Cover Crop Legumes: Nitrogen-Fixing Soil Builders
layout: base.njk title: Cover Crop Legumes description: Growing Cover Crop Legumes in West Virginia (Zone 6b/7a) category: legume
For: The Loop Farmstead, New Martinsville WV 26155
Zone: 6b/7a
Soil: Heavy clay
Understanding Cover Crop Legumes
Types of Legume Cover Crops
Clovers: - Crimson clover (annual) - Red clover (biennial) - White clover (perennial) - Sweet clover (biennial) - Subterranean clover (annual)
Vetches: - Hairy vetch (winter annual) - Common vetch (winter annual) - Monantha vetch (winter annual)
Other Legumes: - Alfalfa (perennial) - Cowpeas (summer annual—covered in field peas) - Austrian winter peas (covered in field peas) - Soybeans (summer annual) - Sunn hemp (tropical annual)
For West Virginia (Zone 6b/7a): - Winter-hardy: Hairy vetch, crimson clover, red clover, white clover, Austrian winter peas - Summer: Cowpeas, soybeans, sunn hemp (plant after frost) - Perennial: Alfalfa, white clover
Red Clover
Characteristics
Type: Biennial Legume
Planting: Spring or late summer
Winter Hardiness: Excellent (hardy in Zone 6b/7a)
Source: Baker Creek Heirloom Seeds, Johnny's Selected Seeds, cover crop suppliers
Nitrogen Fixation: 100-200 lbs N per acre (excellent)
Biomass: Medium
Physical Traits: - Pink-red flower heads - 2-3 feet tall (second year) - Deep taproot - Persists for 2 years
Why Grow It: Red clover is a nitrogen-fixing powerhouse. It fixes more nitrogen than most clovers. The deep taproot breaks up clay and mines nutrients from deep in the soil profile. It persists for two years, providing long-term soil improvement.
Uses: - Green manure (till under) - Hay (excellent quality) - Grazing - Pollinator support
Clay Soil Performance: Excellent. The deep taproot penetrates clay effectively.
For The Loop Farmstead: Red clover is ideal for clay soil improvement. Plant it for 1-2 years in fields that need building. The nitrogen fixation is exceptional.
Sweet Clover
Characteristics
Type: Biennial Legume
Planting: Spring or late summer
Winter Hardiness: Excellent
Source: Johnny's Selected Seeds, cover crop suppliers
Nitrogen Fixation: 150-250 lbs N per acre (highest of clovers)
Biomass: Very high (5-8 feet tall)
Physical Traits: - White or yellow flowers (depending on variety) - Very tall (5-8 feet in second year) - Deep taproot (5+ feet) - Biennial (flowers and dies in second year)
Why Grow It: Sweet clover is the nitrogen champion. It fixes more nitrogen than any other temperate legume. The biomass is enormous—5-8 feet of growth. The taproot penetrates 5+ feet, breaking up severe compaction and mining deep nutrients.
Uses: - Green manure (massive soil improvement) - Deep soil breaking - Hay (cut before flowering—contains coumarin, which can cause bleeding in livestock if moldy) - Pollinator support (flowers are excellent for bees)
Warning: Sweet clover contains coumarin. When moldy, coumarin converts to dicoumarol, which causes internal bleeding in livestock. Don't feed moldy sweet clover hay. For cover crop use, this isn't a concern.
Clay Soil Performance: Exceptional. The deep taproot breaks through hardpan.
For The Loop Farmstead: Sweet clover is for serious soil building. Plant it in compacted, poor fields. Let it grow for two years. Till it under. The soil transformation will be dramatic.
Common Vetch
Characteristics
Type: Winter Annual Legume
Planting: Fall
Winter Hardiness: Good (slightly less hardy than hairy vetch)
Source: Cover crop suppliers
Nitrogen Fixation: 100-200 lbs N per acre
Biomass: High
Physical Traits: - Similar to hairy vetch - Slightly less aggressive - Purple/pink flowers
Why Grow It: Common vetch is similar to hairy vetch but slightly less aggressive. It's a good alternative if hairy vetch is unavailable. The nitrogen fixation is still excellent.
