What is humic acid?
Soil science
What is humic acid?
By Joe, Founder of Dr Forest · June 2026
It's the dark, medium-weight fraction of the humus in any healthy soil. Too big to get inside a plant, which is exactly why it earns its keep in the ground.
Humic acid is the medium molecular weight, alkali-soluble fraction of humic substances: the carbon-rich organic material that builds up when plant residues are broken down by microbes over very long timescales. It sits in the same family as fulvic acid and humin. The difference is size and where it works. The humic fraction is large, dark and stays in the soil, where it holds onto nutrients and helps build structure. Fulvic material is smaller and gets inside the plant.
Chemically, humic acid is a tangle of aromatic carbon rings hung with acidic groups, mostly carboxyl (–COOH) and phenolic hydroxyl (–OH). Those groups carry a negative charge, so they grip positively charged nutrients like calcium, magnesium, potassium and ammonium and stop them washing away with the rain. The conventional size figure quoted for humic acid is 10,000 to 100,000 daltons (a dalton is the unit chemists use for the weight of a single molecule; a water molecule weighs 18). Treat that as a working range rather than a measured constant, for reasons set out below. What matters in practice is that the humic fraction is large enough to stay outside the plant, so almost all of its work happens in the soil and at the root surface.
It's used as a soil conditioner, a base dressing and a soluble drench. There's a solid body of peer-reviewed evidence behind it, alongside a good deal of marketing that overreaches. This piece is the science, told plainly, including the parts that don't flatter the product.
In short
What it is: the dark, alkali-soluble fraction of humic substances. Humic substances are the carbon-rich organic material that accumulates in soil, peat and leonardite (a soft, oxidised brown coal) through long microbial breakdown.
What it does: raises the soil's cation-exchange capacity (its ability to hold positively charged nutrients so they don't leach), improves water retention and soil structure, can lift microbial activity in soils low in organic matter, and frees up phosphorus locked to calcium and iron. At the root surface it also activates a proton pump and mimics auxin to encourage rooting.
How it works: the fraction is large enough to stay outside the plant and remain in the soil. Carboxyl (–COOH) and phenolic (–OH) groups carry negative charge and bind metal cations: positively charged ions like calcium, magnesium, potassium and ammonium.
Where humic acid comes from
All humic substances start the same way: dead plant matter and microbial residues, broken down by bacteria and fungi over very long timescales. As the easily digested material disappears, what's left behind is carbon-rich and oxygen-rich, with a tangle of acidic functional groups hanging off it. That's humus.
Humus is separated into three fractions according to how soluble the material is in acid and alkali:
- Humin is the fraction that never dissolves. It stays bound to soil minerals and isn't soluble in water at any pH. It contributes to long-term soil structure and water-holding capacity.
- Humic acid is the middle fraction, and the subject of this piece. Soluble in alkali, insoluble in strong acid (below about pH 2). It holds positively charged nutrients on its surface, lifts the soil's cation-exchange capacity and interacts with the root.
- Fulvic acid is the smallest fraction. Soluble in water at any pH. It crosses cell walls, chelates trace minerals and acts as a biostimulant inside the plant.
Figure 01 · Humic substances
The family of humic substances
Three fractions of the same dark organic material, separated by solubility. Humic acid is the middle one. The molecular weight figures are conventional trade and older-literature ranges, not measured constants; the fractions are defined by how they dissolve.
How solid are those molecular weight numbers?
Less solid than most labels imply, including plenty in this industry. Worth knowing before you read the rest.
The three fractions are defined by a laboratory procedure rather than by chemistry. You take soil, peat or leonardite, extract it with alkali, then acidify the extract. Whatever precipitates is called humic acid, whatever stays dissolved is called fulvic acid, whatever refused to dissolve in the first place is called humin. The categories describe how the material behaves in that test.
