CHANGES IN SOIL ACIDITY AND RELATED VARIABLES OVER 25 YEARS IN THE NORTH PENNINE UPLANDS, UK

Soil profiles, first sampled between 1963 and 1973, were resampled in 1991 in an upland area with modertely high deposition of pollutants. O ne hundred horizons from 32 profiles, representing 10 different soil s ubgroups were analysed for pH and seven variables related to pH, using the same laboratory methods on both sampling occasions. To allow comp arisons to be made with results obtained with these old methods, analy sis of the 1991 samples was repeated for some determinands using the m ethods currently used in the analytical laboratory. Organic and A hori zons show a consistent increase in acidity between samplings. Although brown soils and lithomorphic soils have increased in acidity througho ut their depth, gleys and podzols have decreased in acidity at depth, probably because of poor water transmission downwards into these horiz ons. Correlations with other determinands suggest that the dominant pr ocess in the soils is leaching of basic cations and their replacement on exchange sites by protons and probably aluminium ions. A cause of t he increase in soil acidity is likely to be the deposition of atmosphe rically transported pollutants.