[Humic Acid Special] An Introduction to Humic Acid

I. What is humic acid?

Humic acid is a mixture of natural organic macromolecular compounds. It is widely distributed in nature, with the highest abundance found in soils. Soil humic acid is a physicochemically heterogeneous and complex mixture whose molecular weights are broadly distributed. This mixture consists of naturally occurring, high‑molecular‑weight, yellow to black, amorphous, colloidal organic polyelectrolytes that exhibit both aliphatic and aromatic characteristics, and it cannot be represented by a single chemical structural formula.

II. Where does humic acid come from?

1. Soil humic acid is naturally occurring, primarily a mixture of specialized macromolecular organic compounds formed through the action of microorganisms on plant material.

2. Coal humic acid is a mixture of macromolecular organic compounds that arises from the decomposition and transformation of plant matter by microorganisms, followed by prolonged geochemical processes.

III. Types and Characteristics of Commonly Used Humic Acids

Currently, the humic acids commonly used as fertilizers are classified into Lignite humic acid 、 Weathered coal humic acid 、 Peat (Caotan humic acid).

1. Lignite humic acid It is a product of the second stage of coal formation—diagenesis—and by the bituminous coal stage, it no longer contains humic acid. The humic acid content in lignite typically ranges from 1% to 85%. Lignite generally appears brown, with some specimens exhibiting a black hue; based on its degree of darkness, it can be classified into…

Soil-like lignite: It exhibits a relatively shallow degree of coalification, with low carbon content and a high humic acid content, typically exceeding 40%;

Bright brown coal: exhibits a relatively high degree of coalification, with a higher carbon content and a lower humic acid content, typically ranging from 1% to 10%.

Dense lignite: Intermediate between earthy lignite and bright lignite, with a typical humic acid content of around 30%.

2. Weathered coal humic acid This refers to exposed coal, commonly known as “coal weathering.” It typically consists of lignite, bituminous coal, and anthracite that lie close to or are exposed at the Earth’s surface. Over time, these coals undergo weathering under the influence of air, sunlight, rain, snow, windblown sand, and freezing conditions, resulting in the formation of weathered coal. The humic acids produced through this process are called weathered‑coal humic acids, also referred to as naturally regenerated humic acids. The humic acid content in weathered coal can vary considerably, ranging from 5% to 80%.

3. Peat (including turf, muck, etc.) humic acid

Peat is the initial stage of coal formation, also known as muck. In China, peat typically contains 20–40% humic acid. The organic matter in peat consists of undecomposed plant residues and humic acids; in Chinese peat, the humic acid content ranges from 20 to 40%.