Aluminum chemistry of nonallophanic Andosols

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The aim of this study is to expand upon previous investigations about aluminum chemistry by examining aluminum solubility and release rates in soils dominated by Alhumus complexes. Aluminum solubility relationships over the pH range 3-5 were determined in a 30-d equilibration study. Aluminum release rates were measured using a stirred, flow-through reaction vessel with 10^<-3> M acetate buffer adjusted to pH 3.5, 4.0 and 4.5 for pretreated soil samples with Na-pyrophosphate, acid oxalate and KCl solutions.The nonallophanic Andosol samples displayed pAl versus pH slopes of 2.0-2.4 and solubilities considerably lower than those of common Al(OH)_3 phases. The soil samples showed very rapid Al release rates. The Al release rates were greatly dependent on the pH and ion strength of extracting solution. Pyrophosphate treatment reduced Al release rates to very low levels indicating that Al-humus complexes were the primary source of dissolved Al.Some non-allophanic soils 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. Thus, the amount of 1M KCl-extractable Al does not always reflect the amount of exchangeable Al in soils dominated by Al-humus complexes.This study and the literature showed that the amount of KCl-extractable Al is closely related to the amount of organically complexed Al. These data indicates that the dynamics of aluminum in nonallophanic Andosols is largely influenced by the exchange of Al ions with humic substances.