Pedologist

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In Southeast Asia, Ultisols are commonly distributed on sedimentary rocks and Oxisols on ultramafic rocks. To investigate the importance of parent material on the formation and distribution of Ultisols and Oxisols, the effects of parent material on soil physico-chemical and mineralogical properties were analyzed using soils derived from serpentine and sedimentary rocks in East Kalimantan. The pH and concentrations of bases and Fe oxides in soils decreased in the order serpentine>mudstone>sandstone. Oxisols are rapidly formed through loss of Si and relative accumulation of Fe oxides on the easily-weatherable serpentine and the richness of bases and Fe limits lessivage and maintains a high base saturation. On Fe-rich mudstone, Ultisols are formed through lessivage and acidification. On base and Fe-poor sandstone, clay destruction by acidification and lessivage is responsible for clay enrichment of the subsoil. In East Kalimantan, the major pedogenetic processes are lessivage and acidification. Ferralitization is not a common process. The leaching condition and the lower concentrations of bases and Fe in sedimentary rocks than in serpentine could account for the dominance of Ultisols in East Kalimantan. Because serpentine is easily weathered to oxic materials, parent material is also a primary factor responsible for the distribution of Oxisols in this area.

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The objectives of this study were to compare soil chemical properties of gravel fractions (>2 mm) and fine-earth fractions (≦ 2 mm) in a weakly-weathered Regosol derived from basaltic scoria, and to evaluate the relative contributions of these fractions to the chemistry of the whole soil. The gravel fraction was found to contribute to chemical properties of the whole soil as follows: contributions to T-C and Al_0 were 21.3% and 21.4%, respectively; and the contributions to cation exchange capacity (CEC) and each exchangeable cation were 17.4% and 16.7-21.5%, respectively. In the initial stages of formation of Regosols at least over 125 years, it is evident that most soil chemical properties are associated with the fineearth fraction. However, some organic carbon and available nutrients also accumulated concomitantly in gravel fractions. In order to evaluate the chemical properties of Regosols derived from basaltic scoria, which is a porous parent material, it is necessary to recognize that the gravel fraction as well as the fine-earth fraction contributes to the whole-soil properties.

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