What is fulvic acid?
By Joe Barrington, founder of Dr Forest · Published May 2026 · Updated 10 October 2026
Fulvic acid is the smallest, most soluble part of humus, the dark material in healthy soil. Trials show modest gains in roots and stress tolerance, but many products sold as fulvic contain little of it: one 2025 test found real fulvic material in only 14 of 25.
Fulvic acid is one part of humus, the dark material left in soil after plant matter has broken down for a very long time. It is the smallest and most soluble part, so it dissolves easily in water and can carry minerals such as iron and zinc into a plant.
Teaching size ranges for fulvic acid are approximate, not a single molecular size. That small size is why fulvic fractions can act as chelators, holding on to minerals such as iron and zinc, and why a fulvic acid powder for plants goes on as a drench or a fine foliar mist rather than as an NPK feed in its own right.
There is peer-reviewed work behind the useful claims, and a lot of noise on top. What follows is the useful part: where it comes from, how it differs from humic acid, what trials actually measured, and how to spot products that aren’t really fulvic at all.
In short
What it is: the smallest, water-soluble fraction of humic substances. Humic substances are carbon-rich compounds left after microbes break down plant matter. They build up over centuries in soil and peat, and over millions of years in leonardite (a soft, oxidised brown coal).
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, and helps plants tolerate drought, salt and heavy metal stress. Zhang discussed auxin-like and gibberellin-like activity, and that trial did not assay the hormone.
How it works: Fulvic acid is the smaller, more soluble fraction of humic substances. Teaching ranges often put it around 1,000–10,000 daltons. Nardi and colleagues (2021) put the size that can enter a cell near 3,500 daltons. 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 are linked to hormone-like plant signalling at low doses. Spray and drench amounts are in the rate.
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, in large amounts helps the soil hold on to nutrients like calcium, magnesium and potassium 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
Fully soluble fulvic acid powder for sprays and drenches.
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 and has a long shelf life, which is why leonardite is a common research and commercial source — though plenty of peer-reviewed trials (including the tomato study cited below) used fermented crop-residue fulvic materials instead. ISO and AOAC methods help identify and quantify humic/fulvic fractions in fertiliser materials; they are not a seal that “mineral equals genuine.”
2. Biochemical fulvic acid (corn straw, sugarcane bagasse, manure compost, sawdust, fermented plant residues). A newer category. It has worked in some trials, but it is a plant extract, not humus, and the 2025 screening test classes plant extracts as not fulvic. 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 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. There is still no agreed lab test that reliably tells real fulvic acid from these look-alikes. A 2025 Journal of AOAC International survey of 25 commercial fulvic acid products found that only 14 of them contained fulvic fractions on UV spectroscopic testing.
So which is best? For shelf-stable, well-studied chelation, mineral fulvic acid from leonardite is the usual choice and keeps well. 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 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.
The side-by-side comparison is in humic vs fulvic acid.
Figure 03 · Permeability
Why size lets fulvic in
Nardi and colleagues (2021) put the size that can enter a cell near 3,500 daltons.
A useful shorthand: humic acid mostly improves the soil; fulvic fractions tend to be more plant-active. Both can matter. Neither replaces compost.
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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 ways it may work
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. That binding can improve how some micronutrients (especially iron and zinc in trial work) stay available to the plant. It is similar in job to synthetic chelators such as EDTA, not identical, and it does not mean every cation is carried unchanged through membranes — or that fulvic replaces a proper feed.
It switches on the root's proton pumps
Work on rice roots has shown that fulvic acid activates 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. Lv and colleagues tested this in 2024 on rice seedlings grown in a nutrient solution low in phosphorus. Fulvic acid switched on the proton-pump genes and raised phosphorus uptake by about a fifth. When the solution was buffered so it could not turn more acidic, the benefit disappeared. They used a laboratory chemical sold as fulvic acid, not a soil or leonardite extract, so treat it as a clue to how fulvic acid works, not proof for garden soil.
