EFFECTS OF CONCENTRATION OF SUPPORTING SOLUTION ON SOIL SOLUTION ALUMINUM IN VARIABLE CHARGE SOILS

Abstract

Characterization of soil solution aluminum (Al) as a predictor of Al toxicity to plant roots growing in acid soils has received increasing attention during recent years. However, release of Al into soil solution may depend on several soil properties and the concentration of supporting solution employed for displacing soil solution. For this study we obtained subsoil (0.6–0.8 m depth) samples of Bladen (Typic Albaquult) and Wedowee (Typic Hapludult) series from Georgia and a sample Podzolic (Epiaquic Haplustult) soil from Australia. Soil solution was displaced by centrifugation following 4 h equilibration with deionized water or solutions of CaCl2.2H2O at 0.1, 1, 10, or 100 mM concentration. The pH and ionic strength of displaced soil solutions were not influenced by the concentration of CaCl2 in the supporting solution from 0 to 1.0 mM. However, the pH of soil solutions decreased while the ionic strength increased when the concentration of CaCl2 in the supporting solution was increased to 10 or 100 mM. Concentrations of Al in soil solutions displaced by water or 0.1–1.0 mM CaCl2 solutions were much greater in predominantly smectitic-Bladen soil than in kaolinitic-Podzolic or Wedowee soils, although exchangeable Al was 78% greater in the Podzolic soil and only 36% lower in the Wedowee soil than in the Bladen soil. Concentration of Al in soil solution rapidly increased along with a decrease in pH and an increase in ionic strength when soil solution was displaced using 10 or 100 mM CaCl2 solutions. The sorption of calcium (Ca) was very similar to the release of other cations into soil solution in the Bladen soil, but it was greater than the release of other cations into soil solution in the Podzolic and Wedowee soils. Greater sorption of Ca from the supporting solution in the Podzolic and Wedowee soils than in the Bladen soil, due to an increase in surface charges, may be partly responsible for lower concentration of Al in displaced soil solution in the former soils than in the latter soil.