The mechanisms of interaction of variable charge soils and
constant charge soils with hydrogen ions were investigated and
evaluated. The results showed that the input of hydrogen ions into soils could transform into positive charges, soluble Al and
exchangeable acidity. However, the relative contributions of hydrogen ions to the three transformations above are different due to the discrepancies in components and properties of soils. The transformation into positive charges arises mainly from protonation of Fe— OH and Al— OH functional groups on the surface of soils. Therefore, the ability of this transformation is strongly dependent on the contents of Fe oxide, and consequently the transformation in variable charge soils is larger than that in
constant charge soils. The ability of transformation into soluble Al is dependent on the constituents of clay minerals in soils. The increases in soluble Al in soil solutions with increasing inputs of hydrogen ions are of the order: Paleudult > Kanhapludult > Hapludox. When the amounts of added hydrogen ions are less than 15 mmol/L, the amounts of soluble Al in yellow- brown soil is between Paleudult and Kanhapludult, whereas they are slightly less than in Kanhapludult when inputs are more than 15 mmol/kg. The amounts of soluble Al in brown soil are remarkably lower than those in both Paleudult and Kanhapludult, but higher than in Hapludox. The exchangeable acidity in constant charge soils is pronouncedly larger than that in variable charge soils. The influence of inputs of hydrogen ions on exchangeable acidity, on the whole, is small.