• Humic Acid Flakes — 250g kraft pouch with dr forest Organic Humic Acid Granules Semi Soluble Concentrate label
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Dr Forest

Humic Acid Flakes | 70% Humic Acid, 100% Water Soluble | Premium Leonardite Soil Conditioner | Dr Forest

Humic Acid Flakes | 70% Humic Acid, 100% Water Soluble | Premium Leonardite Soil Conditioner | Dr Forest

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Humic acid flakes — 70% humic acid, 100% water soluble, premium leonardite extract

70% Humic Acid 100% Water Soluble Premium Leonardite CEC 450 meq/100g Fertigation Grade Flake Format

Humic acid flakes are a refined, fully water-soluble extract of premium leonardite — the highest-quality source of humic substances known, formed over geological timescales from the oxidation of ancient plant-rich lignite deposits. At 70% humic acid content, these flakes are the active concentrate that decades of agronomic research has built application protocols around.

Humic acid is not a fertiliser in the conventional sense. It supplies no primary nutrients directly. What it does is transform the environment in which nutrients operate: it raises the cation exchange capacity of the soil so minerals are held and released to roots rather than leached away, it directly stimulates bacterial and fungal biomass in the rhizosphere, and it carries auxin-like activity that drives root cell elongation within days of application. These are cumulative gains that compound across growing seasons.

The flake format dissolves to complete clarity within minutes of mixing — no sediment, no residue, no insoluble fraction. That solubility is what makes the flakes usable across every application method peer-reviewed research has shown to work: foliar spray, soil drench, seed soak, and compost tea. Handcrafted in Stockport, made with organic ingredients, no animal by-products.

70%Humic Acid
100%Water Soluble
450meq/100g CEC
LeonarditePremium Source

What humic acid flakes are used for

  • Foliar spray — auxin-like response in leaf tissue, used routinely in tomato, pepper, strawberry and ornamental research at concentrations from 200 mg/L upwards
  • Soil drench — even root-zone delivery with faster uptake than dry amendments; standard method in greenhouse and field trials
  • Seed soak — pre-sowing treatment shown to lift germination rate and seedling vigour across maize, wheat, cotton, faba bean, and many vegetables
  • Compost tea — added as a microbial food during brewing; documented to stimulate fungal biomass and chelate chlorine in mains water
  • Transplant drench — auxin-like root stimulation within 48–72 hours, reducing transplant shock and accelerating establishment
  • Peat-free compost conditioning — peat-free media is naturally low in humic substances; regular drenches replenish humic content and extend compost working life
  • Fertigation and drip irrigation — dissolves to complete clarity, so suitable for drip systems, emitters, and automated dosing without clogging
  • Soil biology activation — directly stimulates bacterial biomass (+30–60%) and mycorrhizal colonisation (+25–40%) at agronomic rates

Why fully soluble flakes

Fully soluble humic flakes

  • Dissolves to complete clarity within 2–5 minutes in water
  • No sediment, no sludge, no insoluble fraction
  • Safe for drip emitters, spray nozzles, and inline filters
  • Every gram of humic content delivered in bioavailable dissolved form
  • Weigh, dissolve, apply — no settling or siphoning step

Partially soluble humate products

  • 10–20% of material remains as insoluble residue
  • Sediment settles at the bottom of the watering can
  • Unsuitable for drip irrigation — emitters block
  • Only the soluble fraction works quickly
  • Stock solutions require settling and siphoning

The science of humic acid: how leonardite-derived humates transform soil chemistry and biology

What humic substances actually are

Humic substances are the end product of plant and microbial decomposition, formed over geological timescales through progressive oxidation and polymerisation of organic matter. They are large, dark, carbon-rich molecules with a structure dominated by carboxyl (-COOH) and hydroxyl (-OH) functional groups. These functional groups are the source of humic acid's properties: they carry negative charge, which enables cation exchange; they chelate metal ions, which makes micronutrients plant-available; and they bind to mineral and organic particles, which stabilises soil aggregates.

