Ferralitization

Abstract

We will use the term ‘ferralitization’ for the processes associated with strong weathering that lead to the formation of Oxisols (USDA) or Ferralsols (FAO. The oxic (USDA) or ferralic B (FAO) horizon has undergone extreme weathering of minerals in alil grainsize fractions. Practically all weatherable minerals have been removed from the sand and silt fractions. In the clay fraction, weathering has caused a dominance of kaolinite, gibbsite, and iron minerals, resulting in a low CEC (<16 cmol(+)/kg at pH7) and a low cation retention (<10 cmol(+)/kg at soil pH). Typical of ‘ferralitic’ weathering is the removal of silica (desilication) from primary silicates and even from quartz. Desilication leads to residual accumulation of (hydr)oxides of Fe, (Mn), and Al (ferralitization), Ferralitic weathering also involves a strong depletion of basic cations. As a result of strong weathering of primary minerals, silt contents (2–20 micrometers) are reliatvely low (silt/clay ratio in most oxisols <0.15). Desilication and ferralitization are slow processes; their effects are sufficiently pronounced to cause the presence of Oxisols (Ferralsols) only in old soils of tropical humid climates. Soils with oxic horizons are common also in ustic and dryer climates, but these were formed during a preceding wet climate and are, as such, paleosols.