Green Fallow
Part of Crop Rotation
A green fallow replaces bare, resting soil with an active growing cover crop that builds fertility, suppresses weeds, protects soil structure, and produces biomass — then is incorporated into the soil before the next main crop. It is the productive upgrade to bare fallow, and often the single most cost-effective soil improvement practice available, requiring only seed as an input.
What Is a Green Fallow?
A green fallow (also called a “green manure” when incorporated) is a crop grown specifically to improve soil rather than for direct harvest. The crop grows for weeks or months, then is killed — by mowing, rolling, or incorporation — and its biomass breaks down in or on the soil, releasing nutrients.
Key distinctions:
- Green fallow: Actively growing cover; the field is producing biological work even though it appears “resting.”
- Bare fallow: Soil is exposed, cultivated periodically to kill weeds. Loses nitrogen through leaching; erodes; produces nothing.
- Mulch fallow: Soil covered with dead plant material (hay, straw) — protects soil surface but adds no living biology.
Objectives by Crop Choice
Different cover crops serve different purposes. Choose based on what the soil and the following main crop needs.
| Objective | Best Cover Crops |
|---|---|
| Nitrogen fixation | Crimson clover, red clover, vetch, field pea, lupin, fenugreek |
| Organic matter addition | Phacelia, rye, oats, buckwheat |
| Compaction busting (deep roots) | Radish (tillage radish), turnip, sunflower |
| Weed suppression | Buckwheat (dense canopy), rye, mustard |
| Pest/nematode suppression | Marigold, mustard, sorghum-sudan hybrid |
| Soil structure improvement | Mixed species (see below) |
| Quick biomass in a short window | Buckwheat (grows fast), phacelia, mustard |
Nitrogen Fixation: The Primary Benefit
Leguminous cover crops (clovers, vetches, peas, beans) form symbiotic relationships with Rhizobium bacteria in soil, which fix atmospheric nitrogen gas (N₂) into ammonia compounds accessible to plants. When the cover crop is incorporated, this fixed nitrogen is released as the biomass decomposes.
Approximate nitrogen fixed by common green fallow legumes:
| Crop | Nitrogen Fixed (kg/ha/season) | Additional Notes |
|---|---|---|
| Crimson clover | 80–150 | Fast-establishing; winter-hardy in zones 6+ |
| Red clover | 100–200 | Perennial; very high N; may be left 2 seasons |
| White clover | 80–150 | Lower growth; good under fruit trees |
| Hairy vetch | 100–200 | Excellent cold hardiness; vigorous growth |
| Field peas | 80–150 | Fast; good winter cover; edible if not incorporated |
| Lupin | 100–180 | High biomass; acidic soil tolerant |
| Fenugreek | 40–80 | Fast in warm conditions; also edible seed |
For comparison, a heavy manure application (30 tonnes/ha of farmyard manure) supplies approximately 60–90 kg of available nitrogen. A good legume fallow matches or exceeds this.
Nitrogen fixation requires Rhizobium bacteria in the soil. If the legume species has not been grown in that soil for several years, inoculate the seed with the appropriate Rhizobium strain before sowing. Mix seed with a water-peat slurry containing the inoculant, allow to dry in shade, and sow immediately. Uninoculated legumes in unfamiliar soils may show poor nodule formation and fix little nitrogen.
Non-Legume Cover Crops: Carbon and Structure
Non-legume cover crops add organic matter (carbon), improve soil texture, and suppress weeds but do not fix nitrogen. They are valuable when nitrogen is adequate but organic matter is low, or when used in a mixture with legumes.
Buckwheat: Very fast (flowers in 6–8 weeks); excellent weed suppressant through dense canopy; shallow roots improve light compaction; phosphorus solubilizer (frees soil-bound phosphorus for following crops). Kill before it seeds — buckwheat can become a weed if allowed to mature.
Phacelia: Attractive deep-purple flowers beloved by bees; fast-growing; excellent organic matter; frost-sensitive so winter-kills cleanly in temperate climates (no need to incorporate — lies as mulch).
Winter rye (Secale cereale): Very hardy; sow in autumn and leave until late spring; excellent weed suppressant through winter; high biomass; produces allelopathic compounds that suppress weed seed germination (helpful before a transplanted crop; allow 3–4 weeks between incorporation and sowing small seeds). Up to 10 tonnes/ha of biomass in good conditions.