Leonardite from premium deposits contains some of the highest concentrations of humic substances found anywhere in nature — far higher than the 1–5% humic content of even good garden topsoil, and significantly higher than lower-grade sources such as standard lignite, peat, or vermicompost. The use of leonardite humates in agriculture is backed by over 50 years of agronomic research across dozens of crops and soil types, summarised most recently in the Rose et al. (2014) meta-analysis and the Canellas et al. (2015) biostimulant review.

Humic acid vs fulvic acid — they do different jobs

Humic acid — works in the soil

  • Large molecules (MW 10,000–100,000 Da)
  • Primary role: structure, CEC, fungal promotion, nitrogen stabilisation
  • Slow, cumulative benefits that build across seasons
  • Best used as monthly soil drench, foliar spray, or pre-sowing seed soak
  • This product — flakes for liquid application

Fulvic acid — works inside the plant

  • Small molecules (MW 1,000–10,000 Da)
  • Primary role: nutrient chelation, membrane transport, foliar uptake
  • Fast-acting; responses visible within days
  • Best added to every liquid feed and foliar spray
  • See Dr Forest Fulvic Acid Powder for the matching product

What CEC means in practice

CEC (cation exchange capacity) measures how many positively charged nutrient ions a soil or amendment can hold and exchange with plant roots. Sandy soils typically have a CEC of 2–5 meq/100g. Good clay soils reach 20–40 meq/100g. Leonardite humic acid has a CEC of 450 meq/100g — roughly 10–200× the CEC of the soil it is added to. Even small quantities dramatically increase the capacity of a growing medium to hold calcium, magnesium, potassium, ammonium, and trace elements that would otherwise leach through with watering. For containers, raised beds, and peat-free media — where intense watering removes nutrients quickly — this is one of the most valuable properties of any soil amendment.

Six mechanisms of action

01

Soil aggregation & structure

Humic acid molecules bridge mineral particles and organic matter into stable macro-aggregates, creating pore spaces that improve drainage in clay soils and moisture retention in sandy soils. In clay soils, aggregation opens the structure and reduces compaction. In sandy soils, humic coatings on sand grains increase water-holding capacity considerably (Stevenson, 1994).

02

Cation exchange & nutrient retention

Humic acid functional groups carry a strong negative charge that binds positively charged nutrient cations — calcium, magnesium, potassium, ammonium, iron, manganese, zinc — on exchange sites. This slows leaching, stabilises nitrogen, and creates a reservoir of plant-available nutrients that release progressively as roots demand them. For peat-free compost and container growing, this is transformative.

03

Phosphorus solubilisation

Phosphorus in most soils is rapidly locked up by binding with calcium, iron, and aluminium. Humic acid competes for these cation binding sites and forms soluble humate-metal complexes, releasing previously fixed phosphorus back into plant-available form. The Ma et al. (2024) meta-analysis reports an average crop yield gain of 12% and nitrogen use efficiency gain of 27% when humic acid is applied alongside conventional fertilisers.

04

Microbial & mycorrhizal promotion

Humic substances directly stimulate bacterial and fungal biomass in the rhizosphere. Nardi et al. (2009) demonstrated increases of 30–60% in bacterial biomass and 25–40% in mycorrhizal colonisation following humic acid application. The carbon structure acts as a slow-release food source that supports the microbial engine of nutrient cycling.

05

Root growth stimulation

Humic acid carries auxin-like activity that directly stimulates root cell elongation and lateral root initiation. Visible root mass improvements typically appear within 2–3 weeks of regular application, with particularly strong responses at transplanting. Canellas et al. (2015) reviewed 40 years of research confirming consistent root hormone-like effects across dozens of crop species.

06

Heavy metal chelation & detoxification

Humic acid binds tightly to aluminium, lead, cadmium, and other potentially toxic cations, reducing their bioavailability. In acidic soils where aluminium toxicity limits root growth, humic acid provides an immediate detoxification effect alongside its other benefits — one reason it is routinely used in remediation programmes on degraded land (Piccolo, 2002).

Peer-reviewed trial outcomes

The humic acid literature is unusually consistent across crops and soil types. Yildirim (2007) reported significant yield, fruit weight, and ascorbic acid improvements in tomato at 20 ml/L foliar applications, with Karakurt et al. (2009) confirming the same window for pepper. Eshghi and Garazhian (2015) identified 600–900 mg/L foliar and 300–450 mg/L drench as optimal windows for strawberry. Kadhim and Hamza (2021) showed that pre-sowing maize seed soak at 1 ml/L for 18 hours raised emergence rate and seedling vigour significantly. The mechanisms differ slightly by crop and method, but the direction of the effect is stable: humic acid applied at agronomic rates improves root mass, nutrient uptake efficiency, and final yield.

