Spatial soil complexity and pedogenesis in Pasoh Forest Reserve in Peninsular Malaysia: two types of Inceptisols

Based on field surveys, the Pasoh Forest Reserve in Peninsular Malaysia has been reported to contain Oxisols and Ultisols, although not confirmed by analytical data. This study aimed to re-evaluate two types of neighboring soils in Pasoh through laboratory analyses to elucidate their pedogenesis and site-specific characteristics. In a 6-ha plot of Pasoh, orange soils on the upper slope (PS1) and pale-yellow soils on the lower slope (PS2) were identified in association with a heterogeneous rock layer. Laboratory analysis revealed a distinct difference in CEC per clay values between PS1 and PS2. However, neither soil met the criteria for an argillic horizon with clay illuviation (PS1) nor an oxic horizon with few weatherable minerals (PS2). Consequently, PS1 and PS2 were classified as Typic Dystrudepts and Oxic Dystrudepts, respectively. The heterogeneity of the rock layer and mica abundance in the fine silt fractions suggest a history of colluviation, resulting in Inceptisols distribution. A notable characteristic of the Pasoh soils was their high K content, likely derived from feldspar and micaceous minerals. These two types of Inceptisols were possibly formed by variations in the topographic slope positions and parent materials. Overall, despite the common perception of highly weathered tropical soils in this region, PS1 and PS2 were classified as Inceptisols, indicating limited weathering. These results underscore the significance of soil rejuvenation in Pasoh and its potential implications for site-specific K cycles.