Aluminum Speciation and Phytotoxicity in Dilute Hydroxy‐Aluminum Solutions

Although it has long been recognized that toxicity of Al to plants is a major growth‐limiting factor in acid soils, relationships between the aqueous chemistry of Al and phytotoxicity are still not fully understood. The effect of added hydroxyl ions remains particularly problematic in that although raising solution pH generally relieves Al toxicity, it is unclear whether this should be ascribed to differential toxicity of the mononuclear hydrolysis products, or to formation of nontoxic polynuclear hydroxy‐Al complexes or solid phase Al(OH) 3 . We used solution culture techniques to examine the relationships between Al speciation and phytotoxicity in dilute, chemically well‐defined systems wherein OH ‐ was the only Al‐complexing ligand. Two‐day‐old wheat ( Triticum aestivum L.) seedlings were transferred to 500‐mL test solutions for 2 d of additional growth at 298 K in the dark, and final root lengths used as the measure of Al toxicity. Quantities of polynuclear hydroxy‐Al were estimated with a modified version of the ferron procedure, and activities of mononuclear species were calculated using the computer program GEOCHEM. In the absence of polynuclear hydroxy‐Al, raising solution pH consistently ameliorated Al toxicity, suggesting that although Al 3+ is not necessarily the sole toxic species, its activity is the best single indicator of Al stress. Our results did not confirm other reports that the sum of the mononuclear species activities is a reliable predictor of Al phytotoxicity. Polynuclear hydroxy‐Al was also demonstrably toxic, supporting isolated reports of the same finding. The common practice of excluding the polynuclear fraction from estimates of toxic Al should be reevaluated. These findings have practical implications for the accurate prediction of toxic Al levels in both solution culture media and soil solutions.