Green manure in rice: the nitrogen hack that isn't fertiliser
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Quick takeaways
- Legume green manures can supply part of a rice crop's nitrogen. In published field trials, green manure gave rice yields comparable to urea (Diekmann et al., 1996; Clément et al., 1998).
- In most systems green manure supplements synthetic nitrogen rather than replacing it; the balance is usually applied as split urea topdressing.
- Timing of incorporation matters: nitrogen release should match the rice crop's demand, and fresh biomass incorporated just before transplanting can harm young seedlings.
- Humic acid does not supply nitrogen. It may help hold ammonium in the root zone, but a 2024 meta-analysis found weaker responses in paddy rice than in upland crops (Ma et al., 2024).
- What we will not claim: that green manure removes the need for all synthetic nitrogen, that it works equally well in all rice systems, or that SoilBoost EA alone replaces urea. Green manure is a nitrogen source; SoilBoost EA is a soil conditioner.
Why this guide exists
Synthetic nitrogen, mainly urea, is one of the larger input costs for rice farmers in Malaysia and Southeast Asia. Urea prices follow natural gas markets and have spiked several times in the past decade, and a large share of applied urea nitrogen can be lost through volatilisation, leaching and denitrification before the crop takes it up.
Green manure crops offer a partial substitute: atmospheric nitrogen fixed biologically, incorporated into the soil and released in the root zone. The practice is long established in Asian rice systems, and its potential and limitations have been well reviewed (Becker et al., 1995), but the research rarely reaches farmers in a usable form.
This guide summarises that research for rice systems in Malaysia and tropical Asia.
1) How green manure nitrogen works in rice
The basic cycle
- Fallow period: After the rice harvest, a leguminous green manure is established in the field. Most species are grown on the drained field; flood-tolerant species such as Sesbania rostrata can also grow under flooded conditions.
- Growth phase: The legume fixes atmospheric nitrogen through its symbiosis with rhizobia and stores it in its nodules and tissue.
- Incorporation: The green manure is chopped and ploughed or otherwise incorporated into the soil before the next rice crop is transplanted.
- Decomposition: Soil microorganisms break down the green manure and mineralise the fixed nitrogen into ammonium that rice roots take up.
How much nitrogen?
Field trials give a sense of the range:
- At four sites in Luzon, Philippines, Sesbania rostrata grown under flooded conditions for 49 days accumulated up to 190 kg N/ha in its shoots, and Aeschynomene afraspera up to 196 kg N/ha. Every green manure treatment raised grain yield over the untreated control, by 1.3 to 1.7 t/ha, and yields were comparable to those with 60 kg N/ha of urea (Diekmann et al., 1996).
- In an IRRI field study comparing ten green manure species with urea, each applied at 80 kg N/ha, most green manure species performed as well as deep-placed urea supergranules. Residual effects on the following dry-season crop were generally modest (Clément et al., 1998).
Not all the nitrogen in a green manure reaches the rice crop. Some is lost or tied up during decomposition, and the amount recovered depends on species, biomass, soil and timing.
2) Which green manure species work best
Sesbania rostrata: the most studied option
Sesbania rostrata is one of the most studied green manures for rice. It forms nodules on its stem as well as its roots, so it keeps fixing nitrogen when the field is flooded. This makes it well suited to paddies where water levels during the fallow are hard to control.
Crotalaria juncea: a high-biomass option
Crotalaria juncea (sunn hemp) grows quickly and produces a large amount of biomass. Incorporate it before the stems become woody, as mature stems break down more slowly.
Mung bean: the farmer-friendly choice
Vigna radiata (mung bean) fixes less nitrogen than Sesbania or Crotalaria but provides a harvestable grain crop. Farmers can sell the mung bean harvest and still plough in the remaining biomass. This economic dual use often decides adoption more than the nitrogen numbers do.
3) Timing: matching nitrogen release to rice demand
A common problem in green manure systems is a timing mismatch: nitrogen is released too early and lost before the rice needs it, or the crop runs short of nitrogen during tillering.
In the IRRI study, little green-manure nitrogen was mineralised after the first 30 days from transplanting, and between 52 and 69 days after transplanting rice took up nitrogen faster with green manure than with urea, which the authors read as better synchrony between nitrogen supply and crop demand (Clément et al., 1998).
