Ferrallitisation is the process in which
rock is changed into a
soil consisting of
clay (
kaolinite) and
sesquioxides, in the form of hydrated oxides of iron and
aluminium. In humid
tropical areas, with consistently high temperatures and rainfall for all or most of the year,
chemical weathering rapidly breaks down the rock. This at first produces clays which later also break down to form
silica. The silica is removed by
leaching and the sesquioxides of iron and aluminium remain, giving the characteristic red colour of many
tropical soils. Ferrallitisation is the reverse of
podsolisation, where silica remains and the iron and aluminum are removed. In tropical
rain forests with rain throughout the year, ferrallitic soils develop. In
savanna areas, with altering dry and wet climates,
ferruginous soils occur.