Chemical and mineralogical properties of volcanic ash soils from Java

Abstract

Six profiles of soils of volcanic ash origin from Java have been examined for their chemical and mineralogical characteristics. The soils consisted of 4 Andosols and 2 Latosols (Indonesian classification). Soil reaction was weakly to moderately acid, except for one, the Sokarini soil, in which the reaction was strongly acid due to the smectitic clay mineralogy and influence of acid gases emitted from a nearby fumarole field. The content of exchangeable cations and percentage of base saturation were high and low in accordance with the soil reaction, respectively. Organic carbon content was relatively high in the Andosols and low in the Latosols. The silt and sand fractions were dominated as a whole by quartz, cristobalite, feldspars, magnetite, and volcanic glass, with appreciable amounts of pyroxenes, amphiboles, hematite, tridymite in some soils. Among the 4 Andosols, two (Tlahap and Karang Kulon) were allophanic with imogolite and the other two were either smectitic (Sokarini) or halloysitic (Sebaluh) in clay mineral composition. The origin of smectite in the Sokarini soil was considered to be detrital due to the contamination of parent ash with accidental or accessory pyroclastics and smectite was also considered to be a transformation product from mafic minerals such as pyroxenes and amphiboles. On the other hand, the formation of large amount of 10-Å halloysite in the Sebaluh soil was ascribed to the Si-rich environment which had been induced during the long period of dry months occurring every year. In contrast, the clays from the Latosols consisted principally of kaolinite and 7-Å halloysite with or without a moderate amount of gibbsite, as a result of “lateritic” weathering. Based on the examination of relevant data together with field observations the 4 soils of the Andosol group were tentatively assigned to Hapludands whereas the 2 soils of the Latosol group to Eutropepts. © Taylor & Francis Group, LLC.