Fulvic Acid: Origin, Structure, Properties & Agricultural Functions

Origin of Fulvic Acid

To understand fulvic acid (FA), we start with humus. Multiple theories explain the formation and chemistry of humus; the theories proposed by Kononov and Stevenson are widely recognized internationally and form the basis of this introduction.

Humus is a complex natural high-molecular aromatic polymer formed by the decomposition and re-synthesis of plant residues (alongside some animal and microfauna remains) under microbial activity and subsequent complex geochemical processes. Primary plant components participating in humus formation include lignin and polyphenols, while cellulose, hemicellulose, starch, tannins, proteins and lipids also contribute. Humus exists widely in nature: within soils, mud, freshwater and marine sediments, organic waste, compost and fermented residues. The highest concentrations are found in peat, lignite and weathered coal.

Based on solubility in different solvents, humus substances are classified into four fractions: fulvic acid, humic acid, hymatomelanic acid and humin. The first three groups are collectively defined as humic substances (HS).

• Humic acid (HA): Soluble in alkaline solutions, insoluble in acids.
• Fulvic acid (FA): Soluble in both alkalis and acids, and partially soluble in ethanol and acetone. Originally named fulvic acid by Swedish chemist Odén in 1919, FA is a vital member of the humic substance family.

Although FA is abundant globally, most naturally occurring FA concentrations are below 1%, making direct extraction uneconomical. Peat and coal materials (lignite and weathered coal) contain high levels of humic substances and serve as the primary raw materials for industrial extraction.

Peat represents the early stage of coal formation, where HA and FA develop under aerobic microbial activity. For this reason, peat-derived fulvic acid (PFA) shares many formation characteristics with soil fulvic acid (SFA) and biologically fermented fulvic acid (BFA). Modern peat still retains large amounts of original plant compounds including cellulose, hemicellulose, lignin and tannins, so its HA and FA remain combined with these non-humic substances.

By contrast, coal-derived fulvic acid (CFA, sourced from lignite and weathered coal) undergoes further transformation. Lignite forms under prolonged anaerobic bacterial action, while weathered coal experiences long-term geochemical changes including high temperature, pressure and oxidative weathering. Nearly all original plant components break down, and the contained HA and FA experience extensive aromatic condensation and structural isomerization.

Peat forms primarily from herbaceous, fern and moss plant material, while lignite and weathered coal originate from woody vegetation. The differences in formation age, geochemical conditions and parent plant materials create distinct chemical compositions, properties and processing requirements for these FA sources.

Chemical Composition & Molecular Structure of Fulvic Acid

The primary elemental components of fulvic acid (FA) are carbon (C), hydrogen (H), oxygen (O), nitrogen (N) and sulfur (S). Elemental ratios differ significantly depending on FA origin.

Table 1 Elemental Composition and Functional Groups of Fulvic Acid from Different Sources

Source	Elemental Composition (Approximate Range, %, daf)					Average H/C Ratio	Functional Groups (Average, mmol/g)
	C	H	N	S	O		Total Acidic Groups	COOH	Phenolic OH
Biochemical FA	45~47	7~8	4~5	1~2	39~41	1.84	5.8	3.3	2.5
Compost FA	47~48	5~7	1~3	1~2	40~42	1.72	6.4	1.3	5.1
Aquatic FA	45~47	5~6	2~3	—	44~46	1.53	—	—	—
Soil FA	44~46	4~6	1~3	0.5~2	43~45	1.42	10.3	8.2	2.1
Peat FA	44~46	4~6	2~3	0.5~1	44~46	1.19	10.4	7.8	2.6
Lignite FA	48~50	3~4	1~2	0.5~1	41~43	0.82	9.0	7.3	1.7
Weathered Coal FA	52~55	2~3	0.7~1.5	0.5~1	38~43	0.65	10.7	9.1	1.6
Weathered Coal HA	54~65	1~3	0.1~0.9	0.3~0.5	37~39	0.53	7.8	7.0	0.8

From the comparative data above:

FA extracted from peat, biological fermentation, water bodies, soil and compost features an H/C atomic ratio above 1.1. Coal-derived FA (especially weathered coal FA) contains higher carbon and lower hydrogen, with an H/C ratio below 1.

The main reactive functional groups of FA are carboxylic groups (-COOH) and phenolic hydroxyl groups. Collectively termed total acidic groups, their concentration directly determines the chemical activity of FA.

Peat FA, coal FA and soil FA fall within the same order of magnitude for functional group content. Their total acidic groups (especially carboxylic acid) are markedly higher than biologically fermented FA and compost FA. Peat FA also contains abundant phenolic hydroxyl groups, indicating comprehensive high activity.

