Amelioration of aluminium phytotoxicity with organic matter

Abstract

In strongly weathered soils, the activity of Al³⁺ in the soil solution can often, but not always, be predicted using gibbsite solubility relationships. Overestimates are common and often attributed to a lack of equilibrium between the solid and the solution phases. Soil organic matter may also be involved because of its strong affinity for aluminium. The main components of the soil organic matter are humic substances and the effect of adding a sample of humic acid on the activity of Al³⁺ was investigated. The addition of the humic acid to an Oxisol from Burundi and an Ultisol from Cameroon decreased the activity of aluminium in the pH range 3.2 to 4.6. At pH 4.2, the activity of Al³⁺ in the Oxisol was 38 µM, but the addition of 2% humic acid decreased it to 11 µM. The corresponding values for the Ultisol were 11 µM decreasing to 2 µM with the addition of humic acid. In both soils, plots of pAl³⁺ against pH were linear. The stability lines for the soils treated with the humic acid were mostly parallel to those of the untreated soils. The displacement of the lines was 0.6 pAl³⁺ units in the Oxisol and 0.7 pAl³⁺ units in the Ultisol. The displacement decreased the [Al³⁺] [OH^-] ion activity product for both soils. Dissolution of the soil aluminium was relatively rapid. This suggests that the displacement of the stability line was more likely due to decreased solubility rather than to kinetic restrictions. The humic acid also decreased the exchangeable aluminium saturation of the two soils by decreasing the activity of Al³⁺ and increasing the cation exchange capacity of the system. Exchangeable aluminium was held more strongly giving rise to greater selectivity coefficients for (Ca²⁺+Mg²⁺)/Al³⁺ exchange in the soil treated with humic acid. In situations where kinetic restrictions limit the control of Al³⁺ activity by dissolution, the short term control will be achieved by rapid cation exchange. The decrease in exchangeable aluminium saturation will further ensure lower Al³⁺ activities.