Humuskerrokseen

Humuskerrokseen is a term used in soil science to describe the humus layer of soil, the upper, organic-rich part that forms from the accumulation and decomposition of plant and animal matter. In Finnish usage, the word highlights the horizon that lies above mineral soil and often corresponds to the O-horizon in soil classification. The layer is typically dark and loose, with high concentrations of organic carbon and nutrients, and it varies in thickness from a few to several tens of centimeters depending on vegetation, climate, and land use.

Formation and composition occur as litter such as leaves, needles, moss, and dead organisms accumulate and are gradually processed by bacteria, fungi, and soil faunal activity. The humus layer is particularly prominent in forests with abundant litter input and limited mineral soil mixing. Its properties—dark color, high water-holding capacity, and rich organic matter—change with climate and disturbance, being thinner in dry or degraded sites and thicker where decomposition is slower or input is high.

Function and importance include its role in soil structure and stability, carbon storage, nutrient buffering and slow release, and support for a diverse soil biota. It enhances water infiltration and retention, influences pH buffering, and supports plant growth by acting as a nutrient reservoir.

Disturbances and management are crucial considerations. Logging, drainage, soil compaction, fire, and climate change can reduce humus layer thickness and alter its chemistry, affecting fertility and carbon stocks. Sustainable forest and land management seek to protect and, where possible, restore the humus layer to maintain ecosystem services.

See also: humus, soil horizon, O-horizon, soil organic layer.