Many paddy farmers in Malaysia's granary areas (Kedah, Perlis, Selangor, Perak) find that yields respond less and less to extra fertiliser, even when soil tests show adequate nitrogen, phosphorus and potassium. Part of the explanation can be biological rather than chemical: soil organic matter, soil structure and the soil microbial community all affect how efficiently rice roots take up the nutrients that are already there.
What soil biology does in a paddy field
Paddy soils host a large and diverse community of bacteria, fungi, actinomycetes, protozoa and nematodes. Together they form the soil food web: bacteria mineralise organic matter and release plant-available nitrogen; fungi, including arbuscular mycorrhizal fungi where the soil is not flooded, extend the reach of roots; protozoa graze bacteria and recycle the nutrients they hold. The size and activity of this community is usually measured as microbial biomass carbon or as microbial respiration (CO₂ released per kg of soil per day).
Fertiliser on its own is not the main threat to this community. A meta-analysis of paddy rice studies found that mineral fertiliser, on average, increased soil microbial biomass and soil organic carbon rather than reducing them (Geisseler, Linquist and Lazicki, 2017). The bigger risks are low organic matter inputs and practices that remove or destroy the material soil organisms live on.
What limits soil biology in paddies
Several common practices put pressure on paddy soil biology. Long periods of flooding create anaerobic conditions, so aerobic decomposers give way to anaerobic microbes such as methane-producing archaea and sulphate-reducing bacteria, and organic matter breaks down more slowly. Repeated puddling breaks down soil aggregates and can bury organic matter in anaerobic layers. Burning or removing rice straw takes away the main source of fresh organic carbon that the field itself produces. Over many seasons these practices can lower soil organic matter, and with it the food supply for soil organisms.
Humic acid and soil biology
SoilBoost EA is a leonardite-derived humic acid soil conditioner. The scientific literature reviews humic substances mainly as plant growth promoters: they can influence root growth and nutrient uptake, and they interact with soil minerals and organic matter (Nardi, Schiavon and Francioso, 2021). Their effects on the soil microbial community depend on the soil and its management and are less well documented for Malaysian paddy fields, so treat humic acid as one part of an organic matter programme rather than a replacement for it.
Humic acid does not replace straw, compost or manure as a source of organic matter. It is concentrated and easy to handle, so it can be used alongside those inputs. The application rate is set on your quotation for the site.
Cover crops between rice crops
Most granary areas grow two rice crops a year, so the break between crops is often short. Where a field has a longer break, or is taken out of rice for a season, a legume cover crop keeps living roots in the soil and adds nitrogen through biological fixation. Legume roots release sugars and amino acids that feed bacteria and fungi in the rhizosphere, nodule bacteria (Rhizobium, Bradyrhizobium) multiply, and soil fauna recover as fresh organic matter cycles again. When the legume is incorporated before the next rice crop, its biomass becomes a pulse of organic carbon that soil microbes break down, releasing nitrogen and other nutrients.
The legumes Chemiseed supplies, such as Pueraria javanica and Mucuna bracteata, are perennial plantation cover crops. They are best suited to land that stays out of rice long enough for them to establish, so talk to us about your field and water regime before planning a paddy fallow. Cover crops also protect the soil surface between seasons and add roots and residues that help soil structure.
A practical sequence to test on your farm
Rather than changing a whole farm at once, test the approach on one plot next to a comparison plot managed as usual:
- Start with a soil test that includes organic matter, and repeat it each season on both plots.
- Return rice straw to the field instead of burning it, where practical.
- After harvest, apply SoilBoost EA at the rate on your quotation and incorporate it shallowly rather than burying it deep in waterlogged soil.
- If the field has a long enough break, sow a legume cover crop and incorporate it some weeks before the next rice crop so it has time to decompose. Organic matter buried in waterlogged soil decomposes anaerobically and can produce methane instead of releasing nitrogen to the crop.
- Do not cut nitrogen on the assumption that the legume or humic acid will replace it; adjust fertiliser only on soil and plant test results, with your agronomist.
- Compare yields between the two plots over at least two seasons before scaling up.
Caveats
Yield responses vary with soil, water management, variety and season, which is why a side-by-side plot is the best guide. Where fields are cropped continuously there is little time for a cover crop, and recovery relies more on straw return and organic amendments. Water management also matters: fields that are drained periodically tend to support more aerobic decomposition than fields that stay flooded, because waterlogging favours methane-producing archaea over aerobic decomposers.
References
Geisseler, D., Linquist, B.A. and Lazicki, P.A. (2017). Effect of fertilization on soil microorganisms in paddy rice systems: a meta-analysis. Soil Biology and Biochemistry 115: 452-460. https://doi.org/10.1016/j.soilbio.2017.09.018
Nardi, S., Schiavon, M. and Francioso, O. (2021). Chemical structure and biological activity of humic substances define their role as plant growth promoters. Molecules 26(8): 2256. https://doi.org/10.3390/molecules26082256