What is fulvic acid?
Soil science
What is fulvic acid?
By Joe, Founder of Dr Forest · May 2026
It's the smallest, most active fraction of the dark humus you find in any healthy soil. And it's the part that actually gets inside the plant.
Fulvic acid is the lowest molecular weight, most water-soluble fraction of humic substances: the carbon-rich organic compounds that form when plant material has been broken down by microbes over centuries to millions of years. Chemically, it sits inside the same family as humic acid and humin. The difference is size. Fulvic molecules are smaller, lighter, and stay in solution at any pH.
That small molecular size matters. Fulvic acid molecules are typically 1,000 to 10,000 daltons across (a dalton is the unit chemists use for the weight of a single molecule; a water molecule, for comparison, weighs 18). That makes them small enough to slip through plant cell walls. Humic acid molecules are usually too big to make it inside the plant, so they do their work in the soil. Fulvic acid does its work both in the soil and inside the plant.
It's used as a soil amendment, a foliar spray (a fine mist applied to the leaves) and a natural chelator: a molecule that grips onto mineral nutrients and carries them into the plant in a form the roots and leaves can absorb. There's a meaningful body of peer-reviewed evidence behind it, and a fair bit of marketing noise on top. This piece is the science, plainly told.
In short
What it is: the smallest, water-soluble fraction of humic substances. Humic substances are the carbon-rich organic compounds that build up in soil, peat and leonardite (a soft, oxidised brown coal) over centuries of microbial breakdown.
What it does: chelates mineral nutrients (grips them onto its molecular structure and carries them into the plant in a form the roots can use), speeds up nutrient uptake at the root surface, mimics auxin (the natural plant hormone that drives root growth) to encourage lateral roots, and helps plants tolerate drought, salt and heavy metal stress.
How it works: low molecular weight (1,000–10,000 daltons) lets it cross plant cell walls. Two types of acidic group on the molecule, carboxyl (–COOH) and hydroxyl (–OH), bind metal cations: positively charged metal ions like iron, calcium and zinc. Other parts of the molecule mimic plant hormones at low doses.
Where fulvic 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 is a stable mix of carbon-rich, oxygen-rich molecules with a tangle of acidic functional groups hanging off them. That's humus.
Humus contains three loosely defined fractions, separated by how soluble they are in acid and alkali:
- Humin is the largest, least soluble fraction. 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 medium-sized fraction. Soluble in alkali, insoluble in acid (pH below about 2). Holds positively charged nutrients on its surface, lifts the soil's cation-exchange capacity (its ability to hold onto positively charged nutrients like calcium, magnesium and potassium so they don't leach away with the rain) and feeds soil microbes.
- Fulvic acid is the smallest fraction. Soluble in water at any pH. Crosses cell walls, chelates trace minerals and acts as a biostimulant inside the plant: a substance that helps plants grow better without being a fertiliser itself.
Figure 01 · Humic substances
The family of humic substances
Three fractions of the same dark organic matter, separated by molecular size and solubility.
Figure 02 · Provenance
From dead leaves to fulvic powder
Centuries of microbial breakdown, geological compression, then a gentle enzymatic extraction.
Where commercial fulvic acid actually comes from, and which is best
Open the back of any fulvic acid label and the source story falls into one of three categories. They are not the same thing, do not all behave the same way, and one of them is not really fulvic acid at all.
1. Mineral-source fulvic acid (leonardite, lignite, weathered coal, peat). The classical category. Leonardite is the standout: a soft, oxidised form of brown coal sitting geologically between peat and true coal, found in seams in the western US (the Dakotas, New Mexico), Canada, China, India and parts of Europe. Leonardite carries 30 to 80% humic and fulvic acids depending on the seam, which makes it the richest practical mineral source. Mineral fulvic acid is dark, stable, high in oxidised aromatic carbon, and free of proteins, sugars and amino acids. It does not denature, has a long shelf life, and is what almost every peer-reviewed long-term trial in the literature has used. It is also what the international ISO and AOAC standards were written to identify.
