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Soils on Terraces Along the Cauca River, Colombia: II. The Sand and Clay Fractions¹

ABSTRACT

Microscopic examination and elemental analyses of the sand fractions of the terrace soils show that their compositions are too variable to infer anything very noteworthy about the relative rates of primary mineral weathering. However, the parent materials are sufficiently uniform to reveal a relatively continuous progression of notable changes in clay mineralogy from floodplain through the age sequence of terrace soils. Mica, chlorite, vermiculite, smectite, and kaolinite are present in appreciable quantities in the floodplain soils and mica is present in the sand fractions of all terrace soils, but of these only kaolinite is present in more than minute amounts in the sola of any terrace soils. The quantities of 14A minerals with interlayer Al are greatest in the sola of the youngest terrace soils, but they persist in somewhat reduced quantities throughout all except in the subsoil on the highest terrace. Both amorphous aluminosilicates and gibbsite reach maximum concentrations in the lower terrace soils, thence their quantities decline with increasing age. The Fe_o/Fe_d ratio has a similar age trend but peaks sooner, at 0.53 in the higher floodplain soil, and thence decreases steadily with soil age to < 0.01 in the clay fraction of the highest terrace soil. Kaolinite is the major constituent of clay in the sola of all terrace soils. It increases with soil age to approximately 60% of the clay fraction in the highest terrace soil.

¹ Contribution from the Agronomy Dep., Ohio State Univ., based on a Ph.D. Dissertation entitled "A Chronosequences of Soils on Terraces along the Cauca River, Colombia," 1970. Approved for publication as Jour. Art. no. 22–82 of the Ohio Agric. Res. and Develop. Ctr., Wooster, OH 44691. Copies of the Ph.D. Dissertation are available from University Microfilms (order no. 70-19,288), Ann Arbor, Mich.

² Soil Scientist, U.S. Forest Service, 630 Sansome St., San Francisco, CA 94111, and Professor Emeritus, Dep. of Agron., Ohio State Univ., Columbus.

Received for publication February 12, 1982. Accepted for publication February 25, 1983.