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Volcanic Ash-influenced Vertisols and Associated Mollisols of El Salvador: Physical, Chemical, and Morphological Properties¹

ABSTRACT

Vertisols and Mollisols have developed from Quaternary pyroclastic deposits in central El Salvador under ustic moisture and isohyperthermic temperature regimes. Pellusterts generally occupy the nearly level constructional landforms (alluvial plains, coastal plains, and concave alluvial toeslopes), are poorly drained and exhibit darkgray to black colors. Chromusterts and Pellusterts with argillic horizons and secondary silica in lower sola occupy more sloping, erosional upland landforms that are better drained. Associated Argiustolls have formed in loamy or clayey andesitic alluvium on nearly level alluvial plains. Vertisols of El Salvador have many properties in common with Usterts of the USA, i.e., high clay contents, slickensides, tilted parallelpipeds, and deep wide cracks that extend to the surface, but they lack gilgai relief, cyclic horizonation, and with a few exceptions, self-mulching surface horizons. Most have cambic horizons. The amorphous material content of the <2 µm fraction of these soils range from 14 to 28%, pH's range from 5.4 to 7.5, base saturation is >50% (commonly >75%) with Ca and Mg dominant. COLE values are 0.06 to 0.24 and clays are smectitic or kaolinitic. Microfabrics indicate high shrink-swell. Stress-oriented plasmic fabrics are largely vo-masepic, vo-skel-masepic and lattisepic for cambic and Bt horizons indicative of microshear failure. Undulic plasmic fabrics dominate in pumice-rice ash mantles and pyroclastic-rich parent materials. Illuvial channel ferri-argillans and organo-argillans occur in subsoils of the Chromusterts and Argiustolls, respectively. Pedogenesis and plasmolysis increase in the order: andesitic bedrock or alluvium, saprolite (from bedrock), clay-enriched Bw and Bt horizons, and Bw horizons in upper sola with slickensides and stress-oriented plasmic fabrics. The best Bt and silica-enriched (Bq) horizons occur in lower sola below the more physically active upper Bw horizons.

¹ Contribution from Texas Agric. Exp. Stn., College Station. This study was supported in part by the United States Agency for International Development (AID), under a contract, 1A-C-1084, to the Univ. of Florida and a grant, DAN-1311, to Texas A&M Univ. and Semi-Arid Tropics Soil Management CRSP. Contract no. 1311-GSS-1083-00.

² Graduate Student, Professor of Pedology, Professor of Tropical Soils, and Associate Professor of Pedology, respectively. Dep. of Soil & Crop Sciences, Texas A&M Univ., College Station, TX 77843-2474.

Received for publication April 28, 1986.

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