1. Sources and Classification of Humic Acid (HA)

Humic acid is a complex mixture of hydroxybenzene carboxylic acids with varying molecular weights. Lignite, peat, and leonardite are primary sources of HA extraction, with leonardite offering the highest extraction rate, followed by lignite and peat.

In traditional soil science, humic acid is classified into three main types: humic acid (HA), fulvic acid, and humin. Further categorized by solubility:

• Black HA: Insoluble in alkaline solutions, soluble in acidic solutions.

• Brown HA: Soluble only in alkaline solutions.

• Fulvic Acid: Soluble in both alkaline and acidic solutions.

2. Characteristics of Humic Acid

Humic acid is a high-molecular-weight organic acid composed of aromatic rings and functional groups, exhibiting key properties:

• Promotes soil aggregate formation, improving permeability and water-retention.

• Stimulates plant growth and enhances resistance to drought, cold, and pests.

3. Classification of Humic Acid Fertilizers

Classified by production techniques:

• Humates (potassium, sodium, calcium, magnesium, ammonium, boron, etc.)

• HA derivatives (nitro HA, nitro humate, HA complexes)

Classified by application methods:

• Seed dipping/root dipping fertilizers (sodium humate, potassium humate)

• Foliar/root flushing fertilizers (fulvic acid liquid fertilizers)