As a complex natural organic mixture from varied sources, FA cannot be represented by a single fixed molecular formula. A schematic structural model describes FA units as three core components:

1. Core: Predominantly benzene aromatic rings, alongside a small number of alicyclic, heterocyclic rings
2. Bridges & side chains: Methylene (-CH₂–), imino (-NH–), nitrogen bridges, amino groups, enol groups and other connecting structures
3. Functional groups: Carboxylic acid, hydroxyl and other active groups

Multiple structural units assemble into FA molecules via hydrogen bonds, electrostatic attraction, van der Waals forces and metal ion coordination. FA molecules further link with proteins, amino acids, carbohydrates and hydrocarbons to form macromolecules and colloidal particles.

This structural model is an idealized representation. Natural FA varies greatly by source:

Peat FA, aquatic FA, compost FA and soil FA contain aromatic cores and functional groups bound to proteins, polypeptides, saccharides and aliphatic chains.

Coal-derived FA has a simpler structure; nearly all bound proteins and carbohydrates disappear, with far fewer aliphatic chains remaining.

The E₄/E₆ ratio (absorbance ratio at 465 nm and 665 nm) is a standard indicator of aromatic condensation degree. Peat FA shows E₄/E₆ values similar to soil and compost FA, meaning they share comparable humification maturity. Importantly, peat FA has a relatively low average molecular weight, a beneficial trait for agricultural applications.

Comparing HA and FA extracted from the same raw material (e.g., weathered coal): HA has higher carbon content, lower H/C ratio, lower oxygen content, higher aromaticity, larger molecular weight and fewer functional groups. This explains why FA demonstrates superior biological activity.

Fundamental Properties of Fulvic Acid

While FA shares certain traits with humic acid (HA), structural analysis confirms FA is the fraction of humic substances with the lowest aromaticity, smallest molecular size, highest solubility and largest quantity of functional groups. It exhibits greater chemical, physicochemical and biochemical activity than HA.

Physical & Colloidal Properties

Solid FA appears as dark yellow to dark brown powder. Color intensity follows the gradient: soil FA ≈ peat FA < lignite FA < weathered coal FA.

FA density is approximately 1.4 g/cm³. It freely dissolves in water, acidic and alkaline solutions, plus selected organic solvents. Its molecular size ranges roughly 0.15–0.2 nm. Dilute FA solutions behave as true solutions, while concentrated solutions display colloidal characteristics.

As an organic polyelectrolyte, FA increases colloidal zeta potential, expands the electrical double layer and stabilizes colloidal suspensions. If metal ion concentration or acidity becomes excessively high in FA solutions, flocculation and precipitation occur. The threshold concentration for this reaction is defined as the coagulation limit (n). Higher n values mean stronger resistance to electrolyte-induced flocculation. Peat FA has the highest coagulation limit among all FA sources.

FA also acts as a surfactant, lowering water surface tension, reducing contact angle and improving foaming capacity. Surfactant activity follows this order: peat FA > lignite FA > weathered coal FA. For the same raw material, FA has a much higher coagulation limit than HA. Purified humate (sodium humate) solutions readily flocculate, making HA less suitable for liquid fertilizer formulation.

Chemical Properties

1. Weak acidity: The carboxylic and phenolic hydroxyl groups within FA create weak acidity; aqueous FA solutions register a pH of 3–5.
2. Cation exchange: Active hydrogen ions on hydroxyl and carboxyl groups readily exchange with monovalent cations (K⁺, Na⁺, NH₄⁺) and select divalent metal ions (Ca²⁺, Mg²⁺, Fe²⁺), forming humate salts such as potassium fulvate and sodium fulvate. FA can form complexes with clay minerals, phosphates, carbonates, agrochemicals and organic cations.
3. Chelation & complexation: Functional groups of FA act as electron donors that coordinate with multivalent metal ions and organic molecules to form stable chelate complexes. Examples include FA-Zn, FA-Fe, FA-Urea and FA-pesticide complexes. The chelating capacity of FA governs mobility, fixation and bioavailability of substances in nature, and forms the technical foundation for manufacturing high-efficiency liquid fertilizers and low-toxicity agrochemical formulations.
4. Redox activity: HA and FA share similar standard redox potential (~0.7 V), originating from reversible quinone-phenol interconversion. Carbonyl, hydroxyl, amino, nitro and aliphatic structures within FA all participate in redox reactions. FA regulates soil redox balance, stimulates microbial activity, modulates plant physiology, and influences the mobility and toxicity of heavy metals and organic pollutants. Sufficient FA stabilizes soil redox potential within the optimal 0.2–0.7 V range for crop growth.

