humin

Humin is a fraction of humic substances found in soils, sediments, and natural organic matter. It is defined as the portion that remains after exhaustive extraction of soluble humic substances with alkaline solutions and the removal of fulvic and humic acids. Humin is typically insoluble in water and resistant to standard extraction methods, often forming tight associations with mineral surfaces.

In chemical terms, humin has a high molecular weight and a high degree of structural condensation, with condensed aromatic structures that contribute to its recalcitrance. It is less amenable to microbial degradation than humic and fulvic acids and tends to be less mobile in the environment. Because of its strong binding to minerals such as clays and iron/aluminum oxides, humin can persist for long periods in soils.

Formation and associations:

Humin arises through ongoing humification processes that polymerize and condense degraded plant and microbial residues. Its persistence is enhanced by adsorption onto mineral surfaces, which protects it from enzymatic breakdown and helps immobilize organic carbon within the soil matrix.

Functions and significance:

Humin contributes to soil structure and strength by helping to stabilize aggregates. It acts as a long-term carbon reservoir, influencing the soil’s capacity to store carbon. Through mineral associations, humin can affect nutrient retention and the binding of various contaminants, though its low solubility limits its transport.

Analytical context:

Practically, humin is obtained as the residue remaining after separating fulvic and humic acids in conventional HUM fractionation. Definitions of the fraction can vary with methods and instrumentation, reflecting ongoing debates about humin’s precise boundaries and character.