GEMAS: Spatial distribution of the pH of European agricultural and grazing land soils

During 2008 the GEochemical Mapping of Agricultural Soils (GEMAS) project collected 2108 agricultural (ploughed soil, Ap horizon, 0–20 cm) and 2023 grazing land soil samples (Gr, 0–10 cm) evenly