Chemical significance of aluminium extracted from three horizons of an acid forest soil, using chloride salt solutions

Aluminium is usually important in exchange reactions in acid soil, but amounts extracted are strongly affected by procedure. We examined the impact of proton release, following salt addition, on Al removal from three horizons (Oe, Oa and Al2) of an acid, brown forest soil. The Al was first extracted with either 1 M KCl, 1 M NH4Cl or 0.5 M CuCl2 on the three horizons limed to pH 5.5. For any lime addition, both the pH drop following salt addition and the amounts of extracted Al were greater with CuCl2 than with KCl or NH4Cl. For the Oa and Al2 horizons there was a single inverse linear relation between amount of Al extracted and the pH of final extract, independent of the extracting cation. In the Oe horizon CuCl2 extracted constant amounts of Al, whereas the quantity of Al removed by KCl or NH4Cl increased linearly as pH declined below pH 3.6. Extra Al was mobilized following unbuffered KCl extraction, a side effect induced by the extracting procedure itself. Al mobilization increased with increasing H+ additions. In the Oe horizon, equilibrium was established rapidly (less than or equal to 24 h), and decreasing the pH of the soil-KCl suspensions to 2.0 resulted in extracted Al amounts equal to or slightly greater than those obtained with CuCl2 solutions. Al amounts extracted with acidified KCl solutions from the A(12) were close to those obtained using CuCl2 solutions at similar pH in the extract. In the samples from the Al2 layer, increasing the equilibration time resulted in increasing proton consumption and equivalent release of Al ions in the extract.