Humic acid benefits for plants
By Joe Barrington, founder of Dr Forest · Published August 2026 · Updated 10 October 2026
Eight benefits, each tied to a peer-reviewed trial. Plus the crops where the field data is strong, and the trials that found nothing.
Humic acid is the dark, medium-weight fraction of the humus in healthy soil. At garden rates it works mainly on roots. It can help them branch and make better use of a nitrogen feed. It will not change your soil's structure in a season. The benefits of humic acid for plants are real and measurable, but they depend heavily on your soil, the product, and the dose. This piece runs through what the trials found, crop by crop, and where they found little or nothing.
If you want the chemistry first, start with the companion guide on what humic acid is. This piece is about what it does once it is in the ground.
In short
One big review found about 12% more yield when humic acid went on with a nitrogen feed. The authors then disagree with themselves about which soils gain most, so do not use that paper to decide where to put it. Leonardite-based humic acid, the most common type, gave smaller gains than compost-based material in the largest review. It does not replace compost or feed.
The limits: products vary enormously, several good field trials found no benefit at all, and the soil-biology case is much weaker than the marketing suggests. Humic acid is a soil conditioner, not a fertiliser, and not a substitute for organic matter.
Eight benefits of humic acid for plants, and the trials behind them
1. It improves nitrogen-use efficiency
The evidence
Ma, Cheng & Zhang, 2024, Agronomy 14(12):2763.
2. It grows more root
Root growth is one of the better-studied effects. But the big 2014 review found no average root gain from brown-coal humic acid, the kind most garden products use, so treat the trial below as a best case. A greenhouse study on creeping bentgrass found that granular humate worked into the root zone lifted root mass by roughly 45% in the top 10 cm of the profile and roughly 38% in the 10 to 20 cm layer. More root means more contact with soil water and nutrients, which is the foundation under most of the other benefits. The authors were careful about how far that transfers: their plants grew in sand and in solution, with little or no native organic matter, and they expected a smaller response from plants already well supplied with nutrients.
The evidence
Incorporated granular humate raised creeping bentgrass root dry mass from 0.66 to 0.96 g (0 to 10 cm) and 0.26 to 0.36 g (10 to 20 cm) in sand culture, which works out at roughly 45% and 38%. Foliar humate did not consistently help. Cooper, Liu & Fisher, 1998, Crop Science 38(6):1639–1644.
3. It raises the soil's ability to hold nutrients
Humic acid carries carboxyl and phenolic groups that hold a negative charge, and those sites grip positively charged nutrients: calcium, magnesium, potassium, ammonium. This is cation-exchange capacity. A garden dose of 10 to 20 g per square metre will not stop nutrients washing out in any measurable way. One incubation study of 26 different humic acids raised the cation-exchange capacity of the soil they were added to by between 1% and 58%. The caveat is that most of this work is short-term and done under controlled conditions rather than in long field trials.
The evidence
Carboxyl and phenolic groups give humic substances their weak acidity and their capacity to complex metal cations. Nardi, Schiavon & Francioso, 2021, Molecules 26(8):2256. Twenty-six peat- and coal-derived humic acids raised soil cation-exchange capacity by 1% to 58% in incubation. Ampong, Thilakaranthna & Gorim, 2022, Frontiers in Agronomy 4:848621.
4. It can free up locked phosphorus
In acidic soils, phosphate binds to iron and aluminium and goes out of reach. In chalky soils, it precipitates out as poorly soluble calcium phosphates. Humic substances work on both: they bind iron and aluminium, and they can pull phosphate off the sorption sites on iron and aluminium oxides. In calcareous soil they appear to slow the formation of the calcium phosphates rather than simply grabbing the calcium. The reviews on this are hedged, and so is this paragraph, because the authors themselves write "may" rather than "does".
The evidence
Humic-iron and humic-aluminium complexes may bind phosphorus and may desorb phosphate already bound to iron and aluminium oxides. Gerke, 2021, Journal of Plant Nutrition and Soil Science 184(3):329–338. In calcareous soils, humic and fulvic amendment raised phosphorus fertiliser recovery from under 15% to over 40%, attributed to inhibited precipitation of calcium phosphates, in soil kept in the lab with no plants, at 5 g per kilo of soil. That is roughly 1 kg per square metre, 50 to 100 times a garden dose. Delgado et al., 2002, Plant and Soil 245:277–286.