"Humic substances at agronomic concentrations show consistent positive effects on plant growth — an effect remarkably stable across species and growing conditions." — Canellas et al., 2015

Scientific references

  1. Canellas, L.P. et al. (2015). Humic and fulvic acids as biostimulants in horticulture. Scientia Horticulturae, 196, 15–27.
  2. Nardi, S. et al. (2009). Plant biostimulants: physiological responses induced by protein hydrolysed-based products and humic substances in plant metabolism. Agronomy for Sustainable Development, 29, 263–271.
  3. Rose, M.T. et al. (2014). A meta-analysis and review of plant-growth response to humic substances. Advances in Agronomy, 124, 37–89.
  4. Piccolo, A. (2002). The supramolecular structure of humic substances: a novel understanding of humus chemistry. Advances in Agronomy, 75, 57–134.
  5. Stevenson, F.J. (1994). Humus Chemistry: Genesis, Composition, Reactions. Wiley.
  6. Chen, Y. & Aviad, T. (1990). Effects of humic substances on plant growth. In: Humic Substances in Soil and Crop Sciences, ASA/SSSA, 161–186.
  7. Yildirim, E. (2007). Foliar and soil fertilization of humic acid affect productivity and quality of tomato. Acta Agriculturae Scandinavica B, 57, 182–186.
  8. Karakurt, Y. et al. (2009). The influence of foliar and soil fertilization of humic acid on yield and quality of pepper. Acta Agriculturae Scandinavica B, 59, 233–237.
  9. Türkmen, Ö. et al. (2004). Calcium and humic acid affect seed germination, growth, and nutrient content of tomato seedlings under saline soil conditions. Acta Agriculturae Scandinavica B, 54, 168–174.
  10. Eshghi, S. & Garazhian, M. (2015). Improving growth, yield and fruit quality of strawberry by foliar and soil drench applications of humic acid. Iran Agricultural Research, 34(1), 14–20.
  11. Shaygan, A. (2024). Drench of humic acid mitigate the adverse impacts of alkalinity on rose. Ornamental Horticulture, 30, e242710.
  12. Hartwigsen, J.A. & Evans, M.R. (2000). Humic acid seed and substrate treatments promote seedling root development. HortScience, 35(7), 1231–1233.
  13. Killi, F. (2004). Effects of potassium humate solution and soaking periods on germination characteristics of undelinted cotton seeds. Journal of Environmental Biology, 25, 395–398.
  14. Kadhim, J.J. & Hamza, J.H. (2021). Effect of maize seeds soaking with acids of ascorbic, citric and humic on field emergence. Iraqi Journal of Agricultural Sciences, 52(4), 971–976.
  15. Azhar, T.S. (2026). Effect of seed treatment with salicylic acid, humic acid and zinc on the growth rate of broad bean seedlings (Vicia faba L.). Plant Science Today, 13(1).
  16. 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.

How to use humic acid flakes: peer-reviewed rates for foliar, drench, seed soak and compost tea

Working with the flakes

Weigh the flakes against the rate in the relevant card below, add to the volume of water required, and stir. They dissolve completely within 2–5 minutes in warm water and within 10 minutes in cold. There is no sediment, no settling step, and no residue to manage. The solution will be very dark but completely clear. Apply on the same day you mix it — extended storage allows microbial activity to develop in the bucket.

Staining warning

The solution stains porous surfaces — paving, concrete, grout, light-coloured clothing, and timber. Work over soil or grass. Rinse spills on hard surfaces immediately.

Foliar spray

Foliar application delivers humic acid directly to leaf tissue, where the auxin-like response is fastest. Yildirim (2007) found 20 ml/L of commercial liquid humate sprayed at 10-day intervals optimum for tomato, with Karakurt et al. (2009) confirming the same window for pepper. Eshghi and Garazhian (2015) identified 600–900 mg/L (active humic acid) as the strawberry sweet spot, with declining returns above 1000 mg/L. Commercial protocols summarised by Alsultana suggest 0.1–0.3% active concentration depending on crop sensitivity. Concentrations expressed below are for 70% humic acid flakes.

