Aluminium fractionation of European volcanic soils by selective dissolution techniques

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 (Alp), acid oxalate (Alo) and cold NaOH (Aln) extractions. The difference in Alo–Alp, 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 Aln exceeds Alo, indicating the presence of gibbsite or poorly ordered halloysite. To characterize the Al bonded to the organic matter fraction, extractions with pyrophosphate, CuCl2 and LaCl3 were used. Except in one soil from Tenerife (N10), CuCl2 extracted less Al than pyrophosphate. In organic-rich mineral horizons, AlLa, AlCu and Alp increase with organic carbon content. Using unbuffered KCl and LaCl3, the amount of Al extracted increased with decreasing soil pH, but there was no similar relationship with unbuffered CuCl2. The general sequence of efficiency of the extractants was Alp>AlCu>AlLa≥AlK in organic matter-rich horizons. AlCu constituted 30% of Alp in all soils, indicating that CuCl2 extracts a very specific Al fraction. Alp and AlCu are positively related to organic C. o and Alp–AlCu in andic horizons also increase with organic C. Phosphate retention and pH in NaF were related to Alo and Aln. 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 AlCu and components that determine soil CEC (organic matter and allophane), suggesting a possible action of CuCl2 on noncrystalline aluminosilicates. This should be taken into account if CuCl2 is used to estimate Al in humus complexes.