5. It gives soil microbes something to work with, on poor soils only
This is the benefit most often oversold, so here is what the best-designed test of it actually found. In a seven-day laboratory incubation on two soils, humic acid on its own did nothing to microbial activity. Paired with phosphorus fertiliser it produced a small but statistically significant rise in microbial activity in the low-organic-matter soil, and lifted the fatty-acid markers for fungi, bacteria and actinomycetes. In the soil that already held 2.5% organic matter, the phosphorus-only treatment came out ahead of humic plus phosphorus. No plant benefit followed in the same study. Treat the soil-biology claim as a real but small effect on poor ground.
The evidence
Seven-day incubation, two soils at 0.8% and 2.5% organic matter. With phosphorus fertiliser, humic acid gave a small significant rise in microbial activity and in fungal, bacterial and actinomycete markers in the low-organic-matter soil only. Hartz, CDFA FREP project report 07-0174, the project behind Hartz & Bottoms, 2010, HortScience 45(6):906–910.
6. It nudges the root directly
Some humic effects reach the root surface and act like a mild plant hormone. Work on maize roots, using humic acids isolated from cattle-manure earthworm compost, showed they activate the proton pump in the root membrane (the plasma membrane H⁺-ATPase), the pump that drives nutrient uptake. Separate work in Arabidopsis showed humic substances switch on the same auxin-responsive gene (IAA19) that drives lateral root formation. The plant responds as if it had a small dose of its own rooting hormone. The maize study used a vermicompost extract, not a mined humic acid, and source matters a great deal in this field.
The evidence
Humic acids isolated from earthworm compost activated the root plasma membrane H⁺-ATPase and promoted lateral root emergence in maize. Canellas et al., 2002, Plant Physiology 130(4):1951–1957. The auxin-gene signature (IAA19) was confirmed in Arabidopsis. Trevisan et al., 2010, Plant Biology 12(4):604–614.
7. It improves growth under salt stress
Few UK gardens have salty soil. This matters mainly near the coast or beside roads that are gritted in winter. A two-season trial on barley grown in saline soil tested humic acid across four phosphorus rates. Humic acid with a full phosphorus dose raised grain yield by about 65% against the untreated control. Most of that is the phosphorus. Measured against the full phosphorus rate on its own, humic acid added 22%. That is still a real gain, and it comes with the usual caveat: the baseline was a stressed plant, so the figure reflects stress relief rather than a boost on a healthy crop.
The evidence
Humic acid with 100% recommended phosphorus (treatment T1) raised barley grain yield 64.7% over the untreated control, and 22.3% over the full phosphorus rate alone, in saline-sodic soil across two seasons. Humic and fulvic acid were tested separately, never combined. Alsudays et al., 2024, BMC Plant Biology 24:191.
8. It improves nutrient uptake in water culture
The root and uptake effects show up clearly in controlled systems, which is where the mechanism is easiest to isolate. A tomato study grown in nutrient solution tested humic acid at 20 and 50 mg per litre from two commercial sources. The peat-derived product raised root dry weight by 22% at 20 mg per litre and lifted uptake of nitrogen, phosphorus, iron and copper. The leonardite-derived product worked at the higher dose instead, adding 16% to root dry weight and 8 to 9% to shoots, with more nitrogen, phosphorus and iron but no copper effect. Two products, two doses, two different response patterns.
The evidence
Tomato in nutrient solution at 20 and 50 mg/L. Peat-derived humic acid: +22% root dry weight at 20 mg/L, with higher N, P, Fe and Cu uptake. Leonardite-derived: +16% root dry weight and +8 to 9% shoot at 50 mg/L, with higher N, P and Fe. Adani et al., 1998, Journal of Plant Nutrition 21(3):561–575.
What the data shows by crop
Lawns and turf
Turf is where the root response shows up most clearly. The bentgrass work behind benefit 2 found large root increases from humate worked into the root zone, and none from spraying it on the leaves. You cannot dig anything into an established lawn. The options are a drench, or granules brushed in after aerating. Two later studies on sand-based turf found humic products added nothing once the grass was fed (Lindsey and colleagues, 2020 and 2021), so expect little on a lawn you already feed. Temper the expectation if your lawn sits on decent loam with good organic matter, because the trial grew its plants in sand.
