Suitability of the aluminon technique for measuring phytotoxic aluminum in solutions with varying sulfate concentrations

Abstract

Considerable uncertainty prevails concerning a suitable measure that can adequately describe Al phytotoxicity in nutrient and soil solutions. A study was conducted to evaluate the ability of a modified aluminon technique to discriminate between phytotoxic and non‐phytotoxic Al in solutions containing 80 μM Al with varying levels of CaSO₄(625 to 10000 μM), at two pH levels (4.2 and 4.8). The concentration of Al measured by the modified aluminon technique ranged from 18.3 to 77.7 μM,thereby indicating substantial polymerization in some of the solutions. The greatest amount of polymerization occurred at pH 4.8 in the presence of 625 μM CaSO₄. Increasing additions of CaSO₄resulted in an increase in predicted activity of AlSO₄ ⁺at both pH levels. However, with increasing addition of CaSO₄, the predicted activity of Al³⁺decreased at pH 4.2 or remained relatively constant at pH 4.8. The relationship between the sum of predicted activities of monomeric Al (Sa_{Al mono.}) in solution and tap root length of soybean Glvcine max(L.) Merr.] cv. Lee was extremely poor, thereby indicating the inability of the modified aluminon technique to measure phytotoxic Al in solutions employed in the current study. This difficulty was due to failure of the modified aluminon technique to exclude lesser phytotoxic AlSO₄ ⁺species. The activity of Al³⁺was closely related to tap root length (R²= 0.865). The prediction of root length response to Al was further improved (R²= 0.899) by considering the solution Al index as: S3a_A13+ + 2a_Al(OH)2+ + a_A1(OH)+]. There was a poor relationship between tap root length and the concentration of polymeric Al, thus suggesting the lower phytotoxicity of this component under the prevailing solution conditions.