How much N does hairy vetch fix?

Hairy vetch in good stands fixes 100–200 lbs N/ac total (shoots + roots). First-year credit for the following crop is typically 50–120 lbs N/ac. This is the highest N credit of commonly grown winter annual legumes.

Does the cover crop take nitrogen from the soil?

Legumes use both soil N and biologically fixed N. When soil N is high (after heavy manure or fertilizer), the plant uses soil N preferentially and fixes less. On typical fields, 50–80% of legume N comes from fixation.

How do I estimate biomass without destructive sampling?

Use reference photos calibrated to biomass ranges, or the "armload" method: grab biomass from a known area, compress to estimate density. For precision, cut a 1-ft² quadrat, dry at 140°F for 48 hours, and weigh.

What happens if I terminate the cover crop too early?

Early termination (before bloom) means less biomass and proportionally less N fixation. The N per pound of biomass is also slightly lower at vegetative stages. However, early termination allows more soil moisture conservation for the cash crop.

Can non-legume cover crops provide nitrogen?

Non-legumes (cereal rye, oats, radishes) scavenge and recycle existing soil nitrogen but do not add new N from the atmosphere. High-carbon residues like rye can temporarily immobilize N, requiring additional fertilizer for the following crop.

How does winter kill affect N credit?

Winter-killed legumes (like some varieties of crimson clover in Zone 5) decompose over winter and early spring, releasing N early. This can be an advantage for early-planted crops but risks N loss through leaching before crop uptake.

Cover Crop Nitrogen Fixation Calculator

Estimate nitrogen fixed by legume cover crops based on biomass production. Calculate N credits for the following cash crop.

Cover Crop Species

Termination Method

Above-Ground Biomass

lbs DM/ac

Shoot N Concentration

BNF Fixation %

First-Year Availability

N Fertilizer Price

$/lb N

Total Field Acres

Total Shoot N

140.0 lbs/ac

3.5% N in 4,000 lbs/ac biomass

Biologically Fixed N

105.0 lbs/ac

75% from biological N fixation

Soil-Derived N

35.0 lbs/ac

N absorbed from soil (not credits)

1st Year N Credit

57.8 lbs N/ac

55% availability — subtract from N recommendation

2nd Year Residual

15.8 lbs N/ac

~15% additional credit the following year

Fertilizer Replacement

$37.54/ac

At $0.65/lb N

Total Field N Credit

2,310 lbs N

Across 40 acres

Total Field Value

$1,502.00

Fertilizer savings across 40 acres

Nitrogen Flow Breakdown (per acre)

Total Shoot N: 140.0 lbs

Fixed 75%

Soil 25%

Fixed N available as credit:

1st Year: 57.8 lbs

Later / Lost

Fertilizer Equivalents (per acre)

Fertilizer Product	N Content	Equivalent Amount
Urea (46-0-0)	46%	125.5 lbs/ac
Anhydrous Ammonia (82-0-0)	82%	70.4 lbs/ac
UAN 28%	28%	206.4 lbs/ac
UAN 32%	32%	180.6 lbs/ac
Ammonium Sulfate (21-0-0)	21%	275.2 lbs/ac

Biomass Production Scenarios

Biomass (lbs/ac)	Total N	Fixed N	1st Yr Credit	Value/ac
2,000	70.0 lbs	52.5 lbs	28.9 lbs	$18.77
3,000	105.0 lbs	78.8 lbs	43.3 lbs	$28.15
4,000 ✓	140.0 lbs	105.0 lbs	57.8 lbs	$37.54
5,000	175.0 lbs	131.3 lbs	72.2 lbs	$46.92
6,000	210.0 lbs	157.5 lbs	86.6 lbs	$56.31

Cover Crop Species Reference

Species	N %	BNF %	Typical Biomass	Best Region
Hairy Vetch	3.5%	70-80%	3,000-5,000 lbs	Transition zone
Crimson Clover	2.8%	55-65%	2,000-4,000 lbs	South / Southeast
Red Clover	3.2%	65-75%	2,500-4,500 lbs	Upper Midwest
Austrian Winter Pea	3.8%	75-85%	2,500-4,000 lbs	Northern regions
White Clover	2.5%	50-60%	1,500-3,000 lbs	Cool, moist
Cowpea	2.0%	45-55%	2,000-4,000 lbs	Southern US

Planning notes, formulas, and examples

About the Cover Crop Nitrogen Fixation Calculator

The Cover Crop Nitrogen Fixation Calculator estimates the amount of nitrogen biologically fixed by a legume cover crop and the resulting first-year nitrogen credit for the following cash crop. Legume cover crops form symbiotic relationships with Rhizobium bacteria that convert atmospheric N₂ into plant-usable ammonia.

