Abstract
The extraction of aluminum by a 1_M potassium chloride solution (KCl-Al) has been adopted as a standard method for determining the salt exchangeable forms of Al from acid soils. It is generally assumed that the extracted Al ions are mainly derived from those on the soil's permanent negative charges. However, it was recently confirmed that the Al solubility of non-allophanic Andosols is largely controlled by the Al-humus complexes. We compared the KCl-Al values and the amounts of permanent negative charges of A horizon soils, and examined the relationships between the KCl-Al values and soil pH values or the amounts of organically-complexed Al. Ten of 14 soil samples used for the laboratory determination showed higher KCl-Al values than the amounts of permanent negative charges (negative charges in 1m_M KCl, pH=5), indicating that the KCl-Al values also included a part of the organically-complexed Al besides Al ions on the permanent negative charges. The dataset consisting of previously reported and newly obtained data (105A horizon samples) showed that the KCl-Al values were highly correlated with the pH(KCl) values (r=-0.80, P<0.01), and were also dependent on the pyrophosphate-extractable Al concentration (r=0.22, P<0.05), suggesting that the KCl-Al values were affected by the amounts of organically complexed Al and very dependent on the pH during the extraction by KCl solution. Thus, the KCl-Al value of the Andosols is considered not to be easily exchangeable Al, but to be equilibrated Al in the KCl solution.
