Year 1: Main crop → Fall: plant rye + vetch Year 2: Spring: incorporate green manure → Main crop → Fall: plant oats + peas Repeat.
Intensive garden rotation: Spring: Early crop (peas, lettuce) → Summer: Buckwheat (6-week cover) → Fall: Late crop (kale, garlic) → Winter: Rye + crimson clover → Spring: Incorporate, plant main crop.
Fallow year recovery: If a field is exhausted, dedicate one full year to green manure: Spring: Plant clover + rye mix. Allow to grow all summer. Mow once in midsummer (stimulates regrowth). Incorporate in late fall. The following year’s crop will benefit enormously.
Building Fertility on Poor Land
New or depleted land can be brought into production using successive green manure crops:
- Year 1, Season 1: Plant buckwheat (grows on the poorest soil, mines deep nutrients, adds organic matter). Incorporate at flowering.
- Year 1, Season 2: Plant rye + crimson clover. Overwinter.
- Year 2, Spring: Incorporate. Plant another round of buckwheat or a legume-heavy mix. By mid-Year 2, the soil should support light-feeding crops (beans, lettuce, root vegetables).
- Year 3: Full production with continued green manure rotation in the off-season.
This process transforms nearly any degraded land into productive soil within 2-3 years, using nothing but seeds and labor.
Seed Saving for Green Manure Crops
In a rebuilding scenario, you must save your own green manure seed. Allow a portion of each green manure planting to go to full maturity and set seed:
- Clover: Let a patch flower and dry on the stem. Thresh by rubbing dry flower heads between hands. Seeds are tiny — store in cloth bags in a cool, dry place.
- Vetch: Allow pods to dry on the vine. Harvest when pods rattle when shaken. Thresh by beating in a bag.
- Rye/oats: Let heads mature and dry. Cut, bundle, and thresh by beating against the inside of a barrel.
- Buckwheat: Seeds mature unevenly. Harvest when most seeds are brown. Thresh and winnow.
Save 10-20% of each planting for seed production. This ensures a perpetual supply without needing to acquire seed from outside sources.