Whether large, stable humic macromolecules exist as such in undisturbed soil is disputed. Lehmann and Kleber argued in Nature in 2015 that the available evidence does not support the formation of large-molecular-size, persistent humic substances in soil, and that soil organic matter is better described as a continuum of progressively decomposing compounds. Piccolo's supramolecular model takes a middle position: what behaves like one big molecule is a loose association of smaller ones held together by weak bonds. Both camps accept that alkaline extraction alters the material it recovers.
None of this makes the product useless, and it doesn't undo the field trial results further down this page. Those trials applied the extracted material to real crops and measured what happened, which is a separate question from what the material looks like at the molecular level. It does mean the molecular weight figures on any humic acid label, ours included, describe an operationally defined fraction rather than a single compound with a fixed mass.
Where commercial humic acid actually comes from
Read the back of a humic acid label and the source story falls into a few categories. They aren't the same material, they don't all behave the same way, and one of them isn't really humic acid at all.
1. Mineral-source humic acid (leonardite, lignite, weathered coal, peat). The classical category. Leonardite is the standout: a naturally oxidised, weathered form of lignite, usually found in shallow seams lying above the more compact coal it came from. Deposits occur in the western US, Canada, China, India and parts of Europe. Published humic acid content varies a great deal by deposit, roughly 39 to 85%, which makes it the richest practical mineral source. Mineral humic acid is dark, stable, high in oxidised aromatic carbon and free of proteins and sugars. It has a long shelf life, and it's what much of the agronomic trial literature has used.
2. Compost and vermicompost-derived humic acid. Humic acids can also be extracted from mature compost, manure compost and worm castings. Atiyeh and colleagues did exactly this in 2002, pulling humic acids from pig manure and food waste vermicompost and growing tomato and cucumber seedlings in media dosed with them. Canellas and colleagues, whose proton pump work appears further down this page, used humic acids from cattle manure vermicompost. This material is real and active. It's also more variable batch to batch than mineral humic acid, and lower in total humic content.
3. Lignosulphonate sold as "potassium humate". The one to watch. Lignosulphonates are paper-mill by-products from sulphite pulping, used as concrete plasticisers and dust suppressants. Sold as sodium or potassium lignosulphonate, they are dark, water-soluble, and look superficially like a humic solution. They are lignin breakdown products, not the humic fraction of soil organic matter, and some low-cost "potassium humate" products are essentially repackaged lignosulphonate.
So which source is best?
Here the evidence cuts against the obvious commercial answer, so it's worth saying plainly. The Rose meta-analysis found that humic substances from compost sources significantly outperformed lignite and peat-derived material for growth promotion. Two of the most-cited mechanism papers, Canellas 2002 and Atiyeh 2002, used vermicompost humic acid rather than leonardite. On raw plant growth response, compost-derived material has the better record.
What leonardite offers is different. It's consistent from batch to batch, high in humic content, stable on the shelf, and it's the form behind most of the agronomic trial work underpinning claims about cation exchange and nitrogen use efficiency. If you want a concentrated soil amendment with a declared humic percentage and a known shelf life, leonardite is the practical choice, which is why we use it. If you already run good vermicompost or well-made compost, you have compost-derived humic acid going into the soil already, and you probably need less of anything bought in.
The one to avoid on any grounds is lignosulphonate sold as humate. A reputable supplier will declare the source and the humic acid percentage on a certificate of analysis without being asked. The Dr Forest humic acid flakes are 70% humic acid by mass, 100% water-soluble, extracted from leonardite, with no lignosulphonate.
