Nitrogen fixation and urea credit calculator
Estimate how much nitrogen a tropical legume cover crop can contribute to your plantation each year, and the conservative urea-equivalent credit it may offset. This free calculator covers Mucuna bracteata, Pueraria, Calopogonium, and Centrosema for oil palm, rubber, durian, and other tropical crops.
Nitrogen reference values for tropical cover crops
These are the planning figures the calculator works from. Read them carefully, because nitrogen fixed from the air each year, nitrogen held in standing biomass, and nitrogen recycled through leaf litter are three different quantities that are often confused. We publish the proportion of nitrogen a species draws from the air (%Ndfa), which is well measured, and an annual planning band; we do not present a biomass-nitrogen stock as if it were an annual fixation rate.
| Species | Annual N planning band (kg N/ha/yr) | Nitrogen from fixation (%Ndfa) | Confidence and note |
|---|---|---|---|
| Mucuna bracteata (MB) | 150 to 200 | 67 to 84 percent (15N method, MPOB, oil palm) | High for %Ndfa, medium for the annual figure. Immature high-biomass stands can reach about 210 to 220; the band shown is the well-supported range. MB is nursery-transplanted, not broadcast. |
| Pueraria javanica (PJ) | 100 to 250 | 80 to 93 percent | Medium-high. A Thai rubber study reports about 240 kg N/ha/yr released at about 81 percent Ndfa; the upper band is a best case, not a design value. |
| Calopogonium mucunoides (CM) | 80 to 170 | 40 to 60 percent | Medium. The often-quoted 154 kg N/ha is biomass-nitrogen stock, not annual fixation; the annual planning band is lower. |
| Centrosema pubescens (CP) | 50 to 120 | 50 to 70 percent | Medium-low. The 293 kg N/ha figure is biomass-nitrogen stock (highest of the group), not annual fixation. |
| Calopogonium caeruleum (CC) | 50 to 130 | 50 to 70 percent | Low to medium, data-limited. Persists under deep shade in older stands; treat as approximate and confirm by site. |
| Standard MPOB mix (PJ:CP:CC 4:3:1) | 120 to 190 | Not a simple seed-ratio average | Field nitrogen contribution is not a seed-ratio average; Pueraria typically dominates establishment and biomass. |
The calculator estimates an absolute figure from the biomass, nitrogen content, and fixation share you enter, then converts a conservative share into a urea-equivalent credit using the 46 percent nitrogen content of standard urea. The absolute number it returns is driven by your inputs, not a fixed published claim.
Sources: MPOB Oil Palm Bulletin 60 (15N isotope dilution under oil palm); Vrignon-Brenas et al. 2019, Agronomy for Sustainable Development; Clermont-Dauphin et al. 2016 (Pueraria in rubber, Thailand); Philippine field studies of Calopogonium and Centrosema for the biomass-nitrogen stock figures. Values reviewed September 2026.
How the nitrogen fixation calculator works
The calculator estimates biological nitrogen fixation from a legume cover crop using the biomass the stand produces, the nitrogen concentration of that biomass, and the proportion of plant nitrogen that comes from fixation rather than from the soil (%Ndfa). It multiplies these to estimate nitrogen fixed per hectare per year, then converts a conservative share into a urea-equivalent credit using the 46 percent nitrogen content of standard urea.
How to read the result
Treat the output as a planning range, not a guaranteed input substitution. Only a fraction of fixed nitrogen becomes available to the main crop in any single season, because nitrogen is released gradually as residues decompose. Validate the contribution against your own soil and leaf testing before adjusting a fertiliser programme.
Frequently asked questions
Why do you not publish a single nitrogen figure for Mucuna bracteata?
Because the sound measurement for MB is proportional: 67 to 84 percent of its nitrogen comes from the air, measured by the 15N method. A single absolute kg N/ha/yr value is not well supported, so we show a planning band and let the calculator estimate an absolute figure from your own biomass inputs.
Does fixed nitrogen replace my fertiliser?
It supplements your programme rather than replacing it outright. Released nitrogen is gradual and partial in any one season, so use the urea-equivalent credit as a conservative planning figure and adjust based on your own testing.
How do I get seed and a recommendation for my site?
Tell us your crop, area, and region and our agronomy team will advise on species mix and seeding rate. You can contact us for a quote, or screen species first with the cover crop calculator.