A Discussion of the Changes in Soil Acidity due to Natural Processes and Acid Deposition

Abstract

It is generally accepted that in extremely acid soils, soil solution acidity can change rapidly in response to inputs of sulphate due to the “salt effect”. However, there is little evidence of rapid change in bulk soil acidity due to acid deposition, except where acid inputs are extremely high. Over many decades, soil acidity can be increased due to uptake of base cations by the Vegetation, by humus formation, and by natural as well as acid-deposition-enhanced soil leaching wherever annual mean precipitation exceeds mean annual evaporation. The role of acid deposition in accelerating soil acidification rates can be estimated from elemental budget approaches, but the actual changes in soil acidity due to acid deposition are difficult to document because so few studies combine the measurement of element and proton fluxes with long-term periodic remeasurements of soil acidity. Most empirical observations to date show little change in forest soil acidity from any cause in less than 20 to 30 years (cf. Tyler et al., this volume). However, some increases in exchangeable or soluble Al3+ without concomitant reductions in exchangeable base cations have been noted at the Solling site in West Germany. These Al3+ releases may be due to dissolution of interlayer Al3+ in 2:1 clays. All attempts to monitor long-term changes in soil acidity need to take into account the possibility of seasonal variations due to annual cycling of base-cations by Vegetation. The effects of seasonal uptake and return of base cations by Vegetation upon the base cation Status of surface soils can be considerable and might be confused with long-term trends if not properly accounted for.