Ultisols of lowland Dipterocarp forest in East Kalimantan, Indonesia

Abstract

Thirty five Typic Paleudults and Typic Hapludults under a Lowland Dipterocarp Forest were characterized for clay minerals, free oxides, cation exchange capacity, and exchangeable bases. Kaolinite was predominant in the soil clays followed by quartz, vermiculite and gibbsite and an accessory amount of illite, partially-interlayered-vermiculite (PIV) and goethite in most soils. Goethite formation appeared to take place concomitantly with the weathering of illite into PIV. Lepidocrocite was found only in the soils of the middle to lower slopes. The magnitude of lepidocrocite formation under hydromorphic conditions was associated with the exchangeable Al level. Cristobalite and hematite were found only in the “red” soils, which may have been subjected to some thermal influences associated with coal combustion. The levels of Fed, Ald, and Alo of the soils were, as a whole, positively correlated with the clay content, respectively. Goethite was considered to be a possible source of crystalline Fe oxide (Fed-Feo). The ECEC was correlated with the clay content, and its relation with the CEC at pH 7 indicated the presence of a higher variable charge in the surface soils than in the subsoils. Storage of Mg and K in soil unlike that of Ca paralleled the amount of clay. The exchangeable bases were more largely distributed in the subsoils than so far considered, suggesting their relatively high mobility in the sola. Exchangeable Al content was closely associated with the clay content, unlike the Al saturation percentage, which mostly exceeded 60% and increased with depth.

Key Words:

• clay minerals
• Dipterocarp forest
• exchangeable cations
• free oxides
• Ultisols