How humic acid differs from fulvic acid
The two are usually sold and discussed together because they're the same family of material. They do different jobs because of their different sizes. The companion piece covers the smaller fraction in full: what fulvic acid is.
| Property | Humic acid | Fulvic acid |
|---|---|---|
| Molecular weight (conventional figures) | ~10,000–100,000 Da | ~1,000–10,000 Da |
| Water solubility | Soluble above pH 2 (insoluble in strong acid) | Soluble at any pH |
| Colour in solution | Dark brown to black | Yellow to light brown |
| Enters root cells | No (acts from outside) | Yes (low molecular size fraction) |
| Primary site of action | Soil cation-exchange capacity, structure, root surface | Inside the plant + rhizosphere |
| Cation-exchange contribution | Large, durable | Smaller, more mobile |
| Best application | Soil amendment, base dressing, soluble drench | Foliar spray, root drench, tank-mix with feeds |
| Speed of effect | Slower, longer-lasting | Faster, shorter-lived |
Figure 02 · Molecular size
Why humic acid works from outside the root
Humic substances split by molecular size at the root surface. The large fraction binds to the outside of the cell and signals from there; the small fraction gets in. Threshold after Nardi et al., 2021.
The shorthand most growers use is this: humic acid improves the soil, fulvic acid improves the plant. Slightly oversimplified, since humic acid does reach the root surface and acts there too, but it's a fair starting point.
What humic acid does in the soil
This is the main event. Because the material stays in the ground, most of what it does is soil chemistry and soil biology, and the effects build over a season rather than appearing overnight.
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Figure 03 · Cation exchange
How humic acid holds nutrients in the soil
The negatively charged acidic groups grip positively charged nutrients, so rain doesn't wash them straight through. Schematic illustrative.
Four soil-side effects, in descending order of how well supported they are:
- It raises cation-exchange capacity (CEC). CEC is the soil's ability to hold positively charged nutrients (calcium, magnesium, potassium, ammonium) so they don't leach away with the rain. Humic acid's negatively charged carboxyl and phenolic groups add a large, durable store of exchange sites. This matters most on light, sandy, free-draining soils where natural CEC is low.
- It frees up locked phosphorus. In chalky and high-calcium soils, and in iron- or aluminium-rich clays, phosphate binds into forms plants can't use. Humic acid binds the calcium, iron and aluminium preferentially, releasing some of that phosphorus. Useful where phosphorus availability, rather than phosphorus quantity, is the bottleneck.
- It improves water retention and soil structure. Humic substances help bind soil particles into stable crumbs (aggregates), which improves both drainage and water-holding at the same time. The effect is gradual and cumulative, and it shows up more clearly in soils than in pure sand root zones, where several turf trials have found no water-holding benefit at all.
- It can lift microbial activity, though not reliably. Humic acid is a poor food source next to compost or fresh residues, because the oxidised aromatic carbon that makes it stable also makes it slow to break down. What it does provide is surface, exchange sites and held nutrients that microbes can use. Hartz and Bottoms found humic acid plus fertiliser raised microbial respiration and phospholipid fatty acid content in a low organic matter soil after seven days, with no such effect in a soil already rich in organic matter. Lindsey and colleagues found a similar pattern on sand-based turf: humic products raised microbial activity, but turf quality did not improve.
The honest hedge: humic acid is not a substitute for actual soil organic matter. A scoop of flakes cannot replace years of compost, mulch and worm activity. It is a concentrated dose of one active part of humus, useful for giving a depleted soil a head start. It is not a soil-building strategy on its own.
What humic acid does at the root
Although the material is too large to get inside the plant, it does reach the root surface, and three root-zone mechanisms are reasonably well established in the peer-reviewed literature. These come from humic acid studies specifically, not just general biostimulant work.
Figure 04 · At the root surface
Three mechanisms, well established
What humic acid does where it meets the root, even though it never enters the plant.
It holds nutrients on its surface
Humic acid is rich in carboxyl (–COOH) and phenolic (–OH) groups: acidic clusters that carry a negative charge and bind positively charged metal ions like calcium, magnesium, potassium, ammonium, iron and zinc. Because the material stays put, those nutrients sit on the soil's exchange sites, available to roots but protected from leaching. This is the cation-exchange effect, the single most important thing humic acid does.
