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Mineralogy and Iron Characterization of Plinthitic Soils on Alluvial Landforms in Venezuela¹

ABSTRACT

The iron oxides and clay mineralogy were characterized for several soils of Venezuela which contain plinthite and pseudo-plinthite. Molar ratios of SiO₂/Al₂O₃ (Ki ratios) of the plinthite and pseudo-plinthite zones range from below 2 to above 4 suggesting a wide range in mineral composition. Cation exchange capacity for these zones ranged from 14.6 to 65.6 meq/100 g of clay suggesting that these mottled zones are not always associated with low activity clays. Kaolinite dominated the clay minerals. Montmorillonite was identified in a few pedons.

Goethite was the dominant iron oxide present in the plinthite. The percentage of iron was determined by extracting the soil with citrate-bicarbonate-dithionite and by extracting with ammonium oxalate. These two extractions were ratioed (OX/CBD) yielding values of >0.05 for pseudo-plinthite and values of <0.05 for plinthite in most cases. Soils with plinthite contained most of the iron in the sand- and silt-size fraction following one drying cycle while soils with pseudo-plinthite contained most of the iron in the clay-size fraction.

¹ Contribution from the Dep. of Agronomy, New York State College of Agriculture and Life Sciences, Cornell University, Ithaca, NY 14853. Agronomy Paper no. 1360.

² Former Graduate Assistant, presently Associate Professor, New Mexico State University; and Former Professor, now Director of Soils, USDA-SCS, respectively.