Humusdannelse

Humusdannelse, or humus formation, is the process by which organic matter derived from plants and animals is transformed and stabilized in soil, resulting in humus, the dark, stable component of soil organic matter. It develops as plant litter, root exudates, and microbial remains are broken down by bacteria, fungi, and soil fauna, and progressively reassembled into more complex, resistant compounds. The chemical products of humification include humic substances—humic acids, fulvic acids, and humin—that differ in solubility and molecular size and contribute to the long-term persistence of organic matter in soil.

Humus improves soil structure by promoting aggregate formation, enhances water retention, and increases cation exchange capacity and pH buffering. It also serves as a reservoir of nutrients for plants and a habitat for soil microorganisms.

The rate and characteristics of humusdannelse vary with climate, substrate, and management. Warmer, wetter conditions generally accelerate decomposition and humification, while cool or dry conditions slow the process. Soils with well-aerated, well-mixed organic matter, such as mull humus forms common under temperate forests, tend to have more rapid turnover and greater microbial activity than more acidic, less decomposed mor or moder forms. Vegetation type, litter quality, soil pH, and land use practices—tillage, residue retention, drainage—also influence the amount and stability of humus. In the long term, humusdannelse contributes to soil fertility and acts as a significant component of terrestrial carbon sequestration.