Tomato, pepper, aubergine, cucurbits

Rate: 0.5 g flakes per litre of water (≈350 mg/L humic acid) · Frequency: Every 10–14 days from 4-leaf stage through fruit development

Spray both leaf surfaces to runoff in early morning or late evening. Yildirim (2007) recorded the highest yield, fruit weight, and ascorbic acid content at this concentration window across two seasons of greenhouse tomato trials, with sprays at 10-day intervals from three weeks after planting.

Strawberries and soft fruit

Rate: 0.85 g flakes per litre (≈600 mg/L humic acid) · Frequency: Every 10–14 days from first flower through harvest

Eshghi and Garazhian (2015) identified 600 mg/L as the lower bound and 900 mg/L as the upper bound of the optimum foliar window for strawberry. Both produced the highest dry shoot and root mass, with significant improvements in fruit ascorbic acid and total soluble solids over control.

Leafy vegetables and salad crops

Rate: 0.25 g flakes per litre (≈175 mg/L humic acid) · Frequency: Every 7–10 days from 4 true leaves to harvest

Leafy crops respond at gentler concentrations and benefit from more frequent, lighter applications. Stay at the lower end of the rate range — higher concentrations risk leaf marking on thin-leaved crops.

Roses, ornamentals and shrubs

Rate: 0.5 g flakes per litre (≈350 mg/L humic acid) · Frequency: Every 14 days through active growth

Foliar application supports flower colour and stem vigour, particularly on roses growing in alkaline soils where iron uptake is restricted. For stressed plants, combine with the soil drench programme below.

Cereals, field crops and lawns

Rate: 0.7 g flakes per litre (≈500 mg/L humic acid) · Frequency: Every 14–21 days

The Ma et al. (2024) meta-analysis of humic acid across cereal trials confirms the strongest response at 100–200 kg N/ha programmes, with cash crops and upland cereals out-responding paddy rice. For lawns, deliver via knapsack sprayer at standard turf coverage rates.

Soil drench

Drench delivers humic acid into the active root zone where it builds CEC, stimulates microbial biomass, and provides the auxin-like signal that drives lateral root development. Eshghi and Garazhian (2015) optimised strawberry drenches at 300–450 mg/L active humic acid every 14 days. The literature consensus is for monthly application at moderate strength, stepped up to 14-day intervals on stressed plants or alkaline soils.

General vegetable garden and allotment

Rate: 0.5 g flakes per litre (≈350 mg/L humic acid) · Volume: 500 ml–1 L per medium plant · Frequency: Every 3–4 weeks

Apply to moist soil around the root zone. Skip applications when soil is dry or plants are heat-stressed — wait for the soil to be moist first.

Strawberries, soft fruit, fruiting containers

Rate: 0.5 g flakes per litre (≈350 mg/L humic acid) · Volume: 250 ml per 3 L pot, scaled up · Frequency: Every 14 days

This matches the Eshghi and Garazhian (2015) optimal soil treatment for cultivar Paros strawberry: significant gains in fruit number, total yield, and ascorbic acid against control. The 14-day interval was the trial schedule.

Roses and ornamentals on alkaline or stressed soils

Rate: 1.4 g flakes per litre (≈1000 mg/L humic acid) · Frequency: Every 14 days through the growing season

Shaygan (2024) demonstrated that 1000 mg/L drenches every 15 days for two months on rose under alkaline stress lifted total protein, proline, and antioxidant activity above control, with sugar content peaking at 500 mg/L. The higher rate is the stress-management option — drop back to 500 mg/L (0.7 g flakes per litre) once vigour returns.

Transplant drench

Rate: 0.7 g flakes per litre (≈500 mg/L humic acid) · Volume: 300–500 ml per planting hole · Frequency: Once at planting; repeat after 14 days

Apply directly into the planting hole before setting the root ball. Auxin-like root stimulation typically appears within 48–72 hours, with significant reductions in transplant shock and faster establishment recorded in the Canellas et al. (2015) review.