Alfalfa
Characteristics
Type: Perennial Legume
Planting: Spring or late summer
Winter Hardiness: Good (choose Zone 6b-rated varieties)
Source: Johnny's Selected Seeds, cover crop suppliers, farm supply
Nitrogen Fixation: 150-250 lbs N per acre per year
Biomass: Very high (perennial, multiple cuttings)
Physical Traits: - Purple flowers - 2-3 feet tall - Deep taproot (5-15 feet) - Persists 3-7 years
Why Grow It: Alfalfa is the queen of forage legumes. It's a perennial that fixes nitrogen every year for 3-7 years. The taproot is incredibly deep (5-15 feet), breaking up severe compaction and mining deep nutrients. Alfalfa hay is premium livestock feed.
Uses: - Hay (excellent quality, high protein) - Grazing - Green manure (after 2-3 years) - Deep soil improvement - Nitrogen source for subsequent crops
Clay Soil Performance: Good, but needs decent drainage. Alfalfa doesn't tolerate waterlogging.
Establishment: - Alfalfa is slow to establish (first year is light) - Full production in years 2-4 - Inoculate with alfalfa-specific rhizobia - Soil pH should be 6.5-7.0 (lime if needed)
For The Loop Farmstead: Alfalfa is a long-term investment. Plant it in fields you're taking out of rotation for 3-5 years. Harvest hay for livestock. When you're ready to return to annual crops, till it under—the nitrogen release is massive.
Sunn Hemp (Tropical)
Characteristics
Type: Tropical Summer Annual Legume
Planting: Late spring (after all frost, soil warm)
Days to Maturity: 60-90 days
Source: Cover crop suppliers (tropical varieties)
Nitrogen Fixation: 100-200 lbs N per acre
Biomass: Very high (5-9 feet tall)
Physical Traits: - Tall (5-9 feet) - Yellow flowers - Fast-growing - Killed by first frost
Why Grow It: Sunn hemp is the fastest biomass producer. In 60-90 days, it grows 5-9 feet tall. The nitrogen fixation is excellent. It's nematode-suppressive (reduces root-knot nematodes). It's tropical, so it winter-kills in Zone 6b.
Uses: - Quick green manure - Nematode suppression - Biomass production - Pollinator support
For The Loop Farmstead: Plant sunn hemp after early harvest (June). It grows through summer. Frost kills it. Spring: plant into residue. It's fast, effective, and pest-suppressive.
Nitrogen Fixation: How It Works
The Process: 1. Legume roots secrete compounds that attract rhizobia bacteria 2. Bacteria infect root hairs, form nodules 3. Inside nodules, bacteria convert atmospheric N₂ to ammonia (NH₃) 4. Plant uses ammonia for growth 5. When plant dies, nitrogen is released to soil 6. Next crop uses the nitrogen
Inoculation: - Rhizobia are specific to legume families - Clover inoculant works for all clovers - Vetch inoculant for vetches - Alfalfa needs specific inoculant - Soybeans/cowpeas share inoculant
Without Inoculant: - Legumes may not fix nitrogen - Or fix very little - Always inoculate (it's cheap insurance)
Nitrogen Release: - Legume residue releases nitrogen as it decomposes - 50-80% of fixed N becomes available to next crop - Timing: Greatest release 2-6 weeks after termination
Cover Crop Legumes in Rotation
Before Heavy Feeders: - Corn → Plant hairy vetch/rye in fall → Terminate May → Plant corn - Squash → Crimson clover → Terminate → Plant squash
Soil Building Phase: - Years 1-2: Sweet clover or alfalfa - Year 3: Corn (huge nitrogen boost) - Years 4-6: Rotate vegetables/grains
Permanent Beds: - White clover between beds - Mow periodically - Continuous nitrogen fixation
Orchard Floors: - White clover or red clover - Fix nitrogen for trees - Mow or graze
Economic Value
Replacing Synthetic Fertilizer:
Hairy vetch fixes 150-300 lbs N/acre. Synthetic nitrogen costs ~$0.50-1.00 per lb. Value: - 150 lbs N × $0.75 = $112.50 per acre - In small scale: Significant savings
Seed Cost: - Hairy vetch: $2-4 per lb - Planting rate: 1-2 lbs per 1000 sq ft - Cost per 1000 sq ft: $2-8 - Return: Much higher in nitrogen value
Bottom Line: Cover crop legumes pay for themselves in nitrogen alone. Soil improvement, erosion prevention, and pollinator support are bonuses.