Soil response to acid input in a titration experiment

Authors

• Helinä Hartikainen Department of Applied Chemistry and Microbiology, SF-00014 University of Helsinki, Finland

DOI: https://doi.org/10.23986/afsci.72470

Abstract

Cultivated surface soil samples of an acid Gleysol (soil 1, pH 4.9) and a slightly acid Podzol (soil 2, pH 6.7) were equilibrated for 48 h with oto 144 meq H+ kg-1 by a batch technique designed to simulate reactions of acid load with soil constituents. The pH of the titration suspensions ranged in soil 1 from 5.6 to 3.3, in soil 2 from 7.2 to 4.7. The exchange reaction with base cations on variable charge sites was an important mechanism for H+ inactivation. The quantities of cation equivalents released were, however, lower than the proton equivalents added. Calcium dominated the supernatant solutions, but as related to exchangeable reserves. Mg seemed to be more susceptible to acidification at high soil pH. Protons were also consumed in the mobilization of divalent base cations from a non-exchangeable pool to an exchangeable one. The experimental soils differed in their response of acid cation fractions to proton loading. In the rather neutral soil 2, the quantities of soluble and exchangeable acid cations were very low and not affected by acidification. The Al dissolved by proton attack was immobilized by complexation reactions. This mechanism did not operate in the acid soil 1 where the proton loading markedly increased the exchangeable Al pool and, consequently, the soluble Al in the supernatant solution. This was associated with a simultaneous reduction in the complexed Al and a small increase in complexed Fe. Furthermore, acidification diminished the effective cation exchange capacity (ECEC) decisively less in soil 1 than in soil 2, because the increase in exchangeable Al markedly compensated the reduction in the exchangeable base cations. As compared to freelydrained systems, the batch titration overestimated the release of Al to solution phase.