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 15structure is highly disordered and crystal sizes are much smaller than for specimen and industrial kaolins. The16crystal habit of soil kaolis isdiverse ranging through euhedra lhexagonal and anhedralplaty,spheroidalandtu17bular.The specific surface of soil kaolins is higher than for most industrial kaolins. Consequently, cationandanion 18retentioncapacitiesarehigheralthoughthereisnocompellingevidencethatthehighstructuraldisorderofsoil 19kaolinisassociatedwithagreaterspeci ficreactivityofthekaolinsurface.Asmallamountofferricironsubstitutes 20for octahedral Al but there is generally little or no substitution of other cations. Soil kaolin commonly contains 21minoramountsofpotassiumbutthisispresentinmicalayerswithin,attachedorassociatedwithkaolincrystals 22sothatpotassiumisnotwithin the kaolin structure. Due to its considerable chemical reactivity soil kaolin helps 23provide an effective substrate to support agriculture and other land uses in the tropics.