The development and nature of soils on Rakata

The properties and development of soils derived from pumiceous tuff of the 1883 Krakatau eruption were studied on Rakata island during 1983 and 1989. Field observations on soil profiles of different altitude and topography, and laboratory data, revealed that in a period of 100 years soil development has resulted in the formation of a pronounced A-horizon of 5–15 cm thickness underlaying an AC or weak B horizon. Solum development commonly reaches to a depth of approximately 25 cm. The dominant process of soil formation was decomposition of organic matter and aggregation of soil particles in the surface horizon. There was also a slight indication of iron and fine clay translocation but base cations were accumulating in the surface horizon. Soils at 600 m altitude and higher commonly contained lower base saturation, pH, and weatherable minerals, and higher exchangeable A1 and H than those at lower altitudes. The dominant clay minerals were montmorillonite, kaolinite and other layer silicate clays. It is believed that the clays were not formed pedogenically. Some pedons at higher altitudes were dominated by amorphous material in the clay fraction.Presented at the XVII Pacific Science Congress, May 27–June 2, 1991. Honolulu, Hawaii, USA