2. Biochemical fulvic acid (corn straw, sugarcane bagasse, manure compost, sawdust, fermented plant residues). A newer category, and a real one. Microbial fermentation of crop residues produces a yellow-brown extract with fulvic-like chelation behaviour, plus extra cargo: amino acids, sugars, B vitamins, sometimes endogenous plant hormones. The 2021 Frontiers in Plant Science tomato trial that found a 35% yield increase used corn-straw-derived fulvic acid, and the saline-alkali soil work in Irrigation Science 2025 used a similar material. So it works. The trade-off is stability. Biochemical fulvic acid contains low-molecular-weight organic matter that microbes will eat given the chance, which means shorter shelf life and a tendency to ferment in dilute solution. It is closer to a fast-acting plant extract than a long-stable humic substance.
3. Lignosulphonate-based products sold as fulvic acid. This is the one to watch. Lignosulphonates are paper-mill by-products from sulphite pulping (the chemical process used to break wood into pulp for paper), used as concrete plasticisers, dust suppressants and animal feed binders. Sold as ammonium, sodium, calcium or potassium lignosulphonate, they are yellow-brown, water-soluble, contain similar ring-shaped carbon molecules (phenolics) and look superficially like fulvic acid in a glass of water. They are not. They are lignin breakdown products, not the fulvic fraction of soil organic matter. Some Chinese-export "potassium fulvate" and "bio fulvic acid" products contain little or no genuine fulvic acid and are essentially repackaged lignosulphonate. The industry has known about this for years, and the AOAC and ISO have both published methods specifically designed to flag lignosulphonate adulteration. A 2025 Journal of AOAC International survey of 25 commercial fulvic acid products found that only 14 of them contained genuine fulvic fractions on UV spectroscopic testing.
So which is best? For shelf-stable, well-studied chelation, mineral fulvic acid from leonardite is the default and the one almost all peer-reviewed long-term work has been done on. For fast-acting biostimulant effects on living-soil beds, biochemical fulvic acid from clean crop residues is a reasonable alternative if you accept the shorter shelf life. Lignosulphonate-based "fulvic acid" is not really fulvic acid and should be avoided regardless of the price.
How to tell them apart on a label: look for the source declared ("derived from leonardite", "corn straw biochemical", "sodium/potassium lignosulphonate"), look for ISO 19822 or LAMAR (AOAC 2024.07) test results on the certificate of analysis, and steer clear of vague descriptors like "natural mineral source" with no further detail. A reputable supplier will tell you the source and the test method without being asked. The Dr Forest fulvic acid powder is enzymatically extracted from leonardite, 70% fulvic acid by mass, 99% soluble in cold water; the liquid fulvic acid is recovered from Dutch drinking-water purification (an underground water source filtered through layers of peat). Neither contains lignosulphonate.
How fulvic acid differs from humic acid
The two are often sold and discussed together because they're the same family of compounds. They do different jobs because of their different sizes.
| Property | Fulvic acid | Humic acid |
|---|---|---|
| Molecular weight | ~1,000–10,000 Da | ~10,000–100,000 Da |
| Water solubility | Soluble at any pH | Soluble only above pH 2 (insoluble in acid) |
| Colour in solution | Yellow to light brown | Dark brown to black |
| Oxygen content | Higher (more functional groups per gram) | Lower |
| Chelation strength | Strong, with smaller, more mobile complexes | Strong, with larger, more stable complexes |
| Crosses plant cell walls | Yes | No (mostly soil-bound) |
| Best application | Foliar spray, root drench, tank-mix with feeds | Soil amendment, base dressing, top dress |
| Primary site of action | Inside the plant + rhizosphere | Soil cation-exchange capacity, microbial habitat |
Figure 03 · Permeability
Why size lets fulvic in
Plant cell walls have an effective exclusion limit. Humic acid is too big to pass; fulvic acid slips through.
The shorthand most growers use is this: humic acid mostly improves the soil, fulvic acid mostly improves the plant. Slightly oversimplified, since fulvic acid does work in the rhizosphere too (the thin layer of soil right around the roots), but it's a fair starting point.
What fulvic acid does inside the plant
Three mechanisms are reasonably well established in the peer-reviewed literature. None of them is the whole story by itself, but together they explain why fulvic acid keeps showing up in field trials with measurable effects.
Figure 04 · Inside the plant
Three mechanisms, well established
How fulvic acid acts on the plant once it gets inside.
