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Fulvic Acid

Fulvic acid is the low-molecular-weight, fully water-soluble fraction of humic substances extracted from leonardite or similar oxidised brown coal. It is structurally and functionally similar to Humic Acid but, because of its smaller molecular size and higher oxygen-to-carbon ratio, it remains soluble at all soil pH values, including the acidic conditions where humic acid precipitates.

Composition

  • Origin: alkaline extraction of leonardite (humified brown coal) followed by acidification to separate the humic and fulvic fractions
  • Chemistry: complex mixture of low-MW polyphenolic and carboxylic compounds, typically 1,000-5,000 Da, with high carboxyl (-COOH) and hydroxyl (-OH) functionality
  • Solubility: fully water soluble across the entire pH range
  • Yellow-brown to dark-brown colour in solution

Commercial soil conditioners commonly combine fulvic with humic acid, with the declared humic-plus-fulvic content specified under the FCO biostimulant rules.

Mode of action

Fulvic acid acts primarily as a chelator: its high density of acidic functional groups binds cations such as Fe2+, Zn2+, Cu2+ and Mn2+, keeping them in solution and available to plants in alkaline or calcareous soils. The small molecular size allows fulvic-cation complexes to be absorbed through plant cuticles and translocated systemically, making fulvic acid a strong carrier for foliar and drip-applied micronutrients. It also stimulates root activity and improves cell-membrane permeability.

Target use and dose

Most commonly applied as a foliar spray at 0.5-1 g per litre or through drip irrigation at 250-500 g per acre, both in standalone formulations and combined with micronutrients (Edta Chelated Zinc, Boron Fertilizer). It can be tank-mixed with most water-soluble fertilisers but reacts with strong oxidisers and should be kept separate from chlorine.

Safety and regulatory status

Fulvic acid blends are regulated under the FCO plant biostimulant order; registered products carry a CIB&RC registration number. Non-toxic and non-corrosive at field rates.

See also: Humic Acid, Humic Acid Paddy, Humic Fulvic Acid Input, Leonardite Soil Conditioner, Seaweed Extract.

References

  1. Humic Acids and Fulvic Acids Reference. International Humic Substances Society resource.
  2. Humic Substance. Wikipedia (IHSS reference).