Legume green manure before rice: the rotation nitrogen math

A legume grown before rice can lift the following rice crop and supply part of its nitrogen. A global meta-analysis of legume-rice rotations found that placing a legume in the rotation raised the next rice yield by 15.7 percent on average against a non-legume control, with a median gain of 8.8 percent, across 1,483 paired comparisons from 96 publications in 17 countries (Yao et al., 2025). This article works through what that figure assumes, what the research does not promise, and how a Malaysian grower would put the practice into a real cropping calendar.

What does the data actually say about legume-before-rice?

The 15.7 percent figure is an average; because the median was 8.8 percent, a typical field sees less than the headline. A wider meta-analysis of legume pre-crops across main crops, including rice, found that the yield advantage declines as nitrogen fertiliser rates rise and is larger in low-yielding environments than in high-yielding ones (Zhao et al., 2022).

Read those figures as central tendencies across many sites, not a guarantee for your block. The advantage is largest where soil nitrogen is the limiting factor and where the legume is well nodulated and fully incorporated before it sets seed. On a field already receiving heavy nitrogen, the gain shrinks because nitrogen is no longer the constraint.

Field trials in the Philippines show what a well-grown green manure can contribute on poor lowland soils: a 49-day flooded green manure of Sesbania rostrata or Aeschynomene afraspera accumulated up to 190 and 196 kg N per hectare in its shoots, respectively, and raised rice grain yield over the control by 1.3 to 1.7 t per hectare (Diekmann et al., 1996). An IRRI study of ten green manure species, each applied at 80 kg N per hectare, found that most performed as well as urea supergranules, but that residual effects on the following season's rice were generally modest (Clément et al., 1998).

Why does the nitrogen carry over at all?

A legume green manure fixes atmospheric nitrogen through its root nodule bacteria and stores it in leaf, stem, and root tissue. When that green tissue is ploughed in while still succulent, soil microbes mineralise it over the following weeks and release plant-available nitrogen into the root zone just as the rice crop is establishing. The closer the incorporation timing sits to transplanting or direct seeding, the better the synchrony between nitrogen release and crop demand.

How should a grower adjust nitrogen?

How much bag nitrogen a green manure can replace depends on your soil's existing nitrogen, how well the legume nodulated, how much biomass it made and how completely it was incorporated. The safe approach is to treat the green manure as a partial replacement and confirm it on your own soil before committing.

A sensible first season looks like this:

• Grow the legume and incorporate it during land preparation, before planting rice.
• Cut your baseline nitrogen rate by a conservative margin on a trial strip only.
• Compare tiller count, canopy colour, and yield on the trial strip against your standard block.
• Scale the reduction up the following season if the trial holds.

This staged method protects yield while you learn how much nitrogen your particular soil and legume combination delivers.

Where does this fit a Malaysian cropping calendar?

The rotation suits the window between rice crops, or a fallow that would otherwise grow weeds. A short-duration legume drilled into that gap builds nitrogen and ground cover at the same time, which suppresses weeds and protects the soil surface from monsoon rainfall. The legume is terminated and incorporated during land preparation so the residue has time to break down before the rice goes in.

Two practical cautions. First, incorporation timing matters more than total biomass: a younger, greener crop mineralises faster and more predictably than an older, woody one. Second, the legume needs the right root nodule bacteria to fix nitrogen well in the first place, so inoculation is worth confirming on soils that have not carried that legume before.

What this rotation does not promise

The rice yield uplift and the nitrogen contribution are soil-fertility outcomes measured across many trials. They are not a fixed result for any single field, and they do not replace good agronomy elsewhere in the crop. Phosphorus, potassium, water management, and pest pressure all still govern the final yield. The green manure shifts the nitrogen economics in your favour; it does not carry the whole crop.

FAQ

How long before planting rice should I incorporate the legume? Incorporate during land preparation, before transplanting or seeding, while the legume is still green and succulent. That timing lines up nitrogen release with early crop demand.

Can I cut my nitrogen rate after a green manure? You may be able to apply less, but how much depends on your soil's existing nitrogen, how well the legume nodulated, and how completely you incorporated it, and residual effects on later crops are usually modest (Clément et al., 1998). Trial a conservative reduction on a strip first, then scale.

Does the 15.7 percent yield gain apply to every field? No. 15.7 percent is the average across 1,483 paired comparisons, and the median was 8.8 percent. The gain is largest where nitrogen is the limiting factor and smaller on already nitrogen-rich fields (Zhao et al., 2022).

Talk to an agronomist

If you are planning a legume green manure ahead of rice and want to match the species, seeding rate, and inoculant to your soil, talk to a Chemiseed agronomist about our legume seed range. Request a quote or message us on WhatsApp at +62 877 7837 4714.

Sources

• 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(1-2): 103-108. https://doi.org/10.1007/BF00336000
• Yao, W., Yang, Y., Beillouin, D., Zhao, J., et al. (2025). Legume-rice rotations increase rice yields and carbon sequestration potential globally. One Earth 8(2): 101170. https://doi.org/10.1016/j.oneear.2024.12.006
• Zhao, J., Chen, J., Beillouin, D., Lambers, H., Yang, Y., Smith, P., Zeng, Z., Olesen, J.E. & Zang, H. (2022). Global systematic review with meta-analysis reveals yield advantage of legume-based rotations and its drivers. Nature Communications 13: 4926. https://doi.org/10.1038/s41467-022-32464-0