Aluminium fractionation of European volcanic soils by selective dissolution techniques

Abstract

Several selective dissolution methods were used to differentiate Al forms in 12 soils formed from volcanic materials (64 andic, vitric and organic horizons) in Iceland, Azores (Portugal), Tenerife (Spain) and Italy. The soils differ in many properties because of differences in parent materials, climatic conditions, soil age and history of land use. ‘Inorganic’ fractions of Al were characterized using pyrophosphate (Al p), acid oxalate (Al o) and cold NaOH (Al n) extractions. The difference in Al o–Al p, and Si extracted using acid oxalate, shows important differences among soils from different sources: allophane is richer in alumina in the Andosols from Azores and Tenerife than in those from Iceland and Italy. Cold NaOH generally extracted the same quantity of Al as acid oxalate, but in the soils from Tenerife Al n exceeds Al o, indicating the presence of gibbsite or poorly ordered halloysite. To characterize the Al bonded to the organic matter fraction, extractions with pyrophosphate, CuCl 2 and LaCl 3 were used. Except in one soil from Tenerife (N10), CuCl 2 extracted less Al than pyrophosphate. In organic-rich mineral horizons, Al La, Al Cu and Al p increase with organic carbon content. Using unbuffered KCl and LaCl 3, the amount of Al extracted increased with decreasing soil pH, but there was no similar relationship with unbuffered CuCl 2. The general sequence of efficiency of the extractants was Al p>Al Cu>Al La≥Al K in organic matter-rich horizons. Al Cu constituted 30% of Al p in all soils, indicating that CuCl 2 extracts a very specific Al fraction. Al p and Al Cu are positively related to organic C. Al p/Al o and Al p–Al Cu in andic horizons also increase with organic C. Phosphate retention and pH in NaF were related to Al o and Al n. The soils of Iceland have larger P retention than the others, and vitric horizons from Tenerife have higher pH in NaF and smaller Al pools than the other soils. For many horizons, there is a strong relationship between Al Cu and components that determine soil CEC (organic matter and allophane), suggesting a possible action of CuCl 2 on noncrystalline aluminosilicates. This should be taken into account if CuCl 2 is used to estimate Al in humus complexes.