CASE STUDY
A factorial field trial published in the Indonesian Journal of Agronomy (August 2025) has produced one of the clearest quantified demonstrations to date of a principle long argued by humic acid suppliers but rarely tested this rigorously: humic acid does not simply add to the effect of organic manure — it multiplies it, while allowing growers to apply substantially less manure for an equal or better result. Researchers at IPB University (Bogor Agricultural University) ran a two-factor randomised block trial on organic edamame soybean, combining four rates of chicken manure (0–24 t ha⁻¹) with four rates of humic acid (0–90 kg ha⁻¹) across 48 experimental plots.
For procurement and agronomy teams operating in the Gulf Cooperation Council (GCC) region, the relevance of this trial extends well beyond soybean. Its core finding — that humic acid measurably improves the efficiency of organic fertiliser rather than merely supplementing it — speaks directly to a structural cost problem in GCC agriculture: organic manure is bulky, largely imported or trucked long distances, and expensive to apply at the volumes conventional agronomy recommends. A product category that can reduce required manure tonnage while protecting or improving yield has a direct commercial case in this market.
The study applied chicken manure two weeks pre-planting and delivered humic acid through a seed-coating method combined with Rhizobium inoculation. Soil, vegetative growth, and yield parameters were tracked from planting through harvest at 70–72 days. The chicken manure used was verified against Indonesian Ministry of Agriculture standards for nutrient content (N-total 2.23%, P₂O₅-total 2.95%, K₂O-total 2.27%), giving the trial a defined, reproducible input rather than an unspecified organic amendment.
Humic acid alone, without any manure, increased root length by 68.5% and relative growth rate (RGR) by 111.5% relative to the untreated control. Combined with chicken manure, the same humic acid dose lifted root length by 82.5% and RGR by 288.5%. The best-performing combination — 8 t ha⁻¹ manure with 90 kg ha⁻¹ humic acid — produced the longest roots recorded in the trial (25.95 cm) and the highest RGR (1.01 g g⁻¹ week⁻¹), roughly four times the RGR of manure applied without humic acid at the same rate.
Manure alone at 8 t ha⁻¹ increased flower count by 33.5% and productive node count by 30.8% versus the control. Adding 90 kg ha⁻¹ humic acid to that same manure rate raised those increases to 106.8% and 110.3% respectively — more than triple the response from manure alone. The researchers attribute this to humic acid’s chelation of soil-bound phosphorus and boron, both of which govern flower initiation and are otherwise poorly released by manure application on its own.
The trial’s soil analysis showed that without humic acid, available phosphorus remained relatively low at every tested manure rate. Humic acid’s carboxyl and hydroxyl functional groups chelate the iron, aluminium, and calcium ions that otherwise lock phosphorus into insoluble compounds in the soil — releasing it into a plant-available form regardless of how much manure has been applied. This mechanism is well documented in prior work on phosphate-rock solubilisation and directly explains why phosphorus availability tracked with humic acid dose rather than manure dose in this trial’s soil data.
This distinction matters commercially: growers who increase manure volume to compensate for poor flowering or pod-fill are, according to this data, addressing the wrong variable. The limiting factor was phosphorus release, not phosphorus supply — a chemistry problem that humic acid resolves far more efficiently than additional tonnes of manure.
Plant fresh weight and dry weight followed the same interaction pattern. At the 8 t ha⁻¹ manure rate, adding 90 kg ha⁻¹ humic acid raised fresh weight from 83.0 g to 140.6 g per plant and dry weight from 34.1 g to 46.0 g — both the highest values recorded across all 16 treatment combinations, including those using three times more manure.
Critically for cost-conscious growers, the trial also quantified the downside of manure over-application. At 24 t ha⁻¹ manure with no humic acid, RGR fell to its lowest recorded value and the percentage of empty (unfilled) pods rose to its highest — a pattern the authors attribute to nitrogen imbalance from excess manure interfering with reproductive development. This is direct field evidence that more organic input is not automatically better, and that humic acid allows growers to stay within an efficient, lower-cost manure range while still reaching peak yield.
This trial was conducted on organic edamame in tropical, humic-poor Indonesian soil — conditions that differ materially from the calcareous, alkaline, and often saline soils typical of Bahrain, Saudi Arabia, the UAE, and neighbouring GCC markets. The exact optimum doses reported (9.5 t ha⁻¹ manure, 90 kg ha⁻¹ humic acid) should not be read as a direct GCC field recommendation; soil texture, salinity, irrigation water quality, and crop type will shift the optimal ratio. What the trial does establish, with statistically significant, replicated data, is the underlying mechanism — humic acid unlocks phosphorus and boosts nutrient-use efficiency independently of manure volume — and that mechanism is chemically consistent regardless of geography. It is, if anything, more relevant in GCC conditions, where alkaline soil pH already suppresses phosphorus and micronutrient availability more severely than in the trial’s baseline soil (pH 6.13, slightly acidic).
Scope note: The commercial implications below are extrapolated from the trial’s demonstrated mechanism (humic acid–manure interaction and phosphorus chelation) applied to known GCC soil and logistics conditions. They are not themselves trial-verified outcomes for GCC soils or crops, and should be validated through regional field or pot trials before large-scale programme adoption.
For GCC agricultural distributors, farm operators, and landscaping contractors evaluating humic acid as a manure-efficiency tool, the trial supports three practical conclusions. First, humic acid should be positioned and tested as a fertiliser-efficiency input that reduces the organic manure volume required, not merely as an additive layered on top of existing manure programmes. Second, any regional adoption should begin with small-scale comparative trials — ideally mirroring this study’s factorial design across a narrower, locally relevant range of manure and humic acid rates — given the demonstrated sensitivity of results to soil pH and baseline nutrient status. Third, procurement decisions should evaluate humic acid products specifically on functional group density and water solubility, since the trial’s seed-coating delivery method depended on the humic acid being immediately bioavailable rather than requiring slow microbial release, a distinction with direct implications for product selection.
Humicore supplies leonardite-derived humic acid products to the GCC agricultural and landscaping market, sourced from Ukrainian raw material deposits and formulated for regional soil conditions. Distributors and operators evaluating a manure-reduction or soil-fertility-efficiency programme along the lines described in this research are welcome to contact Humicore to discuss product specification and pilot trial design suited to local soil and crop conditions.
