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Calcium-Aluminum Balance and the Growth of Soybean Roots in Nutrient Solutions

Dep. of Soil Science and Agrometerology, Univ. of Natal, Pietermaritzburg, 3200, South Africa

M. E. Sumner^*

Dep. of Agronomy, Univ. of Georgia, Athens, GA 30602

*Corresponding author.

ABSTRACT

The effect of varying two parameters having contrasting effects has been largely overlooked when describing Al phytotoxicity. Consequently the effects of varying pH (4.2, 4.5, and 4.8), Al (0, 20, 40, 80, and 160 µM Al) and CaSO₄ (625, 1250, 2500, 5000, and 10 000 µM Ca) levels on root length of soybeans (Glycine max (L.) Merr.) were evaluated in a solution culture experiment. At a given pH and Al concentration, an increase in the level of CaSO₄ resulted in a decrease in the activities of Al³⁺, AlOH²⁺ and Al(OH)⁺₂ while the activity of AlSO⁺₄ increased. Among the individual Al monomers, relative root length was most highly correlated with the calculated activity of Al³⁺ (R² = 0.811) followed by AlOH²⁺ (R² = 0.687), while AlSO⁺₄ (R² = 0.338) gave the poorest correlation. Thus, the AlSO⁺₄ species appears to be less phytotoxic. Relative tap root length was poorly correlated with the concentration of monomeric Al determined by the modified Aluminon method, which also assays the less phytotoxic AlSO⁺₄ species under the prevailing conditions. An expression termed CAB <Calcium aluminum balance = [2log(Ca²⁺)] – {3log(Al³⁺) + 2log[Al(OH)²⁺] + log[Al(OH)⁺₂]}>, enabled the most precise assessment to be made of the effects of soluble Ca and Al, together, on root growth (R² = 0.900). The coefficients in the CAB expression were introduced as a weighing to accommodate the assumed importance of valence in ion interactions at the plant root surface. The CAB was also found to be a better index of the solution conditions affecting root growth when applied to data from other published studies.

This research was partly supported by Florida Inst. of Phosphate Research; Foundation for Research and Development, CSIR, South Africa; Georgia Agric. Exp. Stn.; and Univ. of Natal, South Africa.

Received for publication November 23, 1987.

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