SPECIATION AND SOLUBILITY CONTROL OF ALUMINUM IN SOILS DEVELOPED FROMSLATES OF THE RIVER SOR WATERSHED (GALICIA, NW SPAIN)

The Al species in the sold and liquid phases were studied in eight soi ls developed from slates in a watershed subjected to acid deposition. From soil solution data the mechanisms possibly controlling Al solubil ity are also discussed. The soils are acidic, organic matter rich and with an exchange complex saturated with Al. In the solid phase, more t han 75% of non-crystalline Al was organo-Al complexes, mostly highly s table. In the soil solutions, monomeric inorganic. Al forms were predo minant and fluoro-Al complexes were the most abundant species, except in soil solutions of pH<4.8 and AlL/F ratio >3, in which Al3+ predomin ated and sulphate-Al complexes were relatively abundant. The most stab le phases were kaolinite, gibbsite and non-crystalline Al hydroxides. In most samples, Al solubility was controlled by Al-hydroxides. Only i n a few cases (solutions of pH 4-5, Al3+ activity >40 mu mol L-1 and S O4 content >200 mu mol L), Al-sulphates such as jurbanite also could e xert some control over Al solubility. In addition to these minerals, a possible role of organo-Al complexes or the influence of adsorption r eactions of sulphate is considered, especially for samples with very l ow Al3+ content (<0.5 mu mol L-1).