Green Manure
A green manure crop is grown primarily to be incorporated into the soil to add nutrients, organic matter, and improve soil health. Green manures are an essential component of many organic and regenerative farming systems.
A wide range of crops can be used as green manure:
- Legumes: yellow sweetclover, alfalfa, red clover, white clover, peas, Indian Head lentils, black medick, vetches
- Non-legumes: oats, barley, forage grasses, mustard, canola, buckwheat, fall rye
Benefits of Green Manure
Green manures support soil improvement, nutrient cycling, and pest management.
Soil Health Benefits
- Reduce erosion
- Increase soil organic matter
- Improve soil structure and tilth
- Enhance microbial activity
- Reduce compaction
Nitrogen Fixation
Legume green manures such as alfalfa and sweetclover can fix more than 200 lb N/ac, significantly reducing nitrogen fertilizer requirements for subsequent crops.
Weed and Pest Suppression
- Fall rye and oats are highly competitive and effective at smothering weeds.
- Some species (e.g., yellow sweetclover, mustard) release allelopathic compounds that inhibit germination and early weed growth.
- Green manures disrupt insect and disease cycles and provide habitat for beneficial insects like bees and parasitic wasps.
Contribution of Nitrogen
Nitrogen contribution depends on crop type and timing of incorporation:
Green Manure Type |
Approximate N Contribution |
Pure legumes incorporated at blossom |
>100 lb N/ac |
Grass-legume mixtures (blossom stage) |
50–100 lb N/ac |
Grass/legume residue after harvest |
<50 lb N/ac |
Decomposition rate varies with soil texture, temperature, and moisture.
Incorporation
- Best incorporated into the top 3–4 inches using a discer-type implement to promote optimal decomposition.
- Incorporation deeper than 6 inches slows breakdown and should be avoided.
As microbes decompose green manure residue:
- Nutrients are released gradually over multiple years
- Organic acids produced during breakdown help lower soil pH, enhancing plant-available phosphorus
Selecting Legumes for Soil Improvement
Different legumes suit different soil and moisture conditions:
Red Clover
- Excellent choice for low-lying or saturated soils
- Tolerates wet and acidic conditions better than alfalfa
- Short-lived perennial with poor winter hardiness → minimal volunteer issues
- Common in zero-till and organic systems for quick establishment and soil improvement
Yellow Sweetclover
- Best for coarse-textured and drier soils
- Biennial—produces forage in year 1; forage + seed in year 2
- Minimal volunteer concerns when tilled in seeding year
- Choose low-coumarin varieties for grazing
- Provides high biomass → excellent for soil organic matter and hay yields (often surpassing alfalfa)
Mixed Clover Stands
- Many producers use 80% sweetclover + 20% red clover
- Benefits: enhanced hay yield, faster dry-down, and improved soil tilth and fertility
Variety Selection
Red Clover
- Single-cut types preferred in non-hay systems
- Less winter hardy than double-cut types
Sweetclover
- For livestock feed: choose low-coumarin varieties
- Recommended varieties include:
- Norgold (yellow sweetclover)
- Polara (white sweetclover)
Fertility Recommendations (Establishment)
General fertilizer requirements:
- Phosphorus (P₂O₅): 55–75 lb/ac
- Potassium (K₂O): 55–150 lb/ac
- Sulphur (S): 30 lb/ac
Always base fertilizer rates on soil test results and ensure compliance with Manitoba nutrient management regulations.
Seeding and Management
- Seeding rate: 7–10 lb/ac for a strong, nitrogen-fixing stand
- Incorporation timing: Till clover into soil before freeze-up
- Follow good seedbed preparation and timely management practices to maximize benefits
Cost–Benefit Considerations
- It is important to consider the seed cost per pound (pounds per acre seeding rate plus machinery and labour compared to the cost of fertilizer and the benefit gained.
- Potential Nitrogen contribution:
- Red clover: 30–50 lb N/ac
- Sweetclover: 50–100 lb N/ac (at full bloom)
- Actual N contribution varies with:
- Stand density
- Moisture availability
- Soil type
- Length of growing season