Quantitative Aspects of Weathering and Neoformation in Selected Costa Rican Volcanic Soils

Gains and losses of major elements in eight soil profiles developed on volcanic parent material in the perhumid tropical Atlantic lowland of Costa Rica were calculated from total element contents, assuming Ti to be immobile. Initially soil formation (0–500 yr) in Tropopsamments involves dilation of the sandy deposits by incorporation of organic matter and formation of structure and biopores, without detectable gains or losses of elements. In 2000- to 5000-yr-old sandy Hapludands, primary minerals are still abundant, but up to 20% of the mineral soil consists of x-ray amorphous materials. Dilation continues and losses of Mg, Ca, Na, and K, and to a lesser degree Si, become measurable. Within 18 000 yr, a Melanudand develops, which is depleted of primary minerals and especially volcanic glass in the fine-earth fraction. A mixture of short-range order material, metal-humus complexes, and gibbsite and kaolin minerals dominate its upper horizon, while gibbsite, halloysite, and short-range order material are the most important secondary minerals at greater depths. Dilation of the soil mass prevails in the A horizon, while collapse has occurred in the B horizon. The Melanudand has lost considerable amounts of mobile elements: 50 to 85% of Si, Mg, Ca, Na, and K have been leached from the fine-earth fraction. Within 125 000 yr, a Haploperox has formed (thought to have formed from Andisols); it is strongly collapsed and is dominated by gibbsite, kaolin minerals, and Fe (hydr) oxides. The upper meters are devoid of primary minerals except for small opaques. Under the prevailing environmental conditions, weathering and neoformation of primary volcanic minerals lead to almost complete dissolution and leaching of basic cations, probably within 20 000 to 50 000 yr. The Si and P mineral reserves are depleted considerably, but part is still present after such long time periods.