Influence of Volcanic Lithology on Soil Physical Properties in the Watugede Sub-watershed

Lithology is one of the primary geological factors that control landform dynamics and soil development, particularly in tropical volcanic regions where weathering is intense. While previous research has emphasized the role of climate, slope, and land cover on soil properties and land degradation, studies that directly address the influence of lithological variation on soil physical properties through geomorphology at the sub-watershed scale remain limited. This study focuses on the Watugede Sub-watershed in Yogyakarta, Indonesia, which comprises three distinct volcanic formations: Kebobutak, Semilir, and Nglanggeran. These formations represent diverse lithological and geomorphological conditions. The objectives of this study are to (1) identify the lithological and geomorphological characteristics of the Watugede Sub-watershed, and (2) analyze the physical properties of soils that develop above each formation. Lithological data were obtained from geological maps and field surveys, while soil data were collected using purposive sampling at two depths (0–30 cm and 30–60 cm) with four replications. Laboratory analyses included bulk density, specific gravity, texture, permeability, maximum water content, and soil cohesion. The results show that lithological differences strongly influence soil physical characteristics and slope stability. Soils on the Kebobutak Formation have sandy to sandy loam textures with moderate cohesion and permeability, indicating relatively stable conditions but prone to local water saturation in fine tuff layers. The Semilir Formation produces heterogeneous soils with very fast permeability and low cohesion, reflecting susceptibility to erosion and slope deformation. The Nglanggeran Formation yields sandy loam soils with fast to moderate permeability and higher cohesion than those of Semilir, but instability persists on steep slopes due to intense fracturing and weathering. This study contributes to a deeper understanding of lithology–geomorphology–soil interactions in tropical volcanic watersheds, providing a scientific basis for watershed management and slope stability assessment.