It increases root membrane permeability
Studies on maize roots have shown that humic acids activate a tiny biological machine called the plasma membrane H⁺-ATPase: a proton pump that drives nutrient uptake by pushing hydrogen ions out of the root, which makes the soil right at the root surface slightly more acidic and helps minerals dissolve into a form the root can take in. With the pump working harder, ions move into the plant faster. Canellas and colleagues demonstrated this in 2002 using humic acids isolated from cattle manure vermicompost, and it has been replicated since.
It mimics auxin and triggers root growth
Auxin is the natural plant hormone that controls rooting and lateral branching. Humic substances contain fragments that interact with auxin signalling at the root, prompting the plant to respond as if it had received a small dose of the hormone. Trevisan and colleagues showed in 2010 that humic substances induce lateral root formation in Arabidopsis with the same gene-expression signature (IAA19 and the DR5 auxin reporter) as a low dose of auxin itself. The practical effect is a flush of lateral roots and longer root hairs.
Humic acid improves the soil. Fulvic acid improves the plant. Both are useful. Neither replaces compost.
The biostimulant question: what the evidence says
Humic acid is classed as a plant biostimulant under the EU fertilising products regulation 2019/1009: a substance that stimulates plant nutrition processes regardless of nutrient content, with the goal of improving nutrient use efficiency, abiotic stress tolerance or quality. Great Britain kept its own fertiliser rules after leaving the EU and has no equivalent biostimulant category, so on this side of the Channel the word is a scientific description rather than a legal one. Whether biostimulants deliver in the field has been argued over for decades.
The most useful humic-specific synthesis is the meta-analysis by Ma, Cheng and Zhang, published in Agronomy in 2024. A meta-analysis pools the results of many other studies to look for overall trends. This one pooled crop trials testing humic acid amendments and found an average crop yield increase of 12%, a 27% improvement in nitrogen use efficiency, and a 17% increase in nitrogen uptake. Effects were strongest where annual rainfall was above 300 mm and mean annual temperature above 10 °C, and in soils of moderate pH (6 to 8) or low total nitrogen. They were weaker in alkaline soils and in soils already high in nitrogen.
The broader humic-substances picture comes from Rose and colleagues, published in Advances in Agronomy in 2014, which collated and pooled the published literature on applied humic substances. Shoot dry weight rose by 22 ± 4% and root dry weight by 21 ± 6%. Variation between studies was large, and the strongest predictors of response were the source of the humic substances and the rate applied, with plant type and growing conditions mattering less.
Figure 05 · The evidence base
Four numbers from the humic-acid literature
Three pooled meta-analysis averages and one single-trial result, shown together for scale. Results vary widely; soil condition and source material both matter.
The effect is biggest where soils need it most. Variability between products is real.
Two caveats deserve attention. First, the variability between commercial products is large. Source material, extraction method and humic content all matter, and not every product behaves the same way in field trials. Second, the effect depends heavily on whether the soil and management are already good. The Cooper bentgrass work makes this point cleanly: incorporated humate raised root mass by 45% at 0 to 10 cm in sand culture, but no foliar-applied humic acid source consistently beat the control in either sand or solution culture, and phosphorus uptake was unaffected in solution culture. The authors read that as evidence that humic acids do less for plants already well supplied with nutrients. Hartz and Bottoms (2010) tested five commercial humic acid products and found no measurable benefit on romaine lettuce in a greenhouse pot study or on processing tomato in Californian field trials.
The honest summary: the evidence base is real, the average effect is meaningful, the variability is high, and the response is biggest where soils need it most.
Where humic acid sits in a feeding programme
Humic acid is not a fertiliser. It contains very little nitrogen, phosphorus or potassium of its own. What it does is make the rest of the programme work better, by holding nutrients in the root zone, building the soil that feeds the plant, and helping roots take up what's there.
The most useful pairings, based on the published evidence:
- With fulvic acid. The conventional combination, for a reason. Humic for the soil, fulvic for the plant; they work on different timescales and at different points. Dr Forest sells both, used together at planting and through the season.
