Green Manure
Part of Soil Science
Green manuring is the practice of growing a crop specifically to be cut and incorporated into the soil while still green and leafy, rather than harvested. The fresh biomass decomposes rapidly, releasing nutrients, adding organic matter, and dramatically improving soil structure. For a civilization rebuilding agricultural capacity without synthetic fertilisers, green manures are one of the highest-return practices available — they fix atmospheric nitrogen, break pest and disease cycles, prevent erosion on bare ground, and can double organic matter in depleted soils within two to three seasons.
What Green Manures Do
A well-chosen green manure delivers multiple benefits simultaneously:
| Benefit | Mechanism | Quantifiable Impact |
|---|---|---|
| Nitrogen fixation | Legumes host Rhizobium bacteria in root nodules | 50–200 kg N/ha per season |
| Organic matter | All species add biomass | 1–5 t/ha dry matter per season |
| Weed suppression | Dense canopy excludes light | 60–90% weed reduction |
| Erosion control | Soil cover prevents rain impact | Eliminates bare-soil loss |
| Pest/disease break | Different crop breaks cycles | Reduces soil-borne pathogens |
| Subsoil aeration | Deep-rooted species create channels | Improves drainage and rooting depth |
| Soil structure | Root action and organic matter | Improved aggregate stability |
Species Selection
Legume Green Manures (Nitrogen-Fixing)
Legumes should be the first choice wherever nitrogen is limiting. They host Rhizobium bacteria that fix atmospheric nitrogen into a form plants can use, transferring 50–200 kg N/ha — equivalent to 100–400 kg of urea.
| Species | Season | Days to Incorporation | N Fixed (kg/ha) | Biomass (t/ha) |
|---|---|---|---|---|
| Cowpea (Vigna unguiculata) | Warm | 45–60 | 80–150 | 2–4 |
| Lablab (Lablab purpureus) | Warm | 60–90 | 100–200 | 3–6 |
| Sunn hemp (Crotalaria juncea) | Warm | 45–60 | 80–180 | 3–5 |
| Mung bean | Warm | 45–60 | 60–120 | 2–3 |
| Common vetch | Cool | 60–80 | 80–150 | 2–4 |
| Winter pea | Cool | 60–90 | 100–170 | 2–5 |
| Faba bean | Cool | 90–120 | 120–200 | 3–6 |
| White clover | Cool/perennial | 60–90 | 100–200/yr | 2–4/yr |
| Hairy vetch | Cool | 60–80 | 90–160 | 2–4 |
Inoculant Requirement
Legume nitrogen fixation only works if the correct Rhizobium strain is present in the soil. In soils where that legume species has never been grown, the bacteria may be absent. Inoculants (dried bacteria applied to seed before planting) dramatically increase fixation. Where commercial inoculant is unavailable, incorporate soil from a field where that crop was previously grown successfully at 2 t/ha.
Non-Legume Green Manures (Biomass and Structure)
Non-legumes provide organic matter, weed suppression, and deep root action without nitrogen fixation. Use them in rotation with legumes or where nitrogen is already adequate.
| Species | Season | Key Benefit | Biomass (t/ha) |
|---|---|---|---|
| Buckwheat | Warm | Fast growth, P mobilisation | 2–4 |
| Phacelia | Cool | Excellent bee forage | 1–3 |
| Mustard | Cool | Biofumigant against soil pests | 2–4 |
| Oats | Cool | Dense biomass, weed suppression | 3–6 |
| Rye | Cool | Allelopathic weed control | 3–7 |
| Sorghum-sudan hybrid | Warm | Very high biomass | 5–10 |
| Radish (forage/tillage radish) | Cool | Deep taproot breaks compaction | 2–4 |
Timing and Incorporation
When to Cut
Cut and incorporate green manures at flowering or just before — this is when the ratio of nitrogen to carbon (C:N ratio) is optimal for rapid decomposition and nutrient release.
| Growth Stage | C:N Ratio | Decomposition Rate | N Release |
|---|---|---|---|
| Young vegetative (under 4 weeks) | 10–15:1 | Very fast | Rapid but low volume |
| Flowering | 15–25:1 | Fast | Optimal — high volume, good rate |
| Seed set | 25–35:1 | Moderate | Slower, less N per unit |
| Mature/dry | 40–80:1 | Slow | Ties up N temporarily |
Incorporate at flowering for maximum nitrogen benefit. If the field will not be planted for 6+ weeks, allow seed set for higher biomass; if planting within 2 weeks, incorporate at early flowering.
