CHARACTERIZATION OF PHYTOTOXIC ALUMINUM IN SOIL SOLUTIONS FROM PHOSPHOGYPSUM AMENDED SOILS

Phosphogypsum (PG), an industrial by-product from phosphoric acid plants, is being used as an ameliorant for acid soil infertility. Phosphogypsum is primarily CaSO4 and contains F among several impurities. An increase in SO42- and F- ligands in soil solutions following amendment with PG is important for the alleviation of Al toxicity. In soil solutions containing abundant SO42- and F-, a measure of Al that is not complexed with either of the ligands represents phytotoxic Al. Alleviation of Al toxicity by SO42- in some instances is due to SO42- induced precipitation and/or sorption of Al. Some studies have demonstrated a decrease in Al toxicity by SO42- even though precipitation of Al was not evident. In such cases, formation of the Al-SO4+ ion-pair (predicted by speciation models) has been attributed as the reason for alleviation of Al toxicity. Recent evidence has indicated that the proportion of Al complexed with SO42- was much greater than that predicted by using speciation models, which suggests that alleviation is not fully attributable to the formation of AlSO4+ species. The existing colorimetric Al assay techniques have failed to discriminate Al complexed with SO42- and have therefore, proved to be unsuitable for determination of phytotoxic Al in solutions containing SO42-. Fractionation of Al complexed with SO42- by size exclusion chromatography (SEC) has permitted a more precise characterization of Al complexed with SO42- which has been demonstrated as less phytotoxic than the uncomplexed Al. Therefore, size exclusion chromatography is a promising technique for characterization of phytotoxic Al in solutions in the presence of SO42-. In solutions containing F-, alleviation of Al toxicity is due to formation of Al-F complexes which are less phytotoxic. The 8-hydroxyquinoline method with a 15 second reaction time excludes Al-F complexes and, therefore is a measure of phytotoxic Al in solutions containing F-.