Tomatoes and container crops
The nutrient-solution tomato data is solid for root growth and trace-element uptake. In a pot or grow bag, mix 1 to 2 g into each 10 litres of compost at potting. That is too little to change how much feed the compost holds. Any gain will come through the roots. For the full feeding picture on fruiting crops, the tomato fertiliser guide goes deeper.
Strawberries and soft fruit
One Polish field trial sprayed a humic product on strawberries two to four times. The plants grew leaves over 60% larger and gave a higher yield (Zydlik and Zydlik, 2023). It ran for one season on one variety, so treat it as a hint.
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Four limitations
Products vary enormously
"Humic acid" on a label can mean anything from a 70% leonardite extract to a thin lignite suspension to a relabelled lignosulphonate. Source, extraction method and concentration all change how it performs. Two products with the same percentage on the front can behave differently in the same trial. The 2014 meta-analysis of the whole field found that source and application rate drove most of the variability in the response, with compost-derived humic substances outperforming those derived from lignite and peat. Material from brown coal, which includes leonardite, gave the smallest gain: about 12% in shoot growth and no significant effect on roots. Buy on declared source and a certificate of analysis, not on the headline number.
Stressed plants respond most, which is not quite the same as poor soil
The received wisdom is that poor soil responds most. The evidence is more particular than that. The 2014 meta-analysis found the clearest moderator was plant stress: stressed plants gained about 28% in shoot growth against about 18% for unstressed ones. Growth medium made no significant difference to that response, and soil texture and organic matter were never analysed. It also warns that in sandy soils with little clay, humic acid is more prone to leaching away.
The evidence
Shoot growth rose 28 ± 6% under highly stressful conditions against 18 ± 3% under non-stressful ones; growth medium had no significant effect. Rose et al., 2014, Advances in Agronomy 124:37–89.
Several good trials found nothing
Some well-run trials found nothing. Greenhouse and field trials tested five commercial humic-acid products on lettuce and processing tomato and found no consistent yield or nutrient-uptake benefit. Their doses were small, 1.1 to 3.4 kg of humic acid per hectare in the field. A garden dressing of 10 to 20 g of granules per square metre carries about 70 to 140 kg of humic acid per hectare. The lettuce work used soils deliberately chosen for low phosphorus availability. A separate programme of 35 field trials in Ontario, 20 with fulvic acid and 15 with humic acid, found no effect on dry bean vigour, height, seed weight or yield. These trials are the reason the answer to "will it work on my beds" is "it depends".
The evidence
Five commercial humic-acid products gave no consistent benefit on lettuce or processing tomato, in soils selected for low phosphorus availability. Hartz & Bottoms, 2010, HortScience 45(6):906–910. Twenty fulvic and 15 humic acid field trials found dry bean vigour, height, 100-seed weight and yield no different from the control. Mahoney et al., 2017, Canadian Journal of Plant Science 97(2):202–205.
It is not a fertiliser, and not a substitute for organic matter
Humic acid carries almost no nitrogen, phosphorus or potassium of its own. It makes other inputs work better rather than replacing them. And a scoop of humic acid is no shortcut around compost, cover crops and worm activity. It helps roots use what is already in the soil, so it needs compost or leaf mould, and a feed, to work with.
What the numbers look like together
+22%
average shoot dry weight across pooled humic substance trials
Rose et al., 2014 · meta-analysis
Two meta-analyses and a large body of pooled trials land in roughly the same place: the average gain is about 12% to 22%, mostly through roots, and some trials found none.
How to put this into practice
The benefits above translate into a few simple habits. Lead with soil, because that is where humic acid does most of its work: incorporate it at planting or as a base dressing, and if you did not dig it in, water on one drench at planting. Pair it with your nitrogen feed rather than treating it as a feed in itself. The companion guide on how to apply humic acid has the rates and timings.
Related: Humic Acid Granules · Fulvic Acid Powder · humic and fulvic collection
Frequently asked questions
What are the main benefits of humic acid for plants?
