International Journal of Plant & Soil Science

Volcanic soils in humid tropical environments undergo intensive weathering and pedogenic differentiation, producing marked variation in clay activity and soil organic carbon across landforms. This study evaluated pedogenic differentiation across volcanic landforms in Barong Tongkok, East Kalimantan, Indonesia, using clay activity and carbon dynamics as indicators of soil development. Four representative soil profiles from the extinct volcano, volcanic shield, and lava field geomorphic units were selected for pedogenic evaluation. Morphological and physicochemical properties across genetic horizons were evaluated to assess soil development. Physicochemical data from each genetic horizon were evaluated for particle-size distribution, organic carbon, cation exchange capacity of clay, base saturation, and bulk density. Clay content generally increased with depth and was greater in the more stable landforms, exceeding 80% in several subsurface horizons of the volcanic shield and lava field profiles. Soil organic carbon was concentrated in surface horizons and declined markedly with depth, with surface values ranging from 5.07% to 10.38% across the four profiles. Subsurface clay activity was generally low to moderate, while base saturation showed substantial depletion in the more strongly weathered profiles. These vertical and spatial patterns indicate progressive mineral weathering, clay accumulation, and carbon differentiation across the volcanic toposequence. Overall, the results support the use of clay activity, soil organic carbon distribution, and base saturation as complementary indicators of pedogenic differentiation in humid tropical volcanic landscapes.