CONTENTS AND WEATHERING STATE OF VOLCANIC ASHES AND THEIR EFFECTS ON PHOSPHATE SORPTION IN SOILS OF THE SEMIARID PAMPA OF ARGENTINA

Eolian parent materials of soils of the semiarid Argentinean Pampas sh ow high volcanic ash contents. Weathering products of volcanic ashes c an sorb anions in many soils. The objective of this study was to analy ze the volcanic ash distribution in soils of the semiarid Pampas, its weathering state, and the influence of its weathering products on phos phate sorption. Sixteen soils, classified as Entic Hapludolls and Enti c Haplustolls, were studied at different landscape positions of five s ites placed in a SW-NE-oriented climosequence within the semiarid Pamp a. Results showed that volcanic ashes accumulated more in the subsoil of SW-placed profiles and in the topsoil of NE-placed profiles. Topsoi l accumulated ashes of NE-placed profiles were deposited by the erupti on of Quizapu volcano in 1932. Subsoil accumulated ashes of SW-placed profiles were deposited by older volcanic eruptions. Acid oxalate solu ble Al (Al-o), an index of the degree of volcanic ash weathering, war higher in the subsoil of profiles placed at the SW extremity of the se quence. The larger effect of ash age rather than climatic conditions o n ash weathering rate was deduced. Allophane contents were low in all soils (<0.8%). Prevailing clay minerals were illite and very low cryst allized intergrade illite-smectites. Soils with the highest volcanic a sh content also held the largest amounts of illite-smectite intergrade s. Al-o contents explained 52% of sorbed phosphates, and only 36% of f ixed P. By including the contents of exchangeable Ca2+ and CaCO3 into the model, 73% of sorbed P variations and 58% of fixed P variations we re explained. No measurable effects of volcanic ashes on phosphate dyn amics are expected to exist in soils placed to the north of the studie d region, but ill soils placed to the south, which have high contents of CaCO3 and exchangeable Ca2+, amorphous volcanic ash, weathering pro ducts can retain large amounts of phosphates.