Assessing the phytotoxicity of mononuclear hydroxy‐aluminum

Al 3+ is an important rhizotoxic ion in acid soils around the world. Al 3+ is in equilibrium with mononuclear hydroxy‐Al species, such as AlOH 2+ and AL(OH) 2 + , but the toxicity of these species has not been determined. Polynuclear Al may also coexist with Al 3+ , and one of these species, AlO 4 Al 12 (OH) 24 (H 2 O) 12 74 , is very toxic. In order to determine the toxicity of mononuclear hydroxy‐Al we have reanalysed the results of previously published, solution‐culture experiments and have performed new experiments. Several problems are inherent in these studies. At pH values less than 5.0, the activities of the mononuclear hydroxy‐Al species are low relative to Al 3+ , but attempts to change the ratio by raising the pH generally initiate the formation of polynuclear Al. (We discovered that mononuclear solutions are stable for many days when {Al 3+ }/{H + } 3 ≤ 10 8.8 , where braces denote activities.) We avoided, or accounted for, polynuclear Al in our studies. In addition, we used up‐to‐date equilibrium constants to compute Al species, very simple culture media (generally containing CaCl 2 , AlCl 3 and HCl as the only inputs), short‐term (2d) growth, and an Al‐sensitive wheat variety ( Triticum aestivum L. cv. Tyler) that permitted low Al levels and, consequently, higher pH values without Al polymerization. Experiments were designed in which the solutions were constant in {Al 3+ } and variable in mononuclear hydroxy‐Al or were orthonal (factorial) in {Al 3+ } and {AlOH 2+ }. Linear and nonlinear, simple and multiple, regression analyses of these and previous experiments failed to reveal any toxicity for mononuclear hydroxy‐Al to Tyler wheat.