Changes in physical properties of young and old volcanic surface soils in Costa Rica after clearing of tropical rain forest

Large areas of tropical rain forests are being cleared and used for agricultural purposes. As a result, fragile soil structure degrades which alters soil hydraulic properties. Soil physical methods and micromorphometrical analysis were used to analyse effects of changes in vegetation on soils of different ages. Measurements were made in topsoils of a relatively old Humoxic Tropohumult and of a relatively young Oxic Humitropept, both under forest and under pasture. The crust test was used to measure hydraulic conductivities (K) in relation to pressure head (h). Moisture retention data were obtained by measuring outflow from undisturbed core samples placed in porous cups at certain negative pressure heads. Micromorphometric analysis of thin sections supplied quantitative descriptions of macroporosity patterns and pore types. K_sal of the Humult decreased from 1000 cm day⁻¹ under forest to 50 cm day⁻¹ under pasture. Macroporosity of the forest topsoil consisted mainly of compound packing voids. In the pasture topsoil predominantly vughs and more planar voids were present and macroporosity was significantly lower. K_sat was equal at 70 cm day⁻¹ under both types of vegetation of the Tropept. Compound packing voids as well as vughs were present under both forest and pasture. Under pasture more planar voids appeared and macroporosity decreased significantly. Both total porosity and bulk density, before and after clearing of forest, were not significantly different in the two soil types. Obviously, puddling occurred in both soil types. Structure degradation in the relatively old Humu't was, however, more severe in shorter time than in the relatively young Tropept over a longer time span.