Biological Activity

FA’s biological (physiological) activity arises from its small molecular size, which enables easy penetration into plant cells. Its key biological effects:

1. Participates in plant redox metabolism, promotes ATP synthesis and enhances respiration enzyme activity.
2. Activates synthetic enzymes inside plants, regulates enzyme balance and stabilizes endogenous growth regulators to strengthen stress resistance.
3. Improves cell membrane permeability and boosts nutrient uptake.
4. Enhances photosynthesis, accelerates carbohydrate accumulation, and stimulates synthesis of nucleic acids, chlorophyll, vitamins and defensive metabolites to improve crop health and quality.

All components and functional groups of FA contribute to biological effects. For peat FA, low-molecular aromatic carboxylic acids are the primary active ingredients. Co-existing soluble sugars, amino acids and organic acids provide supplementary benefits. Peat naturally contains over a dozen categories of bioactive compounds including terpenoids, vitamins and natural growth regulators, which coexist with extracted FA and further enhance performance.

Core Agricultural Functions of Fulvic Acid

International researchers summarize humic substances as “reservoirs sustaining life, guardians of the biosphere, and exceptional mediators transporting biological and toxic compounds”. As the most active fraction of humic substances, FA fulfills these roles exceptionally well. Its key agricultural functions are outlined below:

Enhances Plant Stress Tolerance

FA strengthens plant resistance against drought, cold, saline-alkaline conditions, pests, diseases and chemical pollution.

Field trials confirm FA delivers reliable cold resistance for wheat, oilseed rape, rice, legumes and sugarcane, and effectively mitigates low-temperature damage to rice seedlings. On saline-alkaline land, seed treatment or foliar spraying with FA improves yields of wheat and maize. FA suppresses numerous crop diseases including apple canker, cucumber downy mildew, sweet potato root rot and cotton fusarium wilt. It also alleviates toxicity from heavy metals, pesticides and polycyclic aromatic hydrocarbons in soil and irrigation water.

Foliar application of low-concentration FA consistently increases crop yields by 6–25%. Yield improvements are typically larger under harsh growing environments, where stress relief becomes the primary mechanism.

When plants face environmental stress, FA modulates internal proline concentrations and activates protective enzymes including SOD and catalase, directly reinforcing plant stress resistance.

Stimulates Plant Growth & Development

Extremely dilute FA solutions (0.0001% ~ 0.001%) clearly accelerate plant growth; excessive concentrations produce inhibitory effects, demonstrating typical natural biostimulant characteristics.

FA stimulates root and leaf respiration, accelerates root cell division, boosts enzyme synthesis, improves photosynthesis and delays senescence. Test data shows FA treatment doubles amylase activity and increases SOD and catalase activity significantly.

Growth stimulation strength follows this rule: FA > HA, low-molecular-weight FA > high-molecular-weight FA, oxidized FA > conventional FA. Oxidized peat FA delivers the strongest physiological stimulation.

Improves Nutrient Use Efficiency

Combined application of humic substances with fertilizers generally raises nutrient utilization by 10 percentage points or more.

Thanks to superior chelating power and anti-flocculation performance, peat FA is highly suitable for liquid fertilizer production. It increases plant uptake of nitrogen, phosphorus, potassium, calcium, magnesium, trace elements and rare earth elements to a greater extent than HA.

For example, soybeans absorb 23% more iron via foliar application of FA-Fe compared to equivalent iron sulfate. Field statistics show FA liquid fertilizers increase grain crop yields by approximately 10%, while fruits, vegetables and cash crops achieve 10–30% yield gains, largely driven by improved nutrient availability and absorption.

Improves Harvest Quality

FA improves agricultural produce quality in two key ways:

1. Elevates concentrations of beneficial nutrients: nucleic acids, amino acids, proteins, sugars, vitamins and essential mineral elements.
2. Reduces harmful substances: heavy metals, nitrates, nitrites and pesticide residues.

Aggregated trial data confirms humic substances increase fruit and crop sugar content by 3–40%, vitamin C by 20–49%, amino acids by 6–16%. Nitrate levels drop by 23–35%, while heavy metal, arsenic and pesticide residues decline by 10–90%.

Synergizes with Agrochemicals: Boost Efficacy & Reduce Toxicity

When combined with pesticides, FA creates slow-release formulations that enhance pesticide performance, lower phytotoxicity, improve stability and reduce required pesticide dosage.

Research shows FA additives raise pesticide efficiency while cutting application rates by 30–50%. Independent studies testing 11 pesticide combinations with oxidized FA reported positive synergistic effects for 10 formulations: pesticide activity increased substantially, and effective duration extended by over 10 days.

Most pesticides are weakly acidic, matching the pH of aqueous FA solutions. Excellent mutual compatibility enables ion exchange, chelation, hydrogen bonding and physical adsorption, forming new organic composite systems. This opens promising pathways for developing low-dose, high-efficiency sustainable agrochemical products.