QUANTITATIVE ASPECTS OF WEATHERING AND NEOFORMATION IN SELECTED COSTA-RICAN VOLCANIC SOILS

Gains and losses of major elements in eight soil profiles developed an volcanic parent material in the perhumid tropical Atlantic lowland of Costa Rica were calculated from total element contents, assuming Ti t o be immobile, Initially soil formation (0-500 yr) in Tropopsamments i nvolves dilation of the sandy deposits by incorporation of organic mat ter and formation of structure and biopores, without detectable gains or losses of elements, In 2000- to 5000-yr-old sandy Hapludands, prima ry minerals are still abundant, but up to 20% of the mineral soil cons ists 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 mineral s 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, hallovsit e, and short-range order material are the most important secondary min erals at greater depths. Dilation of the soil mass prevails in the A h orizon, while collapse has occurred in the B horizon, The Melanudand h as 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 1 25 000 yr, a Haploperox has formed (thought to have formed from Andiso ls); it is strongly collapsed and is dominated by gibbsite, kaolin min erals, and Fe (hydr) oxides, The upper meters are devoid of primary mi nerals except for small opaques, Under the prevailing environmental co nditions, weathering and neoformation of primary volcanic minerals lea d to almost complete dissolution and leaching of basic cations, probab ility within 20 0001 to 50 000 yr. The Si and P mineral reserves are d epleted considerably, but part still present after such long time peri ods.