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Soil Response to Acid Percolation: Acid-Base Buffering and Cation Leaching

Department of Applied Chemistry and Microbiology, Univ. of Helsinki FIN-00014 Univ. of Helsinki, Finland.

^* Corresponding author (helina.hartikainen{at}helsinki.fi).

ABSTRACT

A gradual soil acidification was simulated in a percolation experiment with two cultivated soils differing markedly in their initial pH. Soil samples were repeatedly eluted with water or 0.002 M H₂SO₄, and the changes in the eluate and soil chemistry at various stages of soil acidification were monitored. Ion exchange properties responded to acid load more readily than did the eluate chemistry. Cation exchange at variable charge sites and dissolution reactions releasing from the nonexchangeable pool were important mechanisms consuming solution H⁺ ions. Alkali and alkaline earth saturation degree that was related to the potential cation exchange capacity (AAES_pot) decreased in conformity with acid load, whereas that related to the effective cation exchange capacity (AAES_eff) did not. In terms of eluate chemistry, two stages of acidification were identified in a circumneutral acid Podzol (CaCl₂, pH 6.7). In the first stage, acidification proceeded with small changes in pH as long as buffering by variable charge exchange sites was available (above soil pH 5). In the second stage, percolate pH dropped drastically and the leaching of Al, Fe, and Mn was intensified because of a marked increase in their exchangeable pool. An acid Gleysol (pH 4.9) showed only the feature of the second stage of acidification. The acid-induced losses of cations in the percolation were markedly lower than in a previous titration experiment. Magnesium was more sensitive to acid than Ca. The monovalent cations were only slightly affected. Response of Mn to acid was similar to the alkaline earth cations, whereas Al was not substantially displaced. Above pH 4.7, complexation by humus reduced the leaching of Al, but below this pH complexed Al began to decrease.