Soil slaking sensitivity as influenced by soil properties in alluvial and residual humid tropical soils

Purpose: In the humid Caribbean region characterized by high intensity tropical rainfall, soil aggregate breakdown and pore blocking due to slaking pressures are major land degradation mechanisms. In this research, we investigated the susceptibility of soils to slaking pressures under rapid wetting as influenced by soil properties and the depositional origin from which the soil is formed using water stable aggregates (WSAr) and percolation stability (PSc) as indices of the strength of aggregate inter-particle cohesion. Materials and methods: Wet sieving and percolation stability analyses were employed to investigate WSAr and pore blocking, respectively. The combined effect of soil properties of clay, organic matter (OM), cation exchange capacity (CEC) and exchangeable sodium percentage (ESP) was used to determine the slaking sensitivity score (SSc) of fourteen physiogeographically important soils in Trinidad, comprising of nine alluvial and five residual soils. Results and discussion: Results showed that irrespective of alluvial or residual depositional nature of the parent material, samples had high SSc with an average WSAr of 37.8% and PSc of 6.0mm/10 minutes. The linear relationships between SSc with WSAr (r2 = -0.12) and SSc with PSc (r2 = -0.012) of all the 14 soils although negative were weak. Clay content accounted for 94.0% of the variation in CEC in alluvial soils and had a strong negative relationships with WSAr (r2 = -0.74) and PSc (r2 = -0.79) in residual soils. Additionally, OM with WSAr (r2 = 0.52) and PSc (r2 = 0.24), and CEC with WSAr (r2 = 0.46) and PSc (r2 = 0.39) showed significant positive linear relationships in residual soil. Conclusions: The predominantly micaceous and kaolinitic clay mineralogy of these soils coupled with the low OM contents, increase the proneness of the soils to slaking. This suggests that clay mineralogy is responsible for the high slaking sensitivity rather than clay content or just the depositional origin of the soils. As CEC increases, an accompanying increase in OM is required to increase inter-particle cohesion and to impart partial hydrophobicity, which in turn decreases mineralogically induced susceptibility of individual aggregates to slaking