A leaching horizon lies under an organic humus layer (L and O horizons) or under a mineral, humus topsoil (Ah horizon); Characteristic for the Ae horizon is a depletion of organic matter as well as iron and aluminum compounds. This process is known as podsolization and characterizes the podsol soil type .
An Ae horizon is a lightened, often gray-bleached zone under the humus topsoil. It is caused by strong soil acidification and the associated relocation of complex iron-humus compounds. Underneath is a leaching zone, an illuvial horizon , which is enriched with the leached substances of the Ae horizon. Depending on the prevailing leaching of humus or iron compounds, a distinction is made between the Bh and Bs horizon.
- Lessivation , washing out of clay colloids