The best-supported benefits are better nitrogen-use efficiency, more root growth, improved nutrient holding, freeing up locked phosphorus, and better growth under salt stress. The largest and most reliable effect is on nitrogen-use efficiency when humic acid is paired with a nitrogen feed. The soil-biology benefit is real but small, and only shows up on soils low in organic matter. The nutrient-holding and phosphorus effects were measured at doses far above a garden dressing.
How much does humic acid increase yield?
See the review note above.
Does humic acid help roots grow?
Yes, this is one of its more reliable effects. Humate worked into the root zone raised root mass by roughly 45% in the top layer of soil in a greenhouse study, and humic substances activate the root proton pump and the auxin gene that drives lateral roots. The root response is stronger from soil incorporation than from foliar spraying.
Is humic acid worth it if my soil is already healthy?
Probably less so. In the largest review, the growing medium made no significant difference to the response, so this is more about the state of the plant than the quality of the ground. If your soil is in good heart and your crop is not struggling, expect a modest effect rather than a dramatic one.
Does humic acid work for lawns?
One greenhouse study on young turf grown in sand found more root where humate was worked into the sand before sowing. Later turf studies found humic products added nothing once the grass was fed. For a lawn, that points to early-season incorporation or a soluble drench. The root study grew its plants in sand, so expect less on an established lawn over good soil.
Is humic acid a fertiliser?
No. Humic acid contains almost no nitrogen, phosphorus or potassium of its own. It is a soil conditioner and biostimulant that helps the plant use other nutrients better. It pairs with a fertiliser rather than replacing one, and it is no substitute for compost and organic matter.
What is the difference between humic acid and fulvic acid for plants?
Humic acid is the larger, heavier fraction and works mainly at the root surface, by holding nutrients and binding metals. Fulvic acid is smaller, gets inside the plant, and acts more as a chelator and biostimulant. The full comparison is in humic vs fulvic acid. In the barley trial cited above the two were tested separately, and each worked on its own, so there is no need to treat them as a package.
How long does humic acid take to work?
Some effects are quick and some are slow. Root and uptake responses can show within a few weeks. At garden rates, do not expect a change in the soil itself. Judge it by how the plants grow over a season.
Founder of Dr Forest. Joe started the business and formulates the blended fertilisers and mixes and packs them in small batches in Stockport.
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
- 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. doi.org/10.2135/cropsci1998.0011183X003800060037x
- 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
- Ampong, K., Thilakaranthna, M.S., Gorim, L.Y. (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
- Gerke, J. (2021). The effect of humic substances on phosphate and iron acquisition by higher plants: qualitative and quantitative aspects. Journal of Plant Nutrition and Soil Science 184(3): 329–338. doi.org/10.1002/jpln.202000525
- Delgado, A., Madrid, A., Kassem, S., Andreu, L., del Campillo, M.C. (2002). Phosphorus fertilizer recovery from calcareous soils amended with humic and fulvic acids. Plant and Soil 245: 277–286. doi.org/10.1023/A:1020445710584
- 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. Incubation detail from the project report behind it: Hartz, T.K. Evaluation of humic substances for improving nutrient efficiency in vegetable production, CDFA FREP project 07-0174.
- 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
- Alsudays, I.M., Alshammary, F.H., Alabdallah, N.M., Alatawi, A., Alotaibi, M.M. et al. (2024). Applications of humic and fulvic acid under saline soil conditions to improve growth and yield in barley. BMC Plant Biology 24: 191. doi.org/10.1186/s12870-024-04863-6
- Adani, F., Genevini, P., Zaccheo, P., Zocchi, G. (1998). The effect of commercial humic acid on tomato plant growth and mineral nutrition. Journal of Plant Nutrition 21(3): 561–575. doi.org/10.1080/01904169809365424
- Mahoney, K.J., McCreary, C.M., Depuydt, D., Gillard, C.L. (2017). Fulvic and humic acid fertilizers are ineffective in dry bean. Canadian Journal of Plant Science 97(2): 202–205. doi.org/10.1139/cjps-2016-0143
- 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
- Zydlik, Z., Zydlik, P. (2023). The effect of a preparation containing humic acids on the growth, yield, and quality of strawberry fruits. Agronomy 13(7): 1872. doi.org/10.3390/agronomy13071872
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