humusfractie

Humusfractie is a term used in soil science to describe the portion of soil organic matter that constitutes humus, the relatively stable, dark and chemically complex residue formed by long‑term microbial transformation of plant and animal material. This fraction includes humic substances—humic acids and fulvic acids—as well as the insoluble part known as humin. Together, they represent the more resistant component of organic matter, in contrast to more labile fractions such as fresh residue and microbial biomass.

Humus develops through humification, a process driven by soil organisms, enzymes and environmental conditions such as temperature, moisture, pH and vegetation. The resulting humus fraction improves soil structure by promoting aggregate formation, enhances water retention and contributes to the soil’s cation exchange capacity and buffering capacity. It also serves as a habitat and energy source for soil life and plays a key role in long‑term carbon storage.

In practice, the humusfractie is often determined by chemical fractionation. For example, alkaline extraction isolates humic substances (humic acids and fulvic acids) from soil; upon acidification, humic acids precipitate while fulvic acids remain in solution. The remaining insoluble material is humin. The sum of humin, humic acids and fulvic acids represents the humus fraction of the soil. The size of this fraction influences fertility, resilience to erosion and carbon sequestration potential.

Variability in the humusfractie reflects soil type, management, climate and depth, with higher proportions generally found in darker, better‑structured topsoils enriched by organic inputs.