Contents and weathering state of volcanic ashes and their effects on phosphate sorption in soils of the semiarid pampa of Argentina

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Abstract
Eolian parent materials of soils of the semiarid Argentinean Pampas show high volcanic ash contents. Weathering products of volcanic ashes can sorb anions in many soils. The objective of this study was to analyze the volcanic ash distribution in soils of the semiarid Pampas, its weathering state, and the influence of its weathering products on phosphate sorption. Sixteen soils, classified as Entic Hapludolls and Entic Haplus‐tolls, were studied at different landscape positions of five sites placed in a SW‐NE‐oriented climosequence within the semiarid Pampa. Results showed that volcanic ashes accumulated more in the subsoil of SW‐placed profiles and in the topsoil of NE‐placed profiles. Topsoil accumulated ashes of NE‐placed profiles were deposited by the eruption of Quizapu volcano in 1932. Subsoil accumulated ashes of SW‐placed profiles were deposited by older volcanic eruptions. Acid oxalate soluble Al (Alo), an index of the degree of volcanic ash weathering, was higher in the subsoil of profiles placed at the SW extremity of the sequence. The larger effect of ash age rather than climatic conditions on ash weathering rate was deduced. Allophane contents were low in all soils (o contents explained 52% of sorbed phosphates, and only 36% of fixed P. By including the contents of exchangeable Ca2+ and CaCO3 into the model, 73% of sorbed P variations and 58% of fixed P variations were explained. No measurable effects of volcanic ashes on phosphate dynamics are expected to exist in soils placed to the north of the studied region, but in soils placed to the south, which have high contents of CaCO3 and exchangeable Ca2+, amorphous volcanic ash, weathering products can retain large amounts of phosphates.