- With compost and organic feeds. Humic acid extends the reach of a compost-fed soil by adding exchange sites. It is a concentrate of one active part of what good compost already supplies, useful where you can't get enough compost on in time.
- On soils low in organic matter. This is where the cation-exchange effect matters most, and where the trial data shows the biggest response. On heavy, well-structured, compost-rich ground the effect is smaller. On very sandy, low-clay ground, expect more of it to leach away before it does much.
- With trace element and calcium-magnesium feeds. By holding cations in the root zone, humic acid reduces leaching losses of calcium, magnesium and trace minerals on free-draining soils, so more of what you apply stays put.
Dr Forest humic acid
Leonardite-derived, 100% water-soluble, for soil incorporation or drench. Pair it with fulvic acid for the full effect:
- Humic Acid Flakes, 70%. 100% water-soluble leonardite humic acid, for soil amendment, base dressing and drench.
- Fulvic Acid Powder, 70%. The smaller fraction, for foliar spray and tank-mixing with liquid feeds.
Natural plant food and soil conditioners, blended in small batches. Browse the full humic and fulvic collection.
Frequently asked questions
Is humic acid the same as fulvic acid?
No. They're related fractions of the same humic substances family, but humic acid is larger, soluble only above pH 2, and works mainly in the soil by raising cation-exchange capacity and acting at the root surface. Fulvic acid is smaller, soluble at any pH, and active inside the plant. Most growers use both.
What does humic acid do for soil?
Four main things. It raises cation-exchange capacity so the soil holds positively charged nutrients against leaching. It frees up phosphorus that is locked to calcium, iron or aluminium. It helps bind particles into stable crumbs and improves water retention. And it can lift microbial activity, though mainly in soils that are low in organic matter to begin with. The effects build over a season rather than appearing overnight.
What is humic acid made from?
Mostly leonardite, a naturally oxidised, weathered form of lignite found in shallow seams above the more compact coal it came from. Published humic acid content varies by deposit, roughly 39 to 85%. It can also be extracted from lignite, weathered coal, peat and mature compost or worm castings. A third category sold as 'potassium humate' is sometimes lignosulphonate, a paper-mill by-product that looks similar but is not the humic fraction of soil organic matter.
Is leonardite humic acid better than compost-derived humic acid?
Not for plant growth response. The 2014 Rose et al. meta-analysis found humic substances from compost sources significantly outperformed lignite and peat-derived material for growth promotion, and two of the most-cited mechanism papers used vermicompost humic acid rather than leonardite. Leonardite wins on batch consistency, humic content, shelf stability and the volume of agronomic trial data behind it, which is why most commercial products including ours use it. If you already make good compost or vermicompost, you have compost-derived humic acid going into the soil already.
Does humic acid work in field trials?
Yes, on average, with high variability. The 2024 Ma et al. meta-analysis in Agronomy pooled humic-acid crop trials and found a 12% average yield increase, a 27% improvement in nitrogen use efficiency, and a 17% increase in nitrogen uptake. The 2014 Rose et al. meta-analysis found a 22% average shoot dry weight increase from humic substances, and a bigger response on stressed plants than unstressed ones, 28% against 18%. It found no significant difference between growing media, and did not analyse soil texture or organic matter at all. Hartz and Bottoms (2010) tested five commercial products and found no benefit on romaine lettuce or processing tomato in California.
Is humic acid a fertiliser?
Strictly speaking, no. Humic acid contains very little nitrogen, phosphorus or potassium of its own. It's classed as a biostimulant and a soil conditioner, meaning it improves the soil and how the plant uses other nutrients rather than supplying nutrients itself. It works alongside a fertiliser; it doesn't replace one.
Does humic acid get inside the plant?
Mostly no. The humic fraction is large enough that it stays outside the root cell, so almost all of its work happens in the soil and at the root surface. The smaller fulvic fraction is the one that gets inside the plant. Humic acid still acts at the root, activating the proton pump and prompting lateral root growth, but it does so from outside.