The total nitrogen fixed depends on biomass accumulation, shoot nitrogen concentration (which varies by species), and the proportion derived from biological fixation versus soil uptake. Not all the fixed nitrogen becomes available in the first season — decomposition and mineralization release only a portion, with the remainder contributing to soil organic nitrogen reserves.

This calculator combines species-specific nitrogen content, fixation efficiency, and mineralization timing to produce a practical first-year N credit that you can subtract from the fertilizer recommendation for the subsequent crop. Use this page when a legume cover crop is part of the system and you need a defensible nitrogen credit before buying fertilizer.

When This Page Helps

Legume cover crops can replace $30–$120/ac worth of synthetic nitrogen. This page helps convert biomass and species assumptions into a usable N credit instead of treating every cover crop acre as the same.

How to Use the Inputs

Enter the above-ground biomass at termination (lbs dry matter per acre).
Enter the nitrogen concentration of the biomass (% N, species-dependent).
Enter the estimated fixation percentage (% of shoot N from BNF).
Review the total fixed N and first-year available N credit.
Subtract the credit from your cash crop nitrogen recommendation.

Formula used

Total shoot N = Biomass × (%N / 100)
Fixed N = Total shoot N × (Fixation% / 100)
First-year credit = Fixed N × Availability factor (0.4–0.6)

Typical %N by species:
Hairy vetch: 3.2–4.0%, Crimson clover: 2.5–3.2%
Red clover: 2.5–3.0%, Winter peas: 2.8–3.5%

Example Calculation

Result: 105 lbs N/ac fixed, 53 lbs N/ac first-year credit

Total shoot N = 4,000 × 0.035 = 140 lbs N/ac. Fixed N = 140 × 0.75 = 105 lbs. First-year credit at 50% availability = 105 × 0.50 = 52.5 lbs N/ac. The remaining N enters the soil organic N pool for future years.

Tips & Best Practices

Maximum fixation requires proper Rhizobium inoculation — use species-specific inoculant on new fields.
Biomass at early bloom stage: hairy vetch 3,000–5,000 lbs/ac, crimson clover 2,500–4,500 lbs/ac.
Terminate at early to full bloom for maximum N fixation before seed set.
Grass-legume mixes reduce N credit per acre but improve carbon input and soil structure.
Root N adds 20–30% beyond shoot estimates — this calculator focuses on the conservative shoot-based credit.
Consider a 20–30 lbs N/ac starter application even with high cover crop N credit to bridge early-season mineralization lag.

Species Selection for Maximum N Fixation

Hairy vetch is the gold standard for N fixation in winter annual cover crops, but crimson clover, Austrian winter peas, and berseem clover all contribute meaningful N. In frost-free windows, cowpeas and sunn hemp are warm-season options. Choose species adapted to your climate and planting window for reliable biomass production.

Integration with Cash Crop Rotations

The most common integration is winter annual legume before corn. Plant the legume after harvest of the previous crop (wheat, soybeans, early vegetables) and terminate 2–4 weeks before corn planting. Adjust the corn N rate downward by the estimated cover crop credit. Some farmers reduce N by 50–100 lbs/ac after good vetch.

Economic Analysis

A hairy vetch cover crop costs $30–50/ac (seed + planting). If it provides 75 lbs N/ac credit at $0.65/lb N, the fertilizer replacement value is ~$49/ac. Adding soil health benefits (erosion control, organic matter, water-holding capacity), the return on investment is strongly positive.

Sources & Methodology

Last updated: February 10, 2026

Frequently Asked Questions

Hairy vetch in good stands fixes 100–200 lbs N/ac total (shoots + roots). First-year credit for the following crop is typically 50–120 lbs N/ac. This is the highest N credit of commonly grown winter annual legumes.