What is humic acid? The brown coal in the bag
By Joe Barrington, founder of Dr Forest · Published August 2026 · Updated 4 November 2026
Quick answer. What is humic acid? It's one part of humus, the dark material left once plant matter has fully rotted down. Garden products, including the Dr Forest Humic Acid Granules, are mostly extracted from leonardite, a weathered brown coal. It isn't a fertiliser, as it carries very little NPK. It works at the root, holding nutrients within reach and encouraging more root, and across 93 published studies it raised crop yields by about 12% on average when added to a nitrogen feed. Use it alongside your normal feed and compost, not instead of them.
The best way I can explain humic acid is with compost. You spread compost, the garden does better for it, and a big part of the reason is that compost adds humus. Humic acid is one of the main working parts of that humus, extracted and concentrated. So if you've picked up a bag thinking it's a feed, I'd say plainly: it isn't a fertiliser.
What humic acid is and where it comes from
Bacteria and fungi break down dead plants and microbes over a very long time. What's left is humus, which is where the word humic comes from. Chemists split humus into three parts by how each one behaves in acid and alkali [1]:
- Humin doesn't dissolve at any pH and stays stuck to the soil minerals.
- Humic acid dissolves in alkali and drops out again once the solution turns acid, below about pH 2.
- Fulvic acid dissolves at any pH.
Where commercial humic acid comes from
Most humic acid sold for plants comes from leonardite, a naturally oxidised form of lignite found in shallow seams above the harder coal. Its humic content varies even within one mine: leonardite from the Lee Mine in Thailand ran from 39% to 85% [2]. The Dr Forest Humic Acid Granules are a leonardite extract, 70% humic acid and 100% water-soluble.
Compost and worm castings carry humic acid too, and compost has the better record on plant growth. Pooled trials found humic material from green-waste compost raised shoot growth by about 29%, against about 12% for brown coal, which includes leonardite [3].
Lignosulphonate, a paper-mill by-product, isn't humic acid at all. A 2014 lab study tested it as a possible adulterant of humic products, and adding it pushed up the measured humic and fulvic acid [4]. Whatever you buy, ask for a certificate of analysis that shows the source and the humic percentage.
How humic acid differs from fulvic acid
Fulvic acid is the smaller fraction of humus: it stays dissolved at any pH and can act inside the plant, while humic acid mostly works from outside, at the root surface [1]. Which of the two suits your soil is in my humic vs fulvic acid guide, and for fulvic on its own, see what is fulvic acid.
What humic acid does at the root
Humic material under about 3,500 daltons, a measure of molecule size, gets into root cells easily. The larger humic acid molecules are hardly taken up, so most of the work happens outside the root, at its surface [1].
It holds nutrients in reach. Its carboxyl and phenol groups carry a negative charge and bind metals such as iron, zinc and manganese, which stops them washing away and keeps them available to the plant [1].
It helps roots take nutrients up. Humic acid switches on the proton pumps in the root's outer membrane, which push acid out at the root surface so more minerals dissolve. Canellas and colleagues showed this in maize seedlings with humic acid from worm compost, which also gave longer roots and more side roots [5].
It acts a bit like auxin, the rooting hormone. In Arabidopsis, a small lab plant, humic substances switched on early auxin genes and set off new side roots [6].
What you'd see is more root. In a greenhouse trial, granular humate mixed into sand gave creeping bentgrass 45% more root mass in the top 10 cm [7].
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What humic acid does in the soil
In large amounts, humic acid raises the soil's ability to hold calcium, magnesium, potassium and ammonium, which soil scientists call cation-exchange capacity (CEC). At garden doses the change in the soil itself is small, and the sum shows why.
Take a generous 10 to 20 g of humic acid per square metre, spread through the top 10 to 15 cm of soil. That comes to about 0.05 to 0.15 g per kg of soil, taking topsoil at about 1.3 kg per litre. Even a poor soil at 2% organic matter holds about 20 g of organic matter in every kg, so the dose adds under 1% to what's there.
