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.
How the nitrogen fixation calculator works
The calculator estimates biological nitrogen fixation (BNF) from a legume cover crop using three inputs: the annual above-ground and below-ground 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. It multiplies these to produce an estimate of nitrogen fixed per hectare per year, then converts a conservative share of that figure into a urea-equivalent credit using the 46 percent nitrogen content of standard urea.
The defaults are deliberately conservative and drawn from published tropical-plantation literature. Tropical legume cover crops commonly fix on the order of 100 to 250 kg of nitrogen per hectare per year once well established, with Mucuna bracteata at the higher end of biomass production and Pueraria javanica around a typical 150 kg figure. Calopogonium and Centrosema contribute useful but generally lower amounts. Actual results depend on establishment, ground cover, rainfall, soil fertility, and how much of the residue is retained in the field.
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. The urea-equivalent credit shown is a cautious estimate of what you may be able to offset over time, and it should be validated with your own soil and leaf testing before adjusting a fertiliser programme.
Frequently asked questions
Which cover crops does this cover?
Mucuna bracteata, Pueraria javanica, Calopogonium, and Centrosema, the four tropical legumes most widely used under oil palm, rubber, coconut, durian, and other plantation crops in the Philippines.
How accurate are the nitrogen figures?
They are literature-based ranges, not site measurements. They are useful for comparing species and for sizing the potential contribution of a cover crop, but field results vary with establishment and management. Confirm with soil and tissue analysis.
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 request a consultation or contact us for a quote.