CREATING A LANDSCAPE-SCALE MODEL OF SOIL EVOLUTION ON THE PACIFIC COAST OF COSTA RICA

Geographic soil models are valuable for any kind of land use planning and are most important in agriculture, transportation, community development and conservation planning. Soil mapping in particular is essential in addressing a host of other questions relating to geomorphic processes, environmental concerns, public health, as well as food, water, and energy security. In tropical regions, available soil data may be antiquated, often dating back more than thirty years and the “kaolinite-dominated, nutrient-depleted” paradigm only describes about one third of tropical soils. This study synthesizes research conducted in three sub-regions of Costa Rica’s Pacific coast and analyzes soil properties from a fourth sub-region to create a soil model along the entire coast (spanning 400 km and 2 degrees of latitude). Each sub-region was characterized using X-ray diffraction, bulk soil geochemistry by ICP-AES and ICP-MS as well as soil pH and cation exchange capacity. These data were used to create soil weathering classifications applicable across climatic sub-regions.

Ranging from dry (1500 mm/yr) to wet (4800mm/yr) environments, the four sub regions differ appreciably in weathering rates. Previous studies conclude that soils in tropical wet forest environments (4800mm/yr) evolve to a nutrient-depleted, kaolinite-rich Oxisol mineral assemblage in half the time required for soils in a drier tropical forest (1500 to 2200 mm/yr) to do the same (and these drier soils never become as nutrient depleted as the 4800 mm/yr climate zone). To model the spatial variability of soil weathering, this study compares a cokriged interpolation model and a class pedotransfer function weighing climatic, temporal and topographic data with pH, CEC, soil geochemistry and mineralogy. In a country dependent on agricultural exports and a region with active tectonic uplift, soil data and accurate methods for acquiring such data contribute to fundamental knowledge of the land thus influencing current land use and future planning. Furthermore, a spatial model of this type can be applied to questions of terrace correlation in interpreting landscape evolution over Holocene to Pleistocene time scales.