Why allow a gap before transplanting
Fresh biomass incorporated just before transplanting decomposes under flooded conditions and can produce organic acids and hydrogen sulphide that harm young seedlings. Leaving an interval between incorporation and transplanting lets these break down. Incorporating too early carries the opposite risk: nitrogen is released before the crop can use it. Agree the interval for your soil and season with your local extension service or agronomist.
4) Where humic acid fits
SoilBoost EA is a leonardite-based humic soil conditioner. It does not fix or supply nitrogen. Humic substances can add to the soil's cation exchange capacity, which may help hold ammonium in the root zone.
A 2024 meta-analysis of humic acid amendments reported higher nitrogen use efficiency on average, but the response was weaker in paddy rice than in upland cereals and cash crops, and weaker in alkaline soils (Ma et al., 2024). SoilBoost EA was not among the products tested. If you use it in a green manure system, apply it at incorporation at a rate agreed with your agronomist.
5) Practical recommendations by system type
Double-crop irrigated rice (2 seasons a year)
The fallow between crops is short, which limits the options. Fast-growing, flood-tolerant species such as Sesbania rostrata suit this window best. Some farmers relay-sow mung bean into the standing rice shortly before harvest to gain time.
Single-crop rainfed rice (1 season a year)
A longer fallow allows a full green manure crop. Sesbania, Crotalaria or a grain legume such as mung bean can be established after harvest and incorporated before the next wet-season planting. At a rainfed site on marginal soil in the Philippines, S. rostrata green manure gave higher grain yields than urea (Diekmann et al., 1996).
Direct-seeded rice
Direct-seeded rice may be more sensitive to decomposition products than transplanted seedlings, so allow a longer interval between incorporation and seeding.
Frequently asked questions
Q: Can green manure completely replace urea in rice?
A: In most systems, no. Green manure supplies part of the crop's nitrogen and the balance is usually applied as split urea topdressing. How much it supplies depends on species, biomass, soil and timing; in the Philippine trials above, green manure gave yields comparable to 60 kg N/ha of urea (Diekmann et al., 1996).
Q: Won't growing a cover crop during the fallow use water I need for rice?
A: Most green manures are grown on residual soil moisture or rainfall during the fallow. Flood-tolerant species such as Sesbania rostrata and Aeschynomene afraspera can also be grown in flooded fields, as in the Philippine trials (Diekmann et al., 1996).
Q: Is green manure economically worth it?
A: It depends on local urea prices, seed and labour costs and the length of the fallow. Work it through with your own figures: value the nitrogen the green manure is expected to supply at your urea price, then subtract the cost of seed, establishment and incorporation. Gains in soil organic matter come on top but are harder to price.
Q: Does SoilBoost EA replace green manure?
A: No. SoilBoost EA is a soil conditioner. It does not fix or supply nitrogen. The two can be used together.
Sources
- Becker, M., Ladha, J.K. & Ali, M. (1995). Green manure technology: Potential, usage, and limitations. A case study for lowland rice. Plant and Soil 174: 181-194. https://doi.org/10.1007/BF00032246
- Clément, A., Ladha, J.K. & Chalifour, F.-P. (1998). Nitrogen dynamics of various green manure species and the relationship to lowland rice production. Agronomy Journal 90(2): 149-155. https://doi.org/10.2134/agronj1998.00021962009000020005x
- Diekmann, K.H., Ottow, J.C.G. & De Datta, S.K. (1996). Yield and nitrogen response of lowland rice (Oryza sativa L.) to Sesbania rostrata and Aeschynomene afraspera green manure in different marginally productive soils in the Philippines. Biology and Fertility of Soils 21: 103-108. https://doi.org/10.1007/BF00336000
- Ma, Y., Cheng, X. & Zhang, Y. (2024). The impact of humic acid fertilizers on crop yield and nitrogen use efficiency: a meta-analysis. Agronomy 14(12): 2763. https://doi.org/10.3390/agronomy14122763
About this article
This guide is part of Chemiseed's evidence-based content programme. We separate field-supported claims from mechanistically supported ones and are transparent about where evidence gaps remain.
Last updated: October 2026
