Possible control of aluminum solubility by 1 m kcl treatment in some soils dominated by aluminum-humus complexes

Extraction of aluminum with 1 M KCl has been adopted as a standard method for determining salt exchangeable forms of Al from acid soils. We examined the possibility of Al precipitation during KCl extraction in soil samples dominated by Al-humus complexes by comparing Al release rates between non-treated and KCl-treated samples in an acetate buffer adjusted to pH 3.5. Five of 14 samples used in this study showed that the aluminum release rates and the total amount of Al released from KCl-treated residues were greater than or nearly identical with those of non-treated soil samples. This behavior suggests the formation of an acid-labile, surface or interlayer precipitate during the KCl extraction process. Thus, the amount of 1 M KCl-extractable Al does not always reflect the amount of exchangeable Al in soils dominated by Al-humus complexes.