Mixed Cover Crop Cocktails
Sowing 3–5 species together produces multiple benefits simultaneously and reduces the risk of any single species failing. A typical multi-species cocktail:
Example temperate winter cocktail:
- Hairy vetch: 15 kg/ha (nitrogen fixation)
- Winter rye: 50 kg/ha (biomass, weed suppression)
- Crimson clover: 8 kg/ha (nitrogen, early establishment)
- Radish: 5 kg/ha (compaction, quick establishment)
Example warm-season cocktail:
- Cowpea: 40 kg/ha (nitrogen fixation)
- Sorghum-sudan hybrid: 15 kg/ha (biomass, nematode suppression)
- Buckwheat: 30 kg/ha (weed suppression, phosphorus)
- Sunflower: 5 kg/ha (deep roots, organic matter)
Timing and Management
When to Sow
- After early-harvested crops (early potatoes, winter wheat): Sow immediately after harvest to maximize the growing window before autumn frosts.
- After late-harvested crops (maize, late potatoes): Limited options; sow a winter-hardy species (rye, vetch) as soon as possible.
- Before a spring main crop: Sow previous summer or autumn; incorporate 4–6 weeks before planting.
When to Incorporate
Incorporate at or just before flowering — this is when the biomass is maximum and the carbon:nitrogen ratio is most favorable for rapid decomposition. Flowering also indicates the plant is putting energy into seed production rather than vegetative growth — incorporating just before seed set maximizes nitrogen retained in the field.
| Growth Stage | C:N Ratio | Decomposition Rate |
|---|---|---|
| Lush green (early growth) | 10:1–15:1 | Very fast (1–3 weeks) |
| Flowering | 15:1–25:1 | Fast (3–6 weeks) |
| Mature (seed set) | 25:1–40:1 | Slow (6–12 weeks) |
| Straw/dry residue | 50:1–100:1 | Very slow (months–years) |
Incorporate green material; allow mature/dry material to decompose on the surface as mulch before incorporating.
Incorporation Methods
- Digging or plowing: Most thorough; cuts and buries the cover crop mass. Worms and soil bacteria rapidly decompose buried green material.
- Mowing followed by cultivation: Mow low, then cultivate the surface (hoe, rotary hoe) to chop and mix. Suitable for small-scale work without heavy equipment.
- Rolling/crimping: Roll cover crop flat with a weighted roller or crimper; kills most species without burial. Leaves a dense mulch through which transplants can be planted. Avoids soil disturbance — preferred in no-till systems.
After incorporation, wait at least 2–4 weeks before sowing small seeds (carrot, lettuce, parsnip). Decomposing green matter releases ammonia and other compounds that inhibit germination. Transplants and large-seeded crops (beans, peas, squash) are more tolerant and can be planted within 1–2 weeks.
Cover Crops for Specific Problems
| Problem | Recommended Cover Crop | Mechanism |
|---|---|---|
| Severe compaction | Tillage radish + clover | Radish taproots penetrate compacted layer |
| Root-knot nematodes | Marigold (Tagetes patula), sorghum-sudan | Root exudates suppress nematode populations |
| Soilborne disease (Pythium, Fusarium) | Mustard, brassica | Glucosinolate breakdown products fumigate soil |
| Low organic matter | Winter rye + vetch | High biomass + nitrogen input |
| Acidic soil | Lupin, buckwheat | Tolerates pH 5.0–6.0 where clover fails |
| Infertile sandy soil | Phacelia, buckwheat | Tolerates low fertility; improves structure |
Green Fallow Summary
A green fallow grows an active cover crop on resting ground instead of leaving soil bare, converting an unproductive period into a fertility-building investment. Legume fallows (clover, vetch, peas, lupin) fix 80–200 kg of nitrogen per hectare per season — equivalent to heavy manure applications. Non-legumes (rye, buckwheat, phacelia) add organic matter, suppress weeds, and improve soil structure. Mixed cocktails provide multiple benefits simultaneously. Incorporate at flowering stage, wait 2–4 weeks before sowing small seeds, and choose species based on what the following crop needs and what the soil is lacking.