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Modeling Soil Response to Acidic Deposition in Nonsulfate Adsorbing Soils¹

ABSTRACT

A simple semiempirical model for the prediction of changes in soil pH and base saturation with acidic deposition was developed. In steady-state ecosystems acidic deposition results in losses of basic cations from exchange sites. In the model, depletion of exchangeable bases is calculated from the difference between input acidity and output of H⁺ and Al³⁺ in water percolating through the solum. A correction for the decrease in bicarbonate weathering is made but sulfate adsorption is not considered. Estimates including the effect of increased mineral weathering with decreased pH can be made if the natural weathering rate is known and if the order with respect to H⁺ is known for the rate expression for soil weathering. The model predicted laboratory data for the acidification of samples for three soil horizons. For a fourth horizon, that was formed in contact with bedrock and high in weatherable minerals, the model overestimated the effect of the acid. Model predictions using soil data showed a slow decrease in soil pH and base saturation until the pH region of Al buffering was attained and then a new steady-state was achieved. The pH and base saturation in the Al buffering region was similar to that found in very acid soils under forest vegetation.

Key Words: cation exchange buffering • aluminum buffering • weathering

¹ Scientific Journal Series Paper no. 14277, Minnesota Agric. Exp. Stn., Univ. of Minnesota, St. Paul, MN.

² Associate professor and professor, Dep. of Soil Science, Univ. of Minnesota, St. Paul, MN 55108.

Received for publication October 18, 1984.

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