Incorporation Methods
Surface chopping and tillage: Mow or cut stems to 5–10 cm lengths with a machete or scythe. Incorporate with a plough or fork to 10–15 cm depth. Allows rapid aerobic decomposition.
Slashing and surface mulch: Cut and leave on the surface without incorporation. Slower decomposition but conserves soil moisture and protects soil structure. Best in low-rainfall or no-till systems.
Rolling-crimping: Crush standing green manure with a heavy roller to kill it without cutting, leaving a weed-suppressing mat on the surface. Plant cash crops directly through the mat with minimum disturbance.
Timing Before Planting
The decomposing green manure temporarily immobilises nitrogen as microbes multiply to break down the biomass. This brief “nitrogen drawdown” is most pronounced in high-carbon material (over-mature crops).
| Green Manure Maturity | Minimum Interval Before Planting |
|---|---|
| Young legume (vegetative) | 7–10 days |
| Legume at flowering | 10–14 days |
| Mixed legume/grass | 14–21 days |
| Mature grasses or cereal | 21–28 days |
If the green manure has a high C:N ratio, adding a small amount of nitrogen fertiliser (10–15 kg N/ha) at incorporation speeds decomposition and reduces nitrogen drawdown.
Seeding Rates and Establishment
| Species | Seeding Rate | Seeding Depth | Establishment Notes |
|---|---|---|---|
| Cowpea | 30–50 kg/ha | 3–4 cm | Warm soil required (>18°C) |
| Sunn hemp | 20–30 kg/ha | 2–3 cm | Inoculate; fast growing |
| Common vetch | 40–60 kg/ha | 3–5 cm | Mix with oats for support |
| Winter pea | 80–120 kg/ha | 3–5 cm | Trellis or mix with rye |
| Buckwheat | 60–80 kg/ha | 2–3 cm | Fast to establish |
| Oats | 80–120 kg/ha | 3–4 cm | Very cold tolerant |
| Mustard | 5–8 kg/ha | 1–2 cm | Small seeds, fine seedbed needed |
| Phacelia | 8–12 kg/ha | 1–2 cm | Broadcast and rake in |
Green Manure in Rotation Design
Integrate green manures into crop rotations to build soil continuously:
Two-year rotation (warm climate):
- Season 1: Cash crop (maize, sorghum)
- Inter-season: Cowpea green manure (45 days)
- Season 2: Cash crop (tomato, bean) — benefits from N released by cowpea
Three-year rotation (cool climate):
- Year 1: Cash crop (potato, brassica)
- Year 1 after-crop: Mustard green manure (biofumigation)
- Year 2: Winter rye/vetch mix (overwinter)
- Year 2 spring: Incorporate and plant cereals
- Year 3: Legume cash crop (pea, bean) — continue N cycle
Undersown green manure: Plant the green manure beneath a standing cash crop to save time. Sow clover under a maize canopy at 45–60 days after maize planting. The clover establishes slowly under the maize shade, then grows rapidly after maize harvest, covering the soil through winter. Incorporate in spring before the next planting.
Measuring Success
Track the long-term impact of green manures with simple measurements:
- Earthworm count: Dig 30 cm × 30 cm × 30 cm cubes in three spots; count worms. Target: 10+ worms per cube in healthy soil
- Aggregate stability test: Take a dry soil aggregate (1 cm diameter lump) and drop it into water. Well-structured soil holds its shape for 30+ seconds; poor structure dissolves immediately
- Organic matter change: Submit soil samples to a testing laboratory at the start and after 3 years of green manure incorporation for percentage organic matter
Soils with good green manure programs typically show 0.5–1.0% increases in organic matter per decade, roughly doubling the baseline organic matter of depleted agricultural soils within 10–20 years.
Relay Cropping for Time Efficiency
In warm climates with year-round growing potential, relay-crop a fast-growing green manure into the standing cash crop 3–4 weeks before harvest. The green manure establishes under partial shade, explodes in growth after harvest, and is ready to incorporate within 4–6 weeks of the cash crop removal — giving you both a full cash crop and a full green manure cycle in the same season.
Green Manure Summary
Select species based on season and primary need — legumes (cowpea, vetch, faba bean) for nitrogen fixation at 80–200 kg N/ha; non-legumes (rye, sorghum, buckwheat) for high biomass and weed suppression. Inoculate legume seed with appropriate Rhizobium where needed. Incorporate at early flowering for the best nitrogen release and decomposition rate, and allow 10–21 days before planting the following cash crop to avoid nitrogen drawdown. Integrate green manures into rotation by using inter-season slots, undersowing beneath standing crops, or relay-cropping in warm climates. The long-term reward is measurable improvement in organic matter, soil structure, and crop yield without external inputs.