Mineralogical and Thermal Properties of Volcanic Ash-Derived Soils Affecting Sorption Mechanisms.

Volcanic ash-derived soils have characteristic properties that are rarely found in soils derived from other parent materials. Their clay fractions are dominated by non-crystalline components, such as allophane, which confer greater reactivity and surface area that can interact with other soil components such as organic matter and ionic plant nutrients. Because their clay fractions are dominated by X-ray amorphous materials, the classic approach to characterizing their mineralogy has been through selective chemical dissolutions. The use of pure clays to model soil surface chemical behavior has led to great advances in the study of sorption reactions in soils, but an understanding of the complexity of natural systems should also include how organic surfaces interact with soil minerals, since this interaction determines the reactivity of the whole soil. A set of 25 volcanic ash-derived soils from the Pacific coastal plain of Guatemala were selected for a detailed study of the mineral and organic components that might affect the sorption properties of these soils. The results allowed a classification of the soils based on their contents of amorphous extractable minerals, layer-type aluminosilicates identified by X-Ray diffraction, and fractions of organic matter differentiated by their thermal stability. Soils with lower contents of amorphous minerals contained higher amounts of labile organic matter in the presence of 2:1 clay minerals of the smectite group, whereas soils with greater amounts of amorphous minerals had larger recalcitrant and refractory organic fractions in combination with 1:1 clay minerals. The wide differences in mineralogical and thermal properties of these soils imply differentiated mechanisms by which soil surfaces interact with ionic plant nutrients which were investigated in a separate study.