Clay mineralogy of the a horizons of seven ando soils, central Kyushu

Abstract

Abstract Humus content was determined on the quartz andesitic and andesitic Ando soil samples collected in the Aso and Kuju districts, central Kyushu. The mineralogy of the clay fractions was also determined by a combination of successive and selective dissolution—difference infrared spectroscopy, chemical and X-ray diffraction analyses. The amounts of crystalline layer silicates were much higher in soils derived from quartz andesitic ashes than in those from andesitic volcanic ashes. The composition of amorphous clay constituents which are important to the accumulation of humus in Ando soils was similar and no much difference was observed in the carbon content between the two group soils. Allophane and imogolite were absent or nearly absent in all the present-day surface horizons. Hydrous alumina-rich gel-like materials and opallne silica were the major weathering products in them. On the contrary, all other buried surface horizons contained allophane. allophanelike constituents and imogolite. A contrasting difference in the amorphous clay mineralogy between the present-day surface and buried surface horizons was interpreted as indicating that a balance between the release of aluminum from volcanic ashes by weathering and the supply of organic matter control the formation and transformation of amorphous clay constituents in the Al horizons of Ando soils.