Estimate nitrogen credit from green manure legume crops based on species, biomass, and termination timing. Free N credit estimator.
The Green Manure Nitrogen Credit Calculator estimates how much nitrogen a legume green manure crop contributes to the following crop. Legumes fix atmospheric nitrogen through symbiotic bacteria (Rhizobium) in their root nodules. When the legume is terminated and incorporated, this nitrogen becomes available to the next crop through decomposition and mineralization.
The nitrogen credit depends on the legume species, the amount of above-ground biomass produced, and when it is terminated relative to planting the following crop. Early termination (more than 4 weeks before planting) allows more mineralization time but also more potential for N loss. Late termination maximizes biomass but may not mineralize quickly enough.
This calculator uses species-specific biomass nitrogen concentrations and fixation percentages to estimate the first-year nitrogen credit that can be deducted from the synthetic fertilizer recommendation for the following crop. Whether you are a beginner or experienced professional, this free online tool provides instant, reliable results without manual computation.
Green manure legumes can supply 40–200 lbs of nitrogen per acre, replacing a significant portion of synthetic fertilizer needs. Knowing the actual credit prevents over-fertilizing (wasting money) and under-fertilizing (losing yield) after a legume cover crop. Having a precise figure at your fingertips empowers better planning and more confident decisions.
N credit lbs/ac = Biomass (lbs/ac) × %N × Fixation% × Availability factor Where: %N = Nitrogen concentration of the legume biomass (species-dependent) Fixation% = Proportion of N from biological fixation (typically 50–80%) Availability factor = First-year release adjusted for termination timing
Result: 81 lbs N/ac
Crimson clover at 3,000 lbs biomass × 3.0% N = 90 lbs total N. Fixation = 75%. Available N = 90 × 0.75 × 0.4 (early timing) = 81 lbs N/ac credit. Adjust down for late termination or low biomass.
Legumes form symbiotic relationships with Rhizobium bacteria that colonize root nodules. The bacteria convert atmospheric N₂ into ammonia (NH₃) that the plant uses for protein synthesis. In return, the plant provides carbon compounds as an energy source. Effective fixation requires proper inoculation, adequate soil moisture, and soil pH above 5.5.
Biomass is typically estimated by cutting sample quadrats (1–4 sq ft), drying at 140°F for 48 hours, and weighing. Convert to lbs/ac. Visual estimation charts calibrated for your region can also provide reasonable estimates. More biomass generally means more nitrogen credit.
Green manures fit naturally between cash crops in rotation. A winter annual legume (crimson clover, hairy vetch) can be planted after fall harvest and terminated before spring planting. In shorter windows, spring-planted legumes like cowpeas or sunn hemp provide summer N fixation before fall crops.
Crimson clover fixes 80–150 lbs N/ac, hairy vetch 100–200 lbs N/ac, Austrian winter peas 50–120 lbs N/ac, and red clover 75–150 lbs N/ac. Actual fixation depends on biomass, stand density, and growing conditions.
Non-legume cover crops like cereal rye do not fix nitrogen. They scavenge residual soil nitrogen and recycle it, but this is not a net addition. Rye can immobilize nitrogen temporarily due to high C:N ratio.
For maximum N credit, terminate at early bloom (50–75% flowering). Earlier termination produces less biomass. Later termination risks seed set and increases C:N ratio, slowing decomposition.
Early termination (4+ weeks before planting) gives more mineralization time: 40–60% of N is available. At planting: 25–40%. After planting: less than 20% because decomposition is slow and nitrogen may be immobilized.
Yes, when biomass is adequate (>2,000 lbs/ac). Research consistently shows that legume green manures reduce subsequent crop N needs by 40–150 lbs/ac. Start conservatively and adjust based on experience.
Root biomass and nodules contain additional nitrogen not captured in above-ground biomass estimates. This can add 20–40% more N beyond the shoot-based credit. Most credit tables include conservative estimates that partially account for this.