Rice growers in the Philippines often ask whether a legume grown in the dry-season fallow, such as Pueraria javanica (PJ), can supply part of the nitrogen the next rice crop needs. This article sets out what the research supports, what it does not, and how to test a rotation on your own paddies before you change fertiliser rates.
The process behind it is nitrogen cycling. How legume nitrogen becomes available fast enough for rice uptake depends on soil biology.
Legume Nitrogen in the Soil Bank
Pueraria javanica, Mucuna bracteata, Clitoria ternatea, and Calopogonium mucunoides are nitrogen-fixing legumes (Calopogonium mucunoides is short-lived and behaves as an annual where the dry season is marked). Rhizobia bacteria in their root nodules convert atmospheric N₂ into ammonia, which moves into plant tissue. When the legume is incorporated (usually by in-situ mulching or ploughing some weeks before rice transplanting), that plant nitrogen becomes soil organic matter.
How much nitrogen a dry-season legume leaves for the next rice crop depends on the stand, the length of the fallow and the soil, so we do not publish a single figure for it; treat any number as something to measure on your own paddies. At a global scale, a meta-analysis of legume-rice rotations (1,483 paired comparisons from 96 publications in 17 countries) found rice yields after a legume averaged 15.7% higher than in rotations without one, with a median of 8.8% (Yao et al. 2025). That is an average across many sites and systems, not a promise for any one farm.
Not all of the nitrogen in a legume cover crop is immediately available to the next crop. Some remains bound in stable organic matter; only the labile fraction mineralises into plant-available ammonium and nitrate in the weeks after incorporation.
The Mineralisation Bottleneck
If legume nitrogen is already in the soil, why do farmers still apply high rates of urea? Because timing and availability are not the same thing. Legume N sits in organic matter: amino acids, proteins, nucleic acids. Rice needs it as ammonium (NH₄⁺) and nitrate (NO₃⁻). The conversion happens through microbial activity: bacteria and fungi break down organic compounds, release ammonia, and nitrifiers oxidise it to nitrate. This process is called mineralisation.
In cooler, waterlogged soils (January to February in Central Luzon), mineralisation slows, because microbial activity falls as soil temperature drops and oxygen runs short. In well-drained, warm soils during May to August, mineralisation runs faster. But the timing must align with rice demand. Rice nitrogen demand peaks around panicle initiation. If mineralisation is still slow then, the rice will show deficiency symptoms despite abundant organic N nearby.
SoilBoost EA is a soil conditioner, not a nitrogen source, and we do not claim that it speeds up mineralisation in paddy soil. If you want to see whether it adds anything on your farm, test it as a separate treatment, as described below.
How to Test It on Your Own Farm
How a legume rotation changes the nitrogen your rice needs depends on the soil organic matter baseline and irrigation practice, and we cannot name a controlled Philippine trial that measured it. Test it on your own paddies before you change fertiliser rates across the farm.
A simple design: choose paddies of equal size, soil and cropping history. Keep your usual fertiliser programme on one; on the other, grow the legume in the fallow, incorporate it, and set the nitrogen rate from a soil test taken before transplanting. If you also want to try SoilBoost EA, run it as a separate treatment so its effect is not mixed up with the legume's. Record yield from each paddy separately and repeat over at least two seasons before drawing conclusions.
The legume species matters. In regions with a strong amihan (November to March), PJ survives better. In upland farms with erratic irrigation, MB is more drought-tolerant. Clitoria ternatea is short-season and suits farms that want faster turnover to rice.
Integration with Existing Schedules
One example schedule in wet-season rice areas: harvest October, fallow November to January, plant legume January (or delay to February if dry spell is severe), incorporate April, plant rice May. This leaves a few weeks between legume incorporation and rice transplanting for initial mineralisation and soil settling.
If you include SoilBoost EA as a separate treatment, apply it at the rate on the product label or as set on your quotation, and keep a paddy without it so you can compare.
Economics and Risk
Legume seed (PJ), SoilBoost EA, and labour for mulching incorporation all carry a cost; request a current quote for your crop and hectarage to size the total added cost per rotation.
Against this, legume cover contributes nitrogen; adjust your fertiliser programme only after soil and leaf analysis shows what the cover supplies on your block. Do not expect the added cost to be recovered in the first season; whether the rotation pays back, and how quickly, varies by farm, so track costs and yields over several seasons.
Risk is low if the farm already manages legumes successfully. If legumes are new to the farm, start with a small area and observe establishment and decomposition before scaling. Legumes can fail if: (1) the paddies are severely waterlogged year-round (fix with improved drainage), (2) herbicide residues prevent germination, or (3) poor-quality seed is used. Use seed that comes with germination and purity test results for the lot.
What to Measure
Baseline soil test (pre-legume): total nitrogen (Kjeldahl or Leco combustion method), organic matter, pH. Measure legume biomass at incorporation time (cut ten 1 m² plots, dry to constant weight, calculate kg/ha). Re-test soil after the rice harvest to see residual N status. Track rice grain yield by paddies separately. Judge the trial on the rice yield and the nitrogen you applied in each paddy, season after season, rather than on a single threshold.
The shift from urea-heavy protocols to legume-mineralisation protocols is not a replacement; it is a rebalancing. Farms that do this well run part of their nitrogen through legumes and part through fertiliser, which spreads the risk of relying on a single input and the cost of it.
References
Yao, W., Yang, Y., Beillouin, D., Zhao, J., Olesen, J.E., Zhou, J., Smith, P., Zeng, Z., Lambers, H., Rillig, M.C., Wanger, T.C. & Zang, H. (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