Soil Crusting Under Intense Rainfall As Influenced By Cohesive and Wetting Factors.

Deleterious land practices such as deforestation, slash and burn agriculture, and seasonal fires in the Caribbean expose the soil surface to the direct impact of intense tropical rainfall. The susceptibility of the exposed soil to aggregate disintegration and the associated crusting depends on the cohesive strength of soil binding factors and the magnitude of disruptive forces of rapid wetting and raindrop impact produced by the intense rainfall. We hypothesize that clay and organic matter concentrations in soils subjected to varying pre-wetting conditions influence aggregate cohesion and crust formation. We evaluated under intense simulated rainfall (120 mm h^-1) the shear strength, crust strength and erodibility of six Trinidad soils chosen based on three levels of clay and two levels of organic matter. Samples were either left dry or pre-wetted with mist at slow (7.5 mm h^-1) and fast (75 mm h^-1) wetting rates and equilibrated to matric suction of -0.033 MPa and -0.066 MPa, prior to exposure to the intense simulated rainfall. Our results show that cohesive strength did not necessarily increase with increase in clay concentration in low organic matter soils (<30 g kg^-1), since high clay soils (>450 g kg^-1) shear and crust as much as low clay soils (<200 g kg^-1) under the disruptive forces of intense rainfall and fast wetting. At the high organic matter concentration (>30 g kg^-1), however, cohesive strength increased at medium (200-450 g kg^-1) and high clay concentrations, suggesting that high organic matter is required at these clay contents to reduce the surface tension of water entering the clay-organic matter matrix and thus increasing the cohesive bonding of the clay particles. For medium to heavy textured soils of the humid tropics subjected to disruptive forces of intense rainfall, interaction rather than the individual effects of clay and organic matter control their crustability.