Sources of Humic Acid Raw Materials, and the Definition and Classification of Humic Acid

　　The raw materials for humic acids can be classified into two major categories: coal-derived and non‑coal-derived sources. The former primarily includes peat, lignite, and weathered coal; the latter encompasses soils, aquatic environments, fungi, and other non‑coal substances—such as those containing phenols, quinones, and sugars—which, through biological fermentation, oxidation, or synthesis, can yield humic‑acid compounds. Humic acids derived from coal‑based materials, as well as from soils, waters, and fungi, are considered natural humic acids, with coal‑derived materials serving as the principal feedstock for naturally occurring humic acids.

　　Under anaerobic conditions, dead plant residues accumulate without being fully decomposed, forming peat. Through coalification, peat transforms into lignite and bituminous coal. During the underground coalification process, the organic matter of coal‑forming plants undergoes reduction; subsequently, under the influence of surface atmospheric and aqueous environments, it undergoes slow oxidative hydrolysis, giving rise to weathered coal. Throughout this sequence—from peat to lignite to weathered coal—the content and structure of humic acids also evolve, reflecting both the original characteristics of the source vegetation or coal and the extent of reduction and oxidation.

　　As for the definition of humic acid, there is currently no universally accepted consensus. Different disciplines offer varying definitions: initially, soil scientists regarded humic acid as the fraction soluble in dilute alkaline solutions, while coal chemists included in it also the fraction soluble in dilute acids. Today, humic acid is generally understood to be a heterogeneous mixture of amorphous, acidic substances with aromatic structures and similar properties.

　　Humic acids are classified according to their mode of formation and source. Based on their formation process, they are divided into two major categories: natural humic acids and synthetic humic acids. The former includes soil humic acids, aquatic humic acids, and coal‑derived humic acids; the latter comprises biologically fermented humic acids, chemically synthesized humic acids, and oxidatively regenerated humic acids. According to their source, humic acids are further categorized into three types: primary humic acids, regenerated humic acids, and synthetic humic acids. Primary humic acids are those inherently present in the chemical composition of natural substances. Regenerated humic acids refer to humic acids produced through natural weathering or artificial oxidation processes applied to coals of various ranks. Synthetic humic acids generally denote compounds prepared by artificial means from non‑coal materials that closely resemble natural humic acids.