Green Manure: What It Is and the Best Crops for the Tropics

Green manure is a crop grown to be returned to the soil, rather than harvested, to build fertility and organic matter. In the tropics, the leguminous cover crops used in oil palm and rubber double as green manures: they fix nitrogen, add large amounts of biomass, and improve soil structure when slashed, incorporated, or left as a living mulch.

What is green manure?

A green manure is any crop grown primarily to improve the soil rather than to be sold or eaten. The crop is grown for a period, then either slashed and incorporated into the topsoil or left on the surface as a mulch, returning its nitrogen, organic matter, and nutrients to the soil. Legumes are the most common green manures because, in addition to biomass, they fix nitrogen from the air.

How green manure improves soil

• Nitrogen: legume green manures fix atmospheric nitrogen and release it as the residue breaks down.
• Organic matter: biomass and litter feed soil organic matter and the soil food web.
• Structure and water: added organic matter improves aggregation, aeration, and moisture retention.
• Weed suppression: a dense green manure shades out weeds while it grows.

What the nitrogen figures actually are

Green manure nitrogen figures circulate widely and most of them cannot be traced to a primary source. Here is what we can cite, with the method attached, and where we cannot cite anything we say so.

• Pueraria javanica: up to 250 kg N per hectare per year accumulated in aboveground biomass at 85 to 93% Ndfa, over four years in one rubber-intercrop trial in northeast Thailand (Clermont-Dauphin et al. 2016, Agriculture, Ecosystems & Environment 217:79-88). A measured upper reference point from a single site, not a design value. The same trial found the cover reduced the drought resilience of the young rubber on water-limited ground.
• Mucuna bracteata: 67 to 84% of its nitrogen drawn from the atmosphere by 15N isotope dilution (Cheah, Zaharah & Aminuddin 2010, MPOB Oil Palm Bulletin 60). That is a proportion, not a quantity per hectare, and no traceable absolute figure exists.
• Centrosema pubescens: no verified species-specific nitrogen figure is published.
• Calopogonium mucunoides: no verified species-specific nitrogen figure is published.
• Calopogonium caeruleum: no single species-specific annual fixation rate is published for CC from verified evidence; one Papua New Guinea oil palm study estimated 28 to 75% of its nitrogen from fixation (Pipai et al. 2023). We publish no annual figure for it.

Best green manure crops for the tropics

The tropical legume cover crops used in plantations are the workhorse green manures of Southeast Asia and similar climates:

• Mucuna bracteata: vigorous, deep-rooting, and the shade-tolerant species of the group; nursery-raised and transplanted, never broadcast. Published plantings are 320 seedlings per hectare, two per palm point, from about 500 seeds (75 g) on peat (MPOB TT No. 501), and 680 seedlings per hectare at 1.5 m spacing on mineral soil (Samedani et al. 2015). See the MB guide.
• Pueraria javanica: fast cover and strong nitrogen contribution on open ground; thins as a canopy closes. Broadcast at about 4 kg/ha (Tropical Forages).
• Calopogonium mucunoides: rapid early growth, good nurse crop. Broadcast; the seeding rate is set on the quotation.
• Centrosema pubescens: one of the two shade-persistent successors. Broadcast at 3 to 5 kg/ha (Tropical Forages).
• Calopogonium caeruleum: hardy, persistent perennial and the other shade-persistent successor. Usually sown in a mixture at 1 to 1.5 kg/ha with 5 to 7.5 kg/ha PJ (Tropical Forages).

For full detail on each species and selection, see the cover crops guide and the five-species comparison.

How to use a green manure

1. Establish the crop. Broadcast or transplant the chosen legume into the inter-rows or a fallow block.
2. Grow it. Allow it to build biomass and fix nitrogen over the season.
3. Return it to the soil. Slash and incorporate, or leave it as a living or cut mulch, so the nutrients and organic matter feed the soil.

Frequently asked questions

What is green manure?
A crop grown to be returned to the soil for fertility and organic matter, rather than harvested. Legumes are common because they also fix nitrogen.

Best green manure crops for the tropics?
The legume cover crops Mucuna bracteata, Pueraria javanica, Calopogonium mucunoides, Centrosema pubescens, and Calopogonium caeruleum.

Cover crop vs green manure?
A cover crop protects and covers soil; a green manure is grown to be returned to soil for fertility. The same species often serve both roles.

How much nitrogen does a legume green manure add?
There is no single defensible range across the species, and we would rather say that than publish one. The measurements that exist are set out species by species above, with the method attached: a proportion for Mucuna bracteata, a biomass accumulation from one trial for Pueraria javanica, and no species-specific annual figure for Calopogonium caeruleum. For Centrosema pubescens and Calopogonium mucunoides, no verified species-specific nitrogen figure is published. How much nitrogen you actually get depends on how much biomass the crop grows on your site.

Related reading

• Green Manure in Rice: The Nitrogen Hack That Isn't Fertilizer
• Nitrogen Fixation in Tropical Plantations
• The Role of Leguminous Plants: Nature's Soil Fertility Boosters
• Urea Use on Oil Palm

Sources: Clermont-Dauphin et al. (2016), Agriculture, Ecosystems & Environment 217:79-88; Cheah, Zaharah & Aminuddin (2010), MPOB Oil Palm Bulletin 60; MPOB TT No. 501 / Information Series 588 (June 2012); Samedani et al. (2015), International Journal of Agriculture and Biology 17(2): 251-260; Pipai et al. (2023), Plant and Soil 491: 665-680; Tropical Forages database (CSIRO, CIAT, ILRI). Figures are given with method and context; results depend on site, soil, rainfall, and management. Last reviewed October 2026.