It chelates mineral nutrients
Fulvic acid is rich in carboxyl (–COOH) and hydroxyl (–OH) groups: small acidic clusters of atoms that hang off the main molecule and bind tightly to positively charged metal ions like iron, manganese, zinc, copper, calcium and magnesium. The resulting fulvic-mineral complex is small enough to move through cell membranes intact, which means more of the nutrient applied actually reaches the plant. This is the same trick synthetic chelating agents like EDTA pull (a lab-made chemical that does the same job, used in everything from contact lens solutions to industrial cleaners), but with a naturally occurring carbon backbone.
It increases root membrane permeability
Studies on maize roots have shown that humic substances 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, and the finding has been replicated repeatedly since.
It mimics auxin and triggers root growth
Auxin is the natural plant hormone that controls rooting and lateral branching. Fulvic and humic acids contain low-molecular-weight fragments that interact with auxin signalling pathways inside the root, fooling the plant into responding 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) as a low dose of auxin itself. The practical effect is a flush of lateral root growth and longer root hairs.
There's a fourth mechanism that's still being unpicked: fulvic acid appears to prime the plant's antioxidant defences (the same kind of chemistry that mops up cellular damage in animal cells). Drought-stressed tea plants, oats and maize given fulvic acid show stronger activity from the enzymes that handle this clean-up job (superoxide dismutase, peroxidase, catalase, ascorbate peroxidase) and lower levels of malondialdehyde, a chemical that builds up when cells are oxidatively damaged. The plant copes with stress better because its biochemistry is set up to.
What fulvic acid does in the soil
Although the molecule is small enough to enter the plant, plenty of it stays in the soil and works there. The main soil-side effects:
- It buffers pH. Fulvic acid carries acidic groups across a wide pH range, which softens the impact of fertiliser applications on soil pH (it resists big swings in acidity, both up and down) and helps stabilise the rhizosphere.
- It unlocks bound phosphorus. In soils with high calcium or iron, phosphate gets locked up in unavailable forms. Fulvic acid binds the calcium and iron preferentially, freeing the phosphorus for the plant. Useful in chalky and clay soils where phosphorus availability is the bottleneck.
- It supports the soil microbiome. Fulvic acid is itself a carbon source for bacteria and fungi, and the chelated nutrients it carries become available to microbes as well as plants. Studies on humic-treated soils show meaningful increases in microbial biomass carbon and nitrogen.
- It improves nutrient retention in sandy soils. Fulvic acid binds water and cations onto its functional groups, slowing leaching losses on light, free-draining soil. The effect is smaller than humic acid for cation-exchange capacity, but it's not nothing.
The honest hedge: fulvic acid is not a substitute for actual soil organic matter. A teaspoon dissolved in water cannot replace years of compost, cover crops, and worm activity. It is a precision tool for a specific job. Don't expect it to build your soil for you.
Humic acid improves the soil. Fulvic acid improves the plant. Both are useful. Neither replaces compost.
The biostimulant question: what the evidence says
Fulvic acid is classed as a biostimulant under EU regulation: a substance that stimulates plant nutrition processes regardless of nutrient content, with the goal of improving nutrient use efficiency, abiotic stress tolerance, or quality traits. Whether biostimulants deliver, in the field, has been argued over for decades.
The most useful synthesis is the meta-analysis by Rose and colleagues, published in Advances in Agronomy in 2014. A meta-analysis pools the results of many other studies to look for overall trends, and Rose's team pooled hundreds of greenhouse and field trials testing humic substances on shoot biomass and grain yield. The headline finding: an average shoot biomass increase of around 22% across crops, with positive effects most consistent in greenhouse trials on light-textured soils.
A larger, more recent meta-analysis by Li and colleagues, published in Frontiers in Plant Science in 2022, pooled over 1,000 field-trial data pairs across 180 studies worldwide. Across all biostimulant categories the average yield benefit was 17.9%, with humic and fulvic acids one of the named subcategories. Soil application gave the highest yield benefit; the response was bigger on degraded or low-organic-matter soils than on well-managed ones.
Figure 05 · The evidence base
Four numbers from the literature
Pulled from peer-reviewed meta-analyses and field trials. Results vary; soil condition matters.
The effect is biggest where soils need it most. Variability between products is real.
Two important caveats. First, the variability between commercial products is large. Source material, extraction method, and concentration all matter, and not every product behaves the same way in field trials. Second, the effect size depends heavily on whether the soil and management are already good. A grower running a well-fed, biologically active soil will see less of a kick than someone working depleted ground.
