Nitrogen fixation and urea credit calculator
Estimate what a tropical legume cover crop could contribute to your plantation each year, from the annual nitrogen figure you plan with, 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 figures behind the calculator, kept apart by kind, because three different quantities are often confused: nitrogen accumulated or fixed each year (kg N/ha/yr), the share of a plant's nitrogen drawn from the air (%Ndfa), and nitrogen standing in the biomass at one time (a stock, kg N/ha). A stock is never used as an annual rate. Only Pueraria has a measured annual figure, from one trial, so the calculator asks for the annual figure you plan with and pre-fills a planning assumption for Pueraria only.
| Species | Measured annual N (kg N/ha/yr) | Nitrogen from fixation (%Ndfa) | Biomass-N stock (kg N/ha) | Figure the calculator starts with |
|---|---|---|---|---|
| Mucuna bracteata (MB) | No single routine annual rate is asserted | 67 to 84 percent (15N isotope dilution under three-year-old oil palm, Jawa Series soil, Malaysia) | Not established | None: enter your own. MB is nursery-transplanted, not broadcast. |
| Pueraria javanica (PJ) | Up to 250, accumulated in above-ground biomass in one trial under young rubber in northeast Thailand; less under a closing canopy or on drought-prone ground | 85 to 93 percent (same trial) | Not reported separately | 150, a planning assumption rather than a measurement |
| Calopogonium mucunoides (CM) | No verified annual rate | Not established | Up to about 154, reported in a Philippine field study; not an annual rate | None: enter your own |
| Centrosema pubescens (CP) | No verified annual rate | Not established | Up to about 293, reported in a Philippine field study; not an annual rate | None: enter your own |
| Calopogonium caeruleum (CC) | No verified annual rate | 25 to 72 percent in one Papua New Guinea oil palm study | Not established | None: enter your own |
| Standard MPOB mix (PJ:CP:CC 4:3:1) | No verified figure for this mix; field contribution is not a seed-ratio average | Not applicable | Not established | None: enter your own |
| Whole legume cover (study record, not a species) | 48 to 102 for legume cover in young oil palm plantations in Papua New Guinea, annualised by the authors from four months of growth | Not applicable | Not applicable | Not used |
The calculator multiplies the annual figure you plan with by an establishment factor and by your area, then converts a conservative share into a urea-equivalent credit using the 46 percent nitrogen content of standard urea. The number it returns is driven by your inputs, not by a published claim, and it will not accept a biomass-nitrogen stock as the annual figure.
Sources: Cheah, Zaharah and Aminuddin (2010), MPOB Oil Palm Bulletin 60 (15N isotope dilution under oil palm); Clermont-Dauphin et al. (2016), Agriculture, Ecosystems and Environment 217:79-88 (Pueraria in rubber, Thailand); Edraiza (2014), Sage 5(1) (biomass nitrogen of Calopogonium and Centrosema); Pipai et al. (2023), Plant and Soil 491:665-680 (legume cover in Papua New Guinea oil palm). The 150 planning assumption for Pueraria is ours, not a published figure. Values reviewed September 2026.
How the nitrogen fixation calculator works
The calculator takes the annual nitrogen figure you plan with, in kilograms of nitrogen per hectare per year, scales it by how well the stand is established and by your area, then converts a conservative share into a urea-equivalent credit using the 46 percent nitrogen content of standard urea. For Pueraria it starts at 150 kg N/ha/yr, a planning assumption rather than a measurement; for the other species and the mix you enter your own figure, because no verified annual rate exists for them.
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: in a 15N isotope-dilution study under three-year-old oil palm on Jawa Series soil in Malaysia, 67 to 84 percent of its nitrogen came from the air. No single routine annual fixation rate is asserted for MB, so the calculator asks for the annual figure you plan with rather than supplying one.
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.