Some rice growers in Central Luzon and the Visayas describe the same pattern: yields have stalled while urea rates have crept up, and the extra nitrogen has not brought the yield back. Soil tests may show nitrogen, phosphorus and potassium at adequate levels, yet the crop tillers less, grain weight is lower and the ratoon crop is weaker. When more fertiliser stops paying, it is worth looking at what else the soil is missing.
One likely contributor is a run-down in soil organic matter, and in the biology it feeds, after many years of continuous cropping, intensive puddling and residue removal. You do not need to guess: a soil test that includes organic matter, and a simple trial on your own paddies, will tell you more than another increase in urea.
What Continuous Rice Monoculture Does
Many Philippine paddies are cropped two or three times a year on the same small farm. Residue is burned or removed for livestock feed or fuel. Soil is puddled each season. Organic inputs are minimal: a handful of compost if the farmer is deliberate, nothing if not. Under this regime, little organic matter goes back into the soil to feed soil microorganisms (the bacteria, fungi and protozoa that help stabilise soil structure, mineralise organic matter and release nutrients).
Research does not support blaming mineral fertiliser itself: a review of long-term trials found that mineral fertiliser use generally increased soil microbial biomass rather than reducing it (Geisseler and Scow 2014). What usually matters more is how much organic matter, from residues, compost and living roots, goes back into the soil each season.
Past a point, more urea does not help. The crop cannot use the surplus, and much of what it does not take up is lost through volatilisation, leaching and runoff.
Humic Acid as a Soil Conditioner
SoilBoost EA is a humic acid soil conditioner. It is not a fertilizer or a biofertilizer: it does not feed soil microorganisms or introduce them. Growers apply it at the rate on the product label or quotation, alongside legume cover and organic inputs as part of a broader programme, where it may support soil structure and root-zone conditions.
Soil biology is rebuilt mainly through regular organic inputs: crop residues returned to the field, compost, and living roots such as a legume cover. A soil conditioner does not restart soil biology on its own, and it is not a substitute for those inputs or for fertilizer.
Our only trial data for SoilBoost EA come from a company-commissioned trial at PCA-Davao on banana seedlings, registered to Kudzu Seeds Trading under FPA Experimental Use Permit No. 3227. There is no rice trial, so test it on your own paddies before you rely on it.
Off-Season Legume as Biological Pump
A pueraria (PJ) or Mucuna bracteata (MB) cover crop established in the fallow month (July to August, between dry-season and wet-season rice, or May to June before wet-season planting) does two things: it fixes nitrogen, and its root exudates (carbohydrates, amino acids, and organic acids released by living roots) feed the microbial community directly. Legume roots die back at plowing; their biomass becomes substrate. The combination of fresh organic matter (legume roots and leaves) plus exudate-stimulated microbial activity restarts the soil food web from the bottom up.
The research supports this direction. A meta-analysis of 60 studies found that cover cropping increased soil microbial abundance by 27%, activity by 22% and diversity by 2.5% compared with bare fallow, with smaller effects where the cover was killed with herbicide or under conservation tillage (Kim et al. 2020). Most of those studies were in annual cropping systems rather than paddy rice. For rice itself, a global meta-analysis of legume-rice rotations found rice yields after a legume averaged 15.7% higher than without one, with a median of 8.8% (Yao et al. 2025). Neither result is a promise for a particular paddy, which is why the trial below matters.
Two-Paddy Trial Protocol
Do not treat your entire farm in one year. Pick two adjacent paddies of similar size and soil type. Treat one as control (continue current management: rice monoculture, no cover, standard urea rate). In the fallow, sow the second paddy to PJ, drilled or planted in holes, at 10 kg/hectare for PJ sown alone (our recommendation); we confirm the rate for your paddy on the quotation. Let the PJ grow through the fallow, then plough it under before land preparation. Apply standard NPK at the same rate as the control paddy. Legume cover contributes nitrogen; adjust your fertiliser programme only after soil and leaf analysis shows what the cover supplies on your block. If you also want to test SoilBoost EA, add it on a third paddy at the label or quotation rate, so its effect is not mixed up with the legume's.
Monitor tiller number at 30 days (count tillers in five 30×30 cm squares per paddy), grain weight at harvest (take 100-grain weights from five random spots), and yield in metric tons. Repeat the next season on the same paddies, alternating treatments if you prefer (cross-over design strengthens the conclusion). By year two, you will have two seasons of your own data. Results vary by site, so judge the treatment by the difference you measure between the two paddies.
Working out your return: Cost of legume seed, SoilBoost EA, and additional labor: request a current quote for your crop and hectarage. Compare that cost with the value of any yield difference you measure between the two paddies. We do not publish an expected yield gain or return for this protocol, because no trial we can cite has measured one.
Start Now for Next Season
If you see stalled yield and rising nitrogen costs, it may be a sign that soil organic matter and biology need attention rather than more urea. A two-paddy trial costs little and shows you, and neighboring farmers, whether the approach helps on your soil. If the treated paddy does better, expand block by block over several seasons, and keep an untreated block for comparison.
References
Kim, N., Zabaloy, M.C., Guan, K. & Villamil, M.B. (2020). Do cover crops benefit soil microbiome? A meta-analysis of current research. Soil Biology and Biochemistry 142: 107701. https://doi.org/10.1016/j.soilbio.2019.107701
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
Geisseler, D. & Scow, K.M. (2014). Long-term effects of mineral fertilizers on soil microorganisms: a review. Soil Biology and Biochemistry 75: 54-63. https://doi.org/10.1016/j.soilbio.2014.03.023