Is humic acid good for clay soil?
It can help, but it is most useful on light, sandy, free-draining soils where natural cation-exchange capacity is low. On heavy clay the priority is usually structure and drainage, where bulky organic matter and, on dispersive sodic clay, gypsum do more. Humic acid adds exchange sites to any soil, but the clearest cation-holding benefit shows up on lighter ground.
Can I use humic acid with other fertilisers?
Yes, and it pairs well with most of them. Humic acid holds the nutrients you apply in the root zone rather than letting them leach, so it tends to make a feeding programme more efficient. It pairs naturally with fulvic acid, with compost and organic feeds, and with trace element and calcium-magnesium feeds. It is a soil conditioner working alongside the feed, not a competitor to it.
Sources
- Ma, Y., Cheng, X., Zhang, Y. (2024). The Impact of Humic Acid Fertilizers on Crop Yield and Nitrogen Use Efficiency: A Meta-Analysis. Agronomy 14(12): 2763. doi.org/10.3390/agronomy14122763
- Rose, M.T., Patti, A.F., Little, K.R., Brown, A.L., Jackson, W.R., Cavagnaro, T.R. (2014). A meta-analysis and review of plant-growth response to humic substances: practical implications for agriculture. Advances in Agronomy 124: 37–89. doi.org/10.1016/B978-0-12-800138-7.00002-4
- Canellas, L.P., Olivares, F.L., Okorokova-Façanha, A.L., Façanha, A.R. (2002). Humic acids isolated from earthworm compost enhance root elongation, lateral root emergence, and plasma membrane H+-ATPase activity in maize roots. Plant Physiology 130(4): 1951–1957. doi.org/10.1104/pp.007088
- Trevisan, S., Pizzeghello, D., Ruperti, B., Francioso, O., Sassi, A., Palme, K., Quaggiotti, S., Nardi, S. (2010). Humic substances induce lateral root formation and expression of the early auxin-responsive IAA19 gene and DR5 synthetic element in Arabidopsis. Plant Biology 12(4): 604–614. doi.org/10.1111/j.1438-8677.2009.00248.x
- Cooper, R.J., Liu, C., Fisher, D.S. (1998). Influence of humic substances on rooting and nutrient content of creeping bentgrass. Crop Science 38(6): 1639–1644.
- Lindsey, A.J., Thoms, A.W., McDaniel, M.D., Christians, N.E. (2021). Evaluation of humic fertilizers on a sand-based creeping bentgrass putting green. Crop Science 61. doi.org/10.1002/csc2.20577
- Atiyeh, R.M., Lee, S., Edwards, C.A., Arancon, N.Q., Metzger, J.D. (2002). The influence of humic acids derived from earthworm-processed organic wastes on plant growth. Bioresource Technology 84(1): 7–14. doi.org/10.1016/S0960-8524(02)00017-2
- Nardi, S., Schiavon, M., Francioso, O. (2021). Chemical structure and biological activity of humic substances define their role as plant growth promoters. Molecules 26(8): 2256. doi.org/10.3390/molecules26082256
- Hartz, T.K., Bottoms, T.G. (2010). Humic substances generally ineffective in improving vegetable crop nutrient uptake or productivity. HortScience 45(6): 906–910. doi.org/10.21273/HORTSCI.45.6.906
- Lehmann, J., Kleber, M. (2015). The contentious nature of soil organic matter. Nature 528(7580): 60–68. doi.org/10.1038/nature16069
- Piccolo, A. (2002). The supramolecular structure of humic substances: a novel understanding of humus chemistry and implications in soil science. Advances in Agronomy 75: 57–134.
- Du Jardin, P. (2015). Plant biostimulants: definition, concept, main categories and regulation. Scientia Horticulturae 196: 3–14. doi.org/10.1016/j.scienta.2015.09.021
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