It supports root growth signalling
Fulvic acid’s smaller molecules are linked in the research to plant signalling that affects rooting and lateral branching — without claiming it is the hormone auxin itself. In hydroponic tomato, fulvic acid at 80 mg/L increased root length by 44% and put out 14% more root tips, a pattern Zhang and colleagues (2021) linked to known hormone-like effects of humic fractions — without a direct auxin assay in that trial. The practical effect is more 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). Some peer-reviewed trials report stronger antioxidant enzyme activity and lower oxidative-damage markers when fulvic acid is used under drought or salt stress — results vary by crop and product, so treat this as a research theme, not a guarantee for your beds.
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 can change pH only at the root surface. Right at the root surface it can make the soil slightly more acidic, which helps some minerals dissolve. At garden rates it will not change the pH of a bed.
- It may free a little phosphorus near the roots. In chalky or iron-rich soil, phosphate binds to calcium or iron. It may free a little of that near the roots. That is not shown at garden rates.
- 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. Humic products and organic amendments more broadly are often linked with higher microbial biomass — the evidence is thinner for fulvic acid alone, so treat this as a tendency, not a promise.
- 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.
Fulvic acid does not replace organic matter in the soil. 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.
What the evidence says
Trials show a modest help with roots and with drought, mostly on stressed plants. The figures, and where they do not apply, are on the benefits page.
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 support root-signalling responses more strongly.
This is why extraction matters. A high-temperature alkaline extraction breaks down the parent humic material indiscriminately and produces a wide molecular weight distribution. Our powder is made by an enzyme process at room temperature. We have not tested it head to head against alkaline-extracted powders, so we make no claim that it works better.
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 are easier for plants to take up when they are chelated. Adding fulvic acid to a trace element spray may help. A 2019 Brazilian coffee-seedling study (Justi, Morais & Silva) found foliar fulvic acid more helpful than soil humic acid for growth under their zinc/boron regime.
- 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.
Related: Buy fulvic acid powder · How to apply fulvic acid (rates) · Humic Acid Granules · 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 and feeds soil microbes. Most growers use both.
Is fulvic acid organic?
Fulvic acid extracted from natural sources like leonardite or peat can be permitted in organic growing, depending on the product and the scheme. The Dr Forest fulvic acid powder is organic certified. 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. A 2025 paper proposed a simple light-absorbance test to screen for fakes, because no agreed test existed. In a 2025 test of 25 products sold as fulvic acid, only 14 contained real fulvic material. The paper does not say what the other 11 were made of.
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. 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 can support plant signalling linked to 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. Some of it gets through the waxy outer layer of the leaf, mostly through tiny pores. Published spray rates vary widely — from under 0.1 g/L in some tomato solution trials to several grams per litre in some orchard and rose trials. For a garden spray, use 0.5–1 g per litre. The full rates are in how to apply fulvic acid.
Does fulvic acid expire?
Fulvic acid powder keeps for years if the lid is tight. It soaks up damp from the air and turns sticky if left open. 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.
Founder of Dr Forest. Joe started the business and formulates the blended fertilisers and mixes and packs them in small batches in Stockport.
Sources
- 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.
- Ali, E.F., Al-Yasi, H.M., Issa, A.A., Hessini, K., Hassan, F.A.S. (2022). Ginger Extract and Fulvic Acid Foliar Applications as Novel Practical Approaches to Improve the Growth and Productivity of Damask Rose. Plants 11(3): 412.
- Nikoogoftar-Sedghi, M., Rabiei, V., Razavi, F., Molaei, S., Khadivi, A. (2024). Fulvic acid foliar application: a novel approach enhancing antioxidant capacity and nutritional quality of pistachio (Pistacia vera L.). BMC Plant Biology 24: 241.
- Lv, X. et al. (2024). Fulvic acid application increases rice seedlings performance under low phosphorus stress. BMC Plant Biology 24: 703.
- Journal of Plant Nutrition 41(19): 2438–2445.
- Justi, M., Morais, E.G. & Silva, C.A. (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 65:1969–1983. DOI 10.1080/03650340.2019.1584396.
- 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): 828–839. Survey of 25 commercial fulvic acid products; only 14 contained fulvic fractions on UV spectroscopic testing.
- Biochemical Sources of Fulvic Acid and its Application in Agriculture (2025). Mini-Reviews in Organic Chemistry 22(4): 400–410 (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.
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