Humic acid and fulvic acid are derived from the same source — leonardite or decomposed organic matter — but they behave very differently in soil and plant nutrition. This guide compares their properties and helps you choose the right product for your crops.
What Are Humic Substances?
Humic substances are the stable end-products of microbial decomposition of organic matter. Two fractions matter for agriculture:
- Humic acid — soluble at alkaline pH (above 6.5); large molecules best suited for soil conditioning
- Fulvic acid — soluble at all pH levels; smallest, most active fraction; small enough to enter plant cells directly
Commercial products are typically potassium humate (humic acid) or potassium fulvate (fulvic acid), extracted from leonardite (oxidized lignite).
Humic Acid vs Fulvic Acid: Key Differences
| Property | Humic Acid | Fulvic Acid |
|---|---|---|
| Molecular weight | 10,000–100,000 Da (large) | 1,000–10,000 Da (small) |
| Solubility | Soluble at pH >6.5 only | Soluble at all pH levels |
| Carbon content | 50–60% | 40–50% |
| Functional groups | Many carboxyl & phenol groups | More active acid groups per gram |
| Soil application | Better for soil structure | Better for foliar & fertigation |
| Plant uptake | Indirect (soil conditioning) | Direct (enters plant cells) |
| Chelating capacity | High for soil micronutrients | Very high, especially in solution |
How Humic Acid Benefits Soil and Crops
Humic acid works primarily through soil conditioning. Its large molecules bind to clay particles, improving soil aggregation and water retention. Key benefits:
- CEC increase: Adds negatively charged sites that hold Ca²⁺, Mg²⁺, K⁺, reducing nutrient leaching
- Chelation of micronutrients: Forms stable complexes with Fe, Zn, Mn, keeping them plant-available even at high pH
- Soil microbial stimulation: Provides carbon source and habitat for beneficial bacteria and fungi
- Water retention: Sandy soils treated with humate hold 20–30% more water
- Root development: Promotes lateral root elongation, improving nutrient uptake surface area
Humic acid is best applied to soil via drip irrigation or soil drenching. It is not suited for foliar spray due to its large molecular size and poor leaf penetration.
How Fulvic Acid Benefits Crops
Fulvic acid is the most biologically active fraction. Its small size and full pH-range solubility allow it to enter plant cells directly. Key benefits:
- Nutrient carrier: Forms soluble complexes with Fe, Zn, Mn and transports them through cell membranes
- Foliar efficiency: Enhances membrane permeability, increasing uptake of co-applied nutrients by 30–60%
- Root hormone stimulation: Triggers biosynthesis of plant growth hormones (IAA, gibberellins)
- Antioxidant effect: Reduces oxidative stress under drought, frost or saline conditions
- Reduces phosphorus fixation in high-calcium soils by chelating Ca²⁺
Fulvic acid is ideal for foliar spray and fertigation at very low concentrations (0.01–0.05%).
Which One Should You Choose?
| Scenario | Recommended Product |
|---|---|
| Heavy clay soil — improve structure & drainage | Humic Acid (potassium humate) |
| Sandy soil — improve water & nutrient retention | Humic Acid (potassium humate) |
| Foliar spray to correct micronutrient deficiency | Fulvic Acid |
| Fertigation with chelated Fe, Zn, Mn | Fulvic Acid (better pH stability) |
| High-pH calcareous soil — unlock P and Fe | Fulvic Acid |
| Pre-planting soil amendment | Humic Acid |
| Both soil structure AND foliar nutrition | Combination (50:50 blend) |
Many agronomists use both together: humic acid as a base soil conditioner 2–4 weeks before planting, and fulvic acid as a foliar/fertigation supplement during the growing season.
Product Specifications
Potassium Humate: ≥70% humic acid (flakes) or ≥50% (granules); K₂O 10–12%; pH 8–10. Leonardite-origin preferred for highest bioactivity. Available as crystal flakes or granules.
Potassium Fulvate: ≥50% fulvic acid; 100% water-soluble at all pH; light yellow to amber color (dark color indicates high humic contamination); pH 5–7. Suitable for fertigation and foliar application.
Application Rates
| Application Method | Humic Acid Rate | Fulvic Acid Rate |
|---|---|---|
| Drip irrigation (per hectare) | 5–10 kg/ha per application | 1–2 kg/ha per application |
| Foliar spray (concentration) | Not recommended | 0.01–0.05% solution |
| Soil drench (transplanting) | 2–3 g/L water | 0.5–1 g/L water |
| Granular fertilizer coating | 1–2% of fertilizer weight | 0.5–1% of fertilizer weight |
| Broadcast amendment | 50–100 kg/ha | — |
Related Products from Unifarm Chemical
Hainan Unifarm Chemical supplies both Humic Acid (potassium humate flakes/granules) and Fulvic Acid (potassium fulvate powder) for agricultural use — extracted from high-grade leonardite with guaranteed humic/fulvic content.
For programs combining humic substances with chelated micronutrients, see our EDTA Fe, EDDHA Fe, EDTA Zn, and EDTA Mn; see also our guide on sodium molybdate for molybdenum nutrition. Request samples, COA or pricing via our Contact page.
Frequently Asked Questions
Is humic acid safe to use with fertilizers?
Yes. Potassium humate is compatible with most NPK fertilizers, calcium nitrate, and micronutrient mixes. Avoid mixing directly with concentrated acidic solutions (pH below 4) as humic acid may precipitate. In fertigation tanks, add humate first, dissolve fully, then add other fertilizers.
Can fulvic acid be used in hydroponic systems?
Yes, potassium fulvate is suitable for hydroponics at low concentrations (0.01–0.02%). It improves mineral uptake and reduces root stress. Use a 100% water-soluble, light-colored product and monitor EC levels as fulvate adds slight conductivity.