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.
Why low molecular weight matters
If you're comparing fulvic acid products, the headline number is usually the percentage of fulvic acid by mass. That tells you the concentration but not the bioactivity. Two 70% fulvic acid powders extracted by different methods can perform very differently in a foliar trial.
The variable that matters most is molecular weight distribution. Smaller molecules cross plant cell walls more readily, chelate more efficiently, and trigger the auxin-mimetic response more strongly. Studies comparing standard commercial fulvic acid with low molecular weight (LMW) variants consistently show LMW outperforms on uptake-related measures, even at lower application rates.
This is why extraction matters. A high-temperature alkaline extraction breaks down the parent humic material indiscriminately and produces a wide molecular weight distribution. A gentler, low-temperature enzymatic extraction targets the smaller, more bioactive fragments. The difference shows up at the leaf surface and at the root hair.
Where fulvic acid sits in a feeding programme
Fulvic acid is not a fertiliser. It contains barely any nitrogen, phosphorus, or potassium of its own. What it does is make the fertiliser you're already applying work harder, by chelating the nutrients into more bioavailable forms and helping the plant absorb them faster.
The most useful pairings, based on the published evidence:
- With trace element feeds. Iron, manganese, zinc, and copper all benefit hugely from chelation. A foliar spray combining fulvic acid with a trace mineral mix gives faster correction of micronutrient deficiencies than the trace mix alone. Coffee studies (Marchezan et al. 2019) showed foliar fulvic acid improved zinc and boron uptake more effectively than soil-applied humic acid.
- With calcium-magnesium feeds. Ca²⁺ and Mg²⁺ are notoriously difficult cations to deliver because they precipitate easily in alkaline conditions and bind tightly to soil colloids (microscopic particles of clay and organic matter). Fulvic acid keeps them mobile and bioavailable. The Dr Forest Powdered Cal-Mag includes fulvic acid for this reason.
- With seaweed and amino acid biostimulants. The mechanisms are complementary. Seaweed delivers cytokinins and gibberellins (plant hormones that drive cell division, stem growth and flowering), amino acids deliver protein-form nitrogen, and fulvic acid improves uptake of all of them. Rock dust feeding programmes in particular benefit from fulvic acid because the mineral release rates are slow and chelation accelerates plant access.
- With humic acid granules. The combination is conventional for a reason. Humic for the soil, fulvic for the plant. Dr Forest sells both as a paired set, used together at planting and through the season.
Dr Forest fulvic acid
Made with organic ingredients, OF&G certified, enzymatically extracted at low temperature for maximum bioactivity. Two formats:
- Organic Fulvic Acid Powder, 70%. 99% water-soluble, ultra-low molecular weight, ideal for foliar spray and tank-mixing with liquid feeds.
- Humic Acid Granules. 65% humic with 10% fulvic, semi-soluble, for soil amendment and top dressing.
Premium plant-based fertilisers, handcrafted in small batches in Stockport. Browse the full humic and fulvic collection.
Frequently asked questions
Is fulvic acid the same as humic acid?
No. They're related compounds in the same humic substances family, but fulvic acid is smaller, more water-soluble, and active inside the plant. Humic acid is larger, soluble only above pH 2, and works mainly in the soil by raising cation-exchange capacity and feeding microbes. Most growers use both.
Is fulvic acid organic?
Fulvic acid extracted from natural sources like leonardite or peat is approved for organic agriculture and certified by bodies including OMRI, Soil Association, and OF&G. The Dr Forest fulvic acid powder is certified by OF&G. Always check the certification on the label rather than the marketing copy.
What is fulvic acid made from?
Three main sources, and they are not interchangeable. Mineral fulvic acid comes from leonardite, lignite, weathered coal or peat. Biochemical fulvic acid comes from fermented crop residues like corn straw, sugarcane bagasse or manure compost. The third category, marketed as 'potassium fulvate' or 'bio fulvic acid', is often potassium or sodium lignosulphonate, a paper-mill by-product that mimics fulvic acid in some tests but is not the same material. The classical, most-studied, most stable form is mineral fulvic acid from leonardite.
Is potassium lignosulphonate the same as fulvic acid?
