Testing aluminum-chelate equilibria models using sorghum root growth as a bioassay for aluminum

Aluminum toxicity is an important limitation to crop yields in the acid soils of southeastern U. S. and other parts of the world due to its detrimental effects on roots. Soluble organics in soil solution ameliorate Al toxicity, a phenomenon which can be studied employing synthetic chelates. Theoretical models predict that Al will be complexed on nearly a one to one molar basis by nitrilotriacetic acid (NTA) at toxic pH levels found in acid soils (4.0 to 4.5). A series of growth chamber experiments were conducted with NTA at various Al and pH levels to test equilibrium models using sorghum Sorghum bicolor (L.) Moench] as a bioassay for the uncomplexed Al. At pH 3.5 neither Al nor NTA affected root growth which was very poor, probably because of H⁺ toxicity. At pH 4.0 and 4.5 root growth was reduced by Al levels, and NTA ameliorated toxicity as predicted by the theoretical model. Root length was reduced at pH 4.0 relative to pH 4.5 indicating that H⁺ ion was toxic at pH 4.0. The bioassay method was successful in confirming the model for Al-chelate equilibria for NTA. Differential pulse polarography was found to be sensitive to the uncomplexed Al and may have potential in determining toxic Al in soil solution.