Effects of aluminum, OH:Al and P:Al molar ratios, and ionic strength on soybean root elongation in solution culture

In a series of experiments, we studied the effects of Al concentrations, OH:Al and P:Al ratios, and ionic strength on soybean root length after 4 days’ growth in nutrient solutions approximating soil solutions. Additions of NaOH and NaH2PO4.2H2O solutions decreased the concentrations of monomeric Al species in solution, Al polymers being formed with Al concentrations ≤50 μM originally in solution but not at concentrations ≥100 μM Al. In nutrient solutions with ionic strengths ≤900 μM, the Al polymers formed remained in solution, but were not detected in solutions with higher ionic strengths. Soybean root elongation was reduced severely with Al concentrations of ≥10 μM. The toxic effect of Al was ameliorated by the addition of OH or P through the reduction of monomeric Al species in solution. A close curvilinear relationship was observed between relative root length (%) (y) and Σ activities (μM) of the monomeric Al species in solution (x) (y = 14.9972 + 99.163 exp(−0.130605x); R = 0.950***), irrespective of the initial Al concentration, the concentration of polymeric Al species in solution, ionic strength, solution nutrient composition, P concentration, or pH. It appeared that one or more of the monomeric Al species was responsible for the toxic effect of Al on soybean root growth, and that the ameliorating effect of OH and P resulted from the reduction in the concentration of monomeric Al in solution, either through polymerization or precipitation of Al