Relationship between plasticity index and soil erodibility: a case study of central Malaysian tropical soils

Nations located in tropical rainforest regions, such as Malaysia, face a heightened risk of soil erosion due to persistently warm and humid climates. Soil erodibility (K), a critical parameter in erosion prediction models, is influenced by intrinsic soil properties. Among these, the role of the plasticity index (PI) as a predictor of K remains underexplored in tropical environments, where high-intensity rainfall accelerates erosion processes. This study aims to quantify the relationship between K and PI for tropical soils in Malaysia. A total of 37 soil samples were collected from two locations, Melaka Tengah and the Sungai Langat Basin, in the central region of Malaysia. Laboratory tests were conducted to determine particle size distribution, Atterberg limits, organic matter content, and soil erodibility factors. Linear regression analysis revealed a significant positive relationship between the natural logarithm of the plasticity index (Ln(PI)) and the erodibility factor (K), with the resulting equation K = 0.0236 Ln(PI) - 0.0365. The model showed a strong fit, with a coefficient of determination (R²) of 0.673 and a p-value less than 0.001, indicating high statistical significance. The study reveals that highly plastic soils may become more erosion-prone under tropical conditions, as the PI-K relationship reflects the dual role of clay content in both enhancing aggregate stability and intensifying erosion beyond certain thresholds. The developed regression model provides a potential predictive tool for estimating erodibility based on plasticity. The study advances understanding by highlighting the distinctive erosional behaviour of central Malaysian tropical soils and presenting a novel, cost-effective predictive tool that integrates plasticity into erosion assessment.