Pedological and geochemical characteristics of highly weathered soils derived from ophiolitic rocks in Samar, Philippines

Limited information is available on the pedology of highly weathered soils derived from ophiolitic rocks in the Philippines. This study evaluated red, highly weathered soils across the ophiolitic landscape of Eastern Samar. Five soil profiles were described and sampled for laboratory analysis. The geochemical composition of the parent materials ranged from mafic to ultramafic. The soils were acidic [pH(H₂O) = 4.97–6.45; pH(CaCl₂) = 3.95–5.96] and contained low concentrations of available P (trace–1.14 mg kg⁻¹), exchangeable K (trace–0.23 cmolc kg⁻¹), Ca (trace–12.66 cmolc kg⁻¹), and Na (trace), together with low Ca/Mg ratios (0.1–41.7). Profile 2 had comparatively high exchangeable Ca concentrations, particularly in deeper horizons. Ca/Mg ratios increased with depth in Profiles 2 and 3 but decreased in Profile 5. In Profiles 1 and 4, Ca/Mg ratios were very low and could not be calculated in several subsurface horizons because exchangeable Ca and/or Mg occurred at trace levels. Extractable Ni concentrations ranged from 0.16 to 22.40 mg kg⁻¹, with the highest concentration occurring in the Bo2 horizon of Profile 4, indicating contributions from ultramafic materials, Ni accumulation, and adsorption onto Fe (oxyhydr)oxides during weathering. The warm, humid climate, highly weatherable ophiolitic parent material, stable, well-drained topography, and original rainforest vegetation promoted the formation of highly weathered Ultisols and Oxisols. Low OM (0.36%–3.60%) and total N (0.04%–0.25%) contents may reflect anthropogenic effects, particularly periodic burning associated with shifting cultivation. The CIA (55.56–99.31), CIW (55.63–99.58), MWPI (0.14–7.44), and Vogt ratio (1.08–63.95) indicated advanced weathering and substantial losses of major cations. These findings demonstrate that ophiolitic soils under humid tropical conditions undergo intense weathering and leaching, resulting in marked profile-scale variation during pedogenic development.