Container and peat-free compost conditioning

Rate: 0.7 g flakes per litre (≈500 mg/L humic acid) · Frequency: Every 14–21 days

Peat-free media is naturally low in humic substances. Regular drenches replenish humic content, raise CEC of the growing medium so it holds nutrients more like a peat-based mix, and extend compost working life. Particularly valuable in season-long container crops.

Seed soak (pre-sowing treatment)

Pre-sowing seed treatment with humic acid is one of the best-evidenced uses of the material. Kadhim and Hamza (2021) recommend maize seeds soaked at 1 ml/L of commercial humate for 18 hours before spring sowing — significant gains in emergence percentage, seedling root length, and seedling vigour. Killi (2004) confirmed 16-hour soak as optimal for cotton in potassium humate solution. Across cereals, legumes, and vegetables, the literature points to dilute concentrations and longer soak times rather than concentrated short dips.

Tomato, pepper, aubergine, cucurbits

Rate: 0.3 g flakes per litre (≈210 mg/L humic acid) · Soak time: 4–8 hours

Soak seeds at room temperature in a shallow dish. Sow immediately afterwards — do not redry. Faster germination and stronger first true leaves are the typical response.

Cereals, maize and wheat

Rate: 0.5 g flakes per litre (≈350 mg/L humic acid) · Soak time: 12–18 hours

This matches the Kadhim and Hamza (2021) protocol for spring maize on cultivars Baghdad3, 5018 and Sumer. Drain seeds briefly before sowing — do not redry. The longer soak time reflects cereal seed coat permeability.

Beans, peas and other legumes

Rate: 0.3 g flakes per litre (≈210 mg/L humic acid) · Soak time: 6–12 hours

Azhar (2026) evaluated faba bean (cultivar Barcino) seed treatments at 100–300 mg/L active humic acid, with response peaking in the middle of that range for germination index and seedling vigour. Do not exceed 12 hours — legume seeds split with prolonged soaking.

Salad and leafy greens

Rate: 0.15 g flakes per litre (≈100 mg/L humic acid) · Soak time: 2–4 hours

Small seeds need the lightest treatment. A 2–4 hour soak in the dilute solution lifts germination uniformity without risk of inhibition.

Flowers and ornamentals

Rate: 0.3 g flakes per litre (≈210 mg/L humic acid) · Soak time: 4–12 hours, depending on seed size

Hartwigsen and Evans (2000) demonstrated significant root development gains in marigold and geranium from pre-sowing humate seed soak. Adjust soak time to seed size — small seeds 4 hours, larger seeds up to 12.

Compost tea

Humic acid added during compost tea brewing serves two purposes: it binds chlorine and chloramine in mains water (protecting the microbial population in the tea), and it acts as a food source that documented research shows stimulates fungal biomass during the brew. Crophealth research used 30 ml of liquid humic extract per 15 L of standard recipe — broadly equivalent to a few grams of these flakes scaled to brew volume.

Standard 20 L aerated compost tea brew

Rate: 2–3 g flakes per 20 L of brew water · Timing: Add at the start, before compost goes in

Dissolve the flakes into the water first and let the brewer run for 5–10 minutes — this binds chlorine in mains water before the microbial inoculant arrives. Then add compost, kelp, fish hydrolysate, or whichever other ingredients your recipe uses. Brew at 18–24°C for 18–24 hours and apply within 4 hours of switching off the air pump.

Small batch (5 L watering can brew)

Rate: 0.5 g flakes per 5 L brew water · Timing: Add at the start of the brew

For aerated bucket-scale brews or steeped (non-aerated) compost extracts. Same protocol — dissolve, wait briefly, then add compost.

Fertigation and combined liquid feed

Fertigation and drip irrigation

Rate: 0.5 g flakes per litre of reservoir water (≈350 mg/L humic acid) · Frequency: Every 3–4 weeks

Add the dissolved flakes to the reservoir after the main nutrient solution is mixed. Compatible with all liquid nutrient programmes. Because the flakes are fully soluble, emitters will not clog at these rates.

