The humus form type F Mull occurs on soils with moderate-to-high base saturation. The litter is predominantly moderately-to-readily decomposable. An F Mull may also develop at sites with poorly decomposable litter such as European beech (Fagus sylvatica) if the soils are characterised by a high base saturation. Mineralisation of litter takes usually more than two years. An Of horizon is present throughout the Ol and A horizons. The Of horizon can often be subdivided into two horizons; the upper horizon is poor in fine humus (Olf horizon), and the lower horizon is rich in fine humus (Ohf horizon). In general, the Of material is loose to stacked and with a thickness of the horizon of 1–3 cm. It can be considerably thicker in areas with increased litter input. The litter of the Of horizon, which is partly interspersed with roots and fungus mycelium, may be connected to itself in layers or glued together, forming packages. In the case of high bioturbation, organo-mineral aggregates are often intermixed in the Of horizon. In general, the mineral A horizon is characterised by signs of high biological activity and by stable aggregates with a subangular blocky or angular blocky structure and less occurrence of granular structure. The boundary between the Ah horizon and the underlying mineral soil horizon is sharp (transition zone < 2 cm) ), particularly in silty and loamy substrates. The distinct features of the F Mull are:
- Ol horizon is present;
- Of horizon is continuous and present all year round; and
- Ax, Au, or Ah horizon is present.
The F Mull can be further differentiated into 5 subtypes. In contrast to the L Mull, the F-Mull occurs at sites with high biological activity but moderately-to-readily decomposable litter.
Subtypes F Mull:
MFT Typical F Mull
MFR Rhizo F Mull
MFF Moist F Mull