How the unique properties of soil kaolin affect the fertility of tropical soils

Abstract The kaolin (s.l.) in many tropical soils is distinctly different from most specimen and industrial kaolins. Crystal structure is highly disordered and crystal sizes are much smaller than for specimen and industrial kaolins. The crystal habit of soil kaolins is diverse ranging through euhedral hexagonal and anhedral platy, spheroidal and tubular. The specific surface of soil kaolins is higher than for most industrial kaolins. Consequently, cation and anion retention capacities are higher although there is no compelling evidence that the high structural disorder of soil kaolin is associated with a greater specific reactivity of the kaolin surface. A small amount of ferric iron substitutes for octahedral Al but there is generally little or no substitution of other cations. Soil kaolin commonly contains minor amounts of potassium but this is present in mica layers within, attached or associated with kaolin crystals so that potassium is not within the kaolin structure. Due to its considerable chemical reactivity soil kaolin helps provide an effective substrate to support agriculture and other land uses in the tropics.