- In a lab incubation, 26 humic acids from peat and coal raised the soil's CEC by 1% to 58% [8].
- In two field studies, run over two and five years, the humic acid applied wasn't enough to change soil texture or structure [8].
- In a second lab test, soil microbes picked up when humic acid went on with fertiliser in a soil at 0.8% organic matter, but not in one at 2.5% [9].
So keep the compost going for the soil and let the humic acid work on the roots. To open up clay or bring a tired bed back, you need barrowloads of compost and mulch; more in what is living soil.
What the trials say
Pooled across field trials from 93 published studies on farm crops, humic acid added to a nitrogen feed lifted yields by about 12% on average, against the same feed alone [10]. It does most for stressed plants [3] and on poor, low organic matter ground; the trial-by-trial figures are in humic acid benefits for plants.
Where humic acid sits in a feeding programme
Humic acid helps roots take up the feed you give them, so keep feeding as normal. Rates, timing and what it mixes with are in how to apply humic acid.
It pairs well with Fulvic Acid Powder: humic for the root zone, fulvic for inside the plant. The humic vs fulvic guide covers using the two together, and both are in the humic and fulvic range.
As for my own garden, I use it for everything.
Frequently asked questions
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. It can also be extracted from peat and mature compost or worm castings.
Is humic acid a fertiliser?
No, it contains very little nitrogen, phosphorus or potassium. It helps the plant use the nutrients you feed; see what a fertiliser is.
What does humic acid do for soil?
In large amounts it helps soil hold on to nutrients, and it can lift microbial activity in soils low in organic matter. At garden rates the change to the soil is small, and its real work is at the root.
Is humic acid good for clay soil?
Not on its own. Bulky organic matter such as compost is what opens up clay, and at garden rates humic acid works at the root rather than on soil structure. For gypsum on clay, see does liquid gypsum work on clay soil.
Is leonardite humic acid better than compost humic acid?
On plant growth, compost has the better record: pooled trials found humic material from green-waste compost raised shoot growth by about 29%, against about 12% for brown coal such as leonardite. Most bagged humic acid is still leonardite, so check the humic percentage on whatever you buy.
How can I tell if a humic acid product is genuine?
Ask for a certificate of analysis that shows the source and the humic acid percentage. Lignosulphonate, a paper-mill by-product, isn't humic acid but can push up the measured humic figure in a lab test.
Joe Barrington, founder of Dr Forest. Joe formulates and blends his own feeds in Stockport, such as All Purpose 4-4-4, and stocks bought-in soil products alongside them, including humic and fulvic acid.
Sources
- Nardi, S. et al. (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
- Ratanaprommanee, C. et al. (2017). Chemical characterization of leonardite and its potential use as a soil conditioner for plant growth enhancement. Asia-Pacific Journal of Science and Technology 22(4). apst.kku.ac.th
- 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 to 89. doi.org/10.1016/B978-0-12-800138-7.00002-4
- Lamar, R.T. et al. (2014). A new standardized method for quantification of humic and fulvic acids in humic ores and commercial products. Journal of AOAC International 97(3): 721 to 730. doi.org/10.5740/jaoacint.13-393
- 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: 1951 to 1957. doi.org/10.1104/pp.007088
- Trevisan, S. et al. (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: 604 to 614. doi.org/10.1111/j.1438-8677.2009.00248.x
- Cooper, R.J. et al. (1998). Influence of humic substances on rooting and nutrient content of creeping bentgrass. Crop Science 38(6): 1639 to 1644. doi.org/10.2135/cropsci1998.0011183X003800060037x
- Ampong, K. et al. (2022). Understanding the role of humic acids on crop performance and soil health. Frontiers in Agronomy 4: 848621. doi.org/10.3389/fagro.2022.848621
- Hartz, T.K.; Bottoms, T.G. (2010). Humic substances generally ineffective in improving vegetable crop nutrient uptake or productivity. HortScience 45(6): 906 to 910. doi.org/10.21273/HORTSCI.45.6.906
- Ma, Y. et al. (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
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