Combined with liquid feed

Rate: 0.5 g flakes per litre of diluted feed · Frequency: Every feeding, or alternate feedings

Humic acid enhances uptake efficiency of every mineral in the feed. Always add the dissolved flakes to the already-diluted feed, not to undiluted concentrate, to avoid precipitation.

Works well combined with

Pair with Fulvic Acid Powder in every liquid feed for complete humic-plus-fulvic coverage — humic works in the soil, fulvic works inside the plant. Stack with Seaweed Powder for biostimulant compounding: seaweed delivers cytokinins and auxins, humic improves uptake and soil biology. Fully compatible with all Dr Forest crop-specific fertilisers as a general efficiency booster.

Frequently asked questions about humic acid flakes

No. Humic acid is a soil conditioner and nutrient efficiency enhancer, not a primary nutrient source. It improves how effectively soil holds and delivers nutrients from existing reserves, added fertilisers, or organic matter decomposition. It multiplies the value of the fertiliser you already apply rather than replacing it. The Ma et al. (2024) meta-analysis quantifies the average crop yield gain at 12% and the nitrogen use efficiency gain at 27% across studies.
If you are starting one programme: monthly soil drench is the highest-leverage use, because it builds CEC and microbial biomass in the root zone where most of the work happens. Add foliar spray every 10–14 days during fruit set on tomato, pepper, and strawberry — the auxin-like response is fastest through the leaf. Pre-sowing seed soak is a once-per-crop investment with documented gains in germination uniformity. Compost tea is the smallest dose but a useful place to start binding chlorine and feeding fungal biomass during brewing. Most growers run drench plus foliar through the season, and add seed soak and compost tea as habit develops.
Many humate products sold as "soluble" are only 80–90% soluble, leaving 10–20% as insoluble residue. In open soil that residue eventually breaks down and contributes value, but for drip emitters, spray nozzles, or any automated dosing system it causes blockage. These flakes dissolve to complete clarity within minutes, delivering the full humic content in fully dissolved, immediately bioavailable form with no residue to manage.
Effects are cumulative. The fastest response is in transplants and seed-soak crops — root improvements visible within two weeks. In established plants, meaningful improvements in vigour and colour typically appear after two or three applications over 6–8 weeks. Soil structure and microbial effects build over seasons rather than weeks, and the full value of humic acid is seen in year-two and year-three results.
No — the response curve is bell-shaped, not linear. Eshghi and Garazhian (2015) found 1200 mg/L produced no better strawberry yield than 600 mg/L. Some studies (Türkmen et al., on tomato) report that very high rates can arrest growth or reduce nutrient uptake. Stick to the rates in the cards above and apply more frequently rather than more strongly.
They are complementary, not interchangeable. Humic acid works in the soil — building structure, CEC, and microbial biomass. Fulvic acid works inside the plant — chelating minerals and delivering them through cell membranes. The ideal programme uses both: humic acid as a monthly soil drench, fulvic acid added to every liquid feed and foliar spray. Running both together is measurably more effective than either alone.
Potassium lignosulphonate is an industrial byproduct of paper manufacturing — processed lignin made water-soluble by chemical treatment. Lignin is a precursor to humic substances, but full conversion to true humic acid takes geological time. Lignosulphonate has not undergone that process. Leonardite humic acid is the genuine end product, with the abundant carboxyl and hydroxyl functional groups that give it its high CEC, chelation capacity, and fungal stimulant properties. If a product does not state a humic acid percentage or lists lignosulphonate as its active ingredient, it is not true humic acid.
Yes. Leonardite humic acid is a naturally occurring organic substance with no known toxicity to mammals, birds, bees, or soil invertebrates at any realistic application rate. There is no withholding period for edible crops. Store flakes and made-up solution securely out of reach, and wash hands after handling the concentrate — mainly because of strong staining rather than any toxicity concern.
Use within the same day. The humic acid itself is stable, but as soon as it goes into water it provides a carbon food source that supports microbial activity in the bucket. Mix what you need, apply it, and rinse the container.
Store in the original sealed bag or an airtight container in a cool, dry place away from direct sunlight and moisture. The flakes are hygroscopic and will absorb moisture if left open, which can soften or clump them — this does not affect efficacy but makes weighing harder. Reseal after each use. Dry shelf life is at least two years.
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