An A Mull can be observed at sites with particularly high biological activity due to medium moisture conditions and on substrates with high base saturation and vegetation yielding very rapidly decomposable litter. Under these conditions, a litter layer is only temporarily present. The A horizon shows features of high bioturbation and is usually thicker than 10 cm. The distinct features of the A Mull are:
- Ol horizon is only temporarily present (autumn to early summer), and
- Olf horizon is absent or only present for a short time.