Productivity of tropical soils is often limited by low nutrient reserves and high acidity. Yet due to population pressures, many farmers must utilize these marginal agricultural lands. A better understanding of base cation biogeochemistry within tropical agroecosystems is needed to better assess long-term sustainability of these systems. Soils developed within Tertiary sediment deposits on the Atlantic slope of the Talamanca Mountains in Costa Rica were sampled. An upland Ultisol and an alluvial Inceptisol under cacao agroforestry systems were chosen to represent contrasting soils of similar age and landscape position. Calcium (Ca), magnesium (Mg), and potassium (K) in primary mineral and exchangeable phases were measured. Mineralogy of the clay and sand fractions was determined. Preliminary data indicate that the upland soil has a low capacity to store and exchange bases in comparison with the alluvial soil. The upland Ultisol has an effective cation exchange capacity (ECEC) in the Bt horizon of 5.5 cmol+/kg, a sum of bases equal to 0.6 cmol+/ kg, and aluminum saturation of 89%. In contrast, the alluvial Inceptisol B horizon has an ECEC of 22.5 cmol+/ kg, sum of bases of 20 cmol+/kg, and aluminum saturation of 0.37%. Total elemental calcium (Ca) in the upland soil Bt horizon is 0.05 % and total magnesium (Mg) is 0.26 %. In contrast, total elemental concentrations in the alluvial soil are 1.25 % Ca and 1.82 % Mg. The upland soil clay fraction is dominated by kaolin. The sand fraction contains magnetite, quartz, hematite, and amphiboles. In contrast, the alluvial soil clay fraction contains smectite and the sand fraction contains quartz and feldspars. These data demonstrate differences in elemental stocks of base cations and weatherable minerals within the two soils. The exchangeable cation and aluminum saturation data also indicate the need for different nutrient management practices in the two areas.