Mullakiht

Mullakiht is a term used in soil science, particularly in Finnish pedology, to denote a specific soil horizon often described as a mull layer. The word combines mulla ("soil") and kiht ("layer"). In this usage, mullakiht refers to a transitional organic–mineral horizon beneath the surface litter in boreal woodland soils and peatlands, created by bioturbation and mixing of decomposed plant material into the mineral soil.

The mullakiht is typically characterized by a darker color and increased humus content compared with underlying mineral horizons, while being more decomposed than the overlying organic layer. It forms where soil fauna, especially earthworms, actively mix finer organic matter with mineral soil, producing a homogeneous, humus-enriched zone. Its thickness can vary and may reflect the degree of biological activity and climate.

Geographically, the term is used mainly in Finland and in some Nordic soil literature. It is not uniformly standardized across international soil taxonomies, and its definition can differ by region or classification system. In English-language pedology, the concept is sometimes described as a mull horizon or a mull humus layer, but terminology varies.

In practice, the presence of a mullakiht is taken as an indicator of biotic soil mixing and a relatively dynamic humus–mineral interface, which can influence nutrient cycling and soil fertility in forested and peatland soils.