No. Lignosulphonates are sulphonated lignin breakdown products from sulphite paper-pulping, sold as concrete plasticisers, dust suppressants and animal feed binders. They are sometimes sold as 'potassium fulvate' or 'bio fulvic acid' because they are yellow-brown, water-soluble and superficially mimic fulvic acid in older colorimetric tests. They are not the fulvic fraction of soil organic matter and do not behave the same way in trials. The AOAC and ISO have published methods (LAMAR, ISO 19822) specifically designed to distinguish genuine fulvic acid from lignosulphonate adulterants. A 2025 Journal of AOAC International survey found only 14 of 25 commercial fulvic acid products contained genuine fulvic fractions.
Is fulvic acid from corn stalks as good as leonardite fulvic acid?
It is a different category. Corn-straw and other biochemical fulvic acids are produced by microbial fermentation of crop residues; they contain genuine chelating activity and have been used in peer-reviewed trials with positive results, including the 2021 Frontiers in Plant Science tomato study that found a 35% yield increase. They also carry extra cargo (amino acids, sugars, B vitamins) that can act as a fast biostimulant. The trade-offs are shorter shelf life, more variability between batches, and a tendency for diluted solutions to ferment if stored. For long-term soil conditioning and tank-mixing with other feeds, mineral fulvic acid from leonardite is the more reliable choice. For fast-acting biostimulant work on living-soil beds, biochemical fulvic acid is a fair alternative.
What does fulvic acid do for plants?
Three main things. It chelates mineral nutrients into plant-available forms. It activates root proton pumps to speed up nutrient uptake. It mimics auxin to encourage lateral root growth and root hair development. There's also reasonable evidence it primes the plant's antioxidant defences against drought and salinity stress.
Is fulvic acid a fertiliser?
Strictly speaking, no. Fulvic acid contains very little nitrogen, phosphorus, or potassium of its own. It's classed as a biostimulant, meaning it improves how the plant uses other nutrients rather than supplying nutrients itself. It pairs with a fertiliser; it doesn't replace one.
Can fulvic acid be applied as a foliar spray?
Yes, and that's one of its main uses. The low molecular weight means fulvic acid molecules pass through the leaf cuticle (the waxy outer layer of the leaf) and enter the plant directly. Most published trials use foliar concentrations between 0.5 and 3 grams per litre, depending on crop and growth stage. Tomato studies suggest 0.8 g/L is the sweet spot for fruiting crops.
Does fulvic acid expire?
Fulvic acid powder is stable indefinitely if kept cool, dry, and sealed. Liquid fulvic acid keeps for several years. Microbial growth in dilute solutions is the main practical issue, so mix fresh batches when you spray rather than storing dilute solution for weeks.
Sources
- Rose, M.T. et al. (2014). A meta-analysis and review of plant-growth response to humic substances: practical implications for agriculture. Advances in Agronomy 124: 37–89.
- Li, J. et al. (2022). A meta-analysis of biostimulant yield effectiveness in field trials. Frontiers in Plant Science 13: 836702.
- Canellas, L.P. et al. (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.
- 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.
- Ahmad, T., Khan, R., Khattak, T.N. (2018). Effect of humic acid and fulvic acid based liquid and foliar fertilizers on the yield of wheat crop. Journal of Plant Nutrition 41(19): 2438–2445.
- 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.
- Zhang, P. et al. (2021). Dose-dependent application of straw-derived fulvic acid on yield and quality of tomato plants grown in a greenhouse. Frontiers in Plant Science 12: 736613.
- Marchezan, C. et al. (2019). Fulvic acid in foliar spray is more effective than humic acid via soil in improving coffee seedlings growth. Archives of Agronomy and Soil Science 65(14).
- Application of fulvic acid in agriculture: an overview (2025). Plant Science Today. Review of 100+ peer-reviewed studies.
- Use of a Spectrophotometric Method for the Detection of Adulterants in Commercial Fulvic Acid Products (2025). Journal of AOAC International 108(6): 828. Survey of 25 commercial fulvic acid products; only 14 contained genuine fulvic fractions on UV spectroscopic testing.
- Biochemical Sources of Fulvic Acid and its Application in Agriculture (2024). Mini-Reviews in Organic Chemistry (Bentham Science). Review of biomass-fermentation fulvic acid sources, including corn straw.
- ISO 19822:2018. Fertilizers and soil conditioners. Determination of humic and hydrophobic fulvic acids concentrations in fertilizer materials. International Organization for Standardization.