Aluminum in soil solution exists in a variety of forms, some of which are toxic to plants. In the current investigation three spectrophotometric techniques-8-hy-droxyquinoline, aluminon, and ferron—were compared for their ability to estimate phytotoxic Al in soil solution. Soil solution Al reacting with 8-hydroxyquinoline in 15 s, ferron in 30 s, and aluminon in 30 min were related to root and shoot growth of subterranean clover (Trifolium subterraneum L. cv. Mt. Barket) and switchgrass (Panicum virgatum L. cv. Cave-in-Rock) in limed and unlimed treatments of 10 acidic subsoil horizons. Across all 10 soils the amount of Al reacting with 8-hydroxyquinoline was generally the lowest, and that reacting with ferron the highest, although overlap among methods did occur. Manganese interference in several of the soil solutions limited the utility of the ferron method. The amounts of Al reacting with 8-hydroxyquinoline and aluminon were significantly related (P < 0.05) to root and shoot growth limitations exhibited by subterranean clover and switch-grass. Activities of Al3+ (aAl3+) calculated from the GEOCHEM program using the Al reacting with 8-hydroxyquinoline and aluminon as the Al inputs were generally the best predictors of root and shoot growth. Especially for subterranean clover, including measures of soil or soil solution Ca and pH with aAl3+ greatly enhanced the predictive capability of regression equations describing root and shoot growth. The concentrations of organic acids and dissolved organic C were low in the soil solutions from this experiment, and both 8-hydroxyquinoline and aluminon gave reasonable estimates of phytotoxic Al. However, the extent of reaction between these reagents and organically complexed forms of Al should be investigated before these techniques are applied to soil solutions in which a significant fraction of soil solution Al is in an organically complexed form.
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