Nitrogen fixation calculator for tropical cover crops
Estimate the nitrogen a tropical legume cover crop could contribute to your plantation each year, in kg N, from the annual nitrogen figure you plan with. This free calculator covers Mucuna bracteata, Pueraria, Calopogonium, and Centrosema for oil palm, rubber, durian, and other tropical crops. Legume cover contributes nitrogen; adjust your fertiliser programme only after soil and leaf analysis shows what the cover supplies on your block.
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Nitrogen fixation reference values for tropical cover crops
These are the figures behind the calculator. 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 | Nitrogen evidence | Biomass (dry matter) | Seeding rate, pure stand | Seeding rate, in mixture |
|---|---|---|---|---|
| Mucuna bracteata (MB) | 67 to 84 percent of plant N from the air (%Ndfa; 15N isotope dilution under three-year-old oil palm, Jawa Series soil, Malaysia). No single routine annual rate is asserted; the calculator asks for your own figure | No verified figure | 320 seedlings/ha, two per palm point, from about 500 seeds (75 g) (MPOB TT No. 501, deep peat); 680 seedlings/ha at 1.5 m spacing (Samedani et al. 2015, mineral soil) | Raised as seedlings, not broadcast |
| Pueraria javanica (PJ) | Up to 250 kg N/ha/yr accumulated in above-ground biomass, 85 to 93 percent from the air, in one trial under young rubber in northeast Thailand; less under a closing canopy or on drought-prone ground. The calculator starts at 150 kg N/ha/yr, a planning assumption rather than a measurement | 5 to 10 t DM/ha in pure stands | About 4 kg/ha | 4 kg/ha in the MPOB TT No. 501 mixture; 5 to 7.5 kg/ha with 1 to 1.5 kg/ha CC |
| Calopogonium mucunoides (CM) | No verified species-specific nitrogen figure is published. The calculator asks for your own figure | 4 to 6 t/ha/yr under cutting every 9 to 12 weeks | Set on the quotation for the site | Calopo:centro:kudzu 5:4:1 mixture at 2 to 5 kg/ha interrow (PROSEA 4) |
| Centrosema pubescens (CP) | No verified species-specific nitrogen figure is published. The calculator asks for your own figure | No verified figure | 3 to 5 kg/ha broadcast | 3 kg/ha in the MPOB TT No. 501 mixture |
| Calopogonium caeruleum (CC) | No single species-specific annual fixation rate is currently published from verified evidence. The calculator asks for your own figure | 3 to 7 t DM/ha/yr in the open; 1 to 1.5 under mature oil palm at 10 percent light | No published rate for CC sown alone; set on the quotation | 1 to 1.5 kg/ha with 5 to 7.5 kg/ha PJ; 1 kg/ha in the MPOB TT No. 501 mixture |
The MPOB TT No. 501 mixture is PJ 4 + CP 3 + CC 1 = 8 kg/ha, with 5 g Rhizobium per kg seed. Where no published rate exists, the seeding rate is set on the quotation for the site. Legume cover contributes nitrogen; adjust your fertiliser programme only after soil and leaf analysis shows what the cover supplies on your block.
For legume cover as a whole, one study in young Papua New Guinea oil palm estimated 48 to 102 kg N/ha/yr, an annual figure extrapolated from shorter-term measurements (Pipai et al. (2023)). That is a study estimate for a mixed cover, not a rate for any single species, and the calculator does not use it.
Nitrogen sources: Cheah, Zaharah and Aminuddin (2010), MPOB Oil Palm Bulletin 60: 22-27 (15N isotope dilution under oil palm); Clermont-Dauphin et al. (2016), Agriculture, Ecosystems and Environment 217: 79-88 (Pueraria in rubber, Thailand); Pipai et al. (2023), Plant and Soil 491: 665-680 (legume cover in Papua New Guinea oil palm). Biomass and seeding columns: Tropical Forages (CIAT and ILRI 2020, tropicalforages.info); MPOB TT No. 501 (Hasnol Othman, Farawahida Mohamad Darus and Zulkifli Hashim 2012); Samedani et al. (2015), International Journal of Agriculture and Biology 17(2): 251-260; PROSEA 4, Forages (1992). The 150 planning assumption for Pueraria is ours, not a published figure. Values reviewed October 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, and returns the nitrogen the cover may contribute in kg N per year. For Pueraria it starts at 150 kg N/ha/yr, a planning assumption rather than a measurement; for the other species you enter your own figure, because no verified annual rate exists for them. It will not accept a biomass-nitrogen stock as the annual figure.
Actual results depend on establishment, ground cover, canopy shade, rainfall, soil fertility, and how much of the residue is retained in the field. The one measured Pueraria figure, up to 250 kg N/ha/yr accumulated in above-ground biomass, came from a single trial under young rubber in northeast Thailand, so expect less under a closing canopy or on water-limited ground.
How to read the result
Treat the output as a planning estimate. Only a fraction of fixed nitrogen becomes available to the main crop in any single season, because nitrogen is released gradually as residues decompose. Legume cover contributes nitrogen; adjust your fertiliser programme only after soil and leaf analysis shows what the cover supplies on your block.
Frequently asked questions
Which cover crops does this cover?
Mucuna bracteata, Pueraria javanica, Calopogonium mucunoides, Centrosema pubescens and Calopogonium caeruleum, the five tropical legumes commonly used under oil palm, rubber, coconut, durian, and other plantation crops in the Philippines.
How accurate are the nitrogen figures?
They are not site measurements. Only Pueraria has a measured annual figure, from one trial; for the other species no verified annual rate exists, and a biomass-nitrogen stock is not an annual rate. Use the result to size the potential contribution of a cover crop, and confirm it with soil and tissue analysis.
Does fixed nitrogen replace my fertiliser?
Legume cover contributes nitrogen; adjust your fertiliser programme only after soil and leaf analysis shows what the cover supplies on your block. Released nitrogen is gradual and partial in any one season.
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 request a quotation. We reply within one business day (Philippine time).