humification

Humification is the process by which organic matter derived from plants, animals, and microbes is transformed into humus, a stable and nutritionally valuable fraction of soil organic matter. It results from a combination of biological activity and chemical reactions that convert readily decomposable compounds into more complex, resistant macromolecules through processes such as polymerization, condensation, and oxidation.

The main products of humification are humic substances, typically categorized as humic acids, fulvic acids, and humin. These substances differ in chemical composition and solubility: humic acids and fulvic acids are soluble in alkaline conditions, with fulvic acids remaining soluble across pH ranges, while humin is insoluble in most conditions. Collectively, humic substances contribute to the dark color of humus and to the structural stability of soil organic matter.

Humification is distinct from mineralization, the latter being the microbial breakdown of organic matter to carbon dioxide, water, and inorganic nutrients. Through humification, a portion of organic matter is stabilized, increasing soil carbon storage and enhancing soil physical and chemical properties, such as soil structure, water retention, and cation exchange capacity, which in turn influence nutrient availability.

Factors influencing humification include climate (temperature and moisture), substrate quality (especially lignin content and C:N ratio), soil pH and aeration, microbial community composition, and residence time. Management practices that introduce or conserve organic matter, such as adding compost or manure, can promote humification and long-term soil health.

Humification plays a central role in the soil organic matter cycle and the global carbon cycle, contributing to nutrient cycling, soil fertility, and carbon sequestration in terrestrial ecosystems.