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Properties and Genesis of Selected Colombian Andosols¹

ABSTRACT

A climosequence of southern Colombian Andosols developing in andesitic volcanic ash was studied through the use of physical, chemical, and mineralogical parameters in an effort to further elucidate pedogenesis of volcanic ash weathering.

The four Andosol profiles were characterized by high organic carbon contents especially at higher elevation. Sodium fluoride pH values suggested that the clay fractions of these profiles were dominated by allophanic colloids. Low bulk densities and high 15-bar H₂O contents reflected the high porosity of Andosols and the hydrophilic nature of allophane, respectively.

Hornblende was the dominant heavy 50-500µ fraction mineral except in some horizons of the 3,510-m profile which were dominated by pyroxenes. Volcanic glass decreased with profile depth and increased with increasing altitude.

Varying amounts of vermiculite, hydrated halloysite, metahalloysite, cristobalite, and allophane were identified by infrared and X-ray analyses. Hydrated halloysite, metahalloysite, and allophane were identified in the fine clay fraction.

A clay mineral weathering sequence of allophane hydrated halloysite was postulated. The maturation of this sequence increased with profile depth and with decreasing altitude. Occurrence of metahalloysite in surface horizons was attributed to periodic higher moisture tensions. It is hypothesized that coarse clay vermiculite is transient or, alternatively, that insufficient time has elapsed for its transformation to other 2:1 phyllosilicates. It is further postulated that vermiculate pedogenesis occurred rather rapidly through modification from a primary mineral source (possibly biotite).

¹ Contribution from the Soil Science Department, Florida Agr. Exp. Sta., Gainesville 32601, as Journal Series Paper 4176. Presented in part before a joint session of Div. S-5 and Div. S-9, Soil Science Society of America, New York, N.Y., Aug. 19, 1971.

² Assistant Professor of Soil Science: Associate Professor in Soil Science Dept., Gainesville, Fla.; and Asesor Direción Agrologica, Instituto Geografico "Agustín Codazzi," Bogotá, Colombia, respectively.

Received for publication November 8, 1971. Accepted for publication February 7, 1972.