The pH Dependency of Aluminum Phytotoxicity Alleviation by Calcium Sulfate

The alleviation of Al toxicity by CaSO₄ is partly due to an increase in formation of less phytotoxic AlSO⁺₄ species. Since ion-pair formation is dependent on the solution pH, the magnitude of alleviation of Al toxicity by CaSO₄ may be influenced by pH. In the present study, the alleviation of Al toxicity (80 µM) to soybean [Glycine max (L.) Me root growth by CaSO₄ (625–10 000 µM) was investigated in dilute nutrient solutions at pH 4.2 or 4.8. The concentration of monomeric Al (by a modified aluminon technique) in these solutions ranged from 69.3 to 77.7 µM. An increase in CaSO₄ in solution (625–10 000 µM) increased the root length by 3- and 2-fold in solutions at pH 4.2 and 4.8, respectively. The predicted activity of Al³⁺ decreased while that of AlSO⁺₄ increased with an increase in added CaSO₄. The magnitude of alleviation of Al toxicity by CaSO₄ was smaller at pH 4.8 than at pH 4.2. This pH dependency is due to lesser formation of AlSO⁺₄ at pH 4.8 than at pH 4.2, together with an increase in formation of Al(OH)⁺₂ at pH 4.8. Root length was poorly correlated with the predicted activity of Al³⁺ (R² = 0.346) or sum of the activities of Al³⁺, hydroxy-Al, and AlSO⁺₄ species (R² = 0.366). However, the relationship was improved when the sum of the activities of Al³⁺, Al(OH)²⁺ and Al(OH)⁺₂ species was considered (R² = 0.624) and further improved (R² = 0.841) when the activities of these species were corrected for their respective valence. A good correlation was also found (R² = 0.88) between root length and calcium aluminum balance; CAB = [2log(aCₐ²⁺)] − [3log(aAₗ³⁺) + 2log(aAₗ₍OH₎²⁺) + log (aAl(OH)⁺₂)] index. Research supported by Florida Institute of Phosphate Research, Foundation for Research and Development, CSIR, South Africa and The Georgia Agric. Exp. Stn. Contribution from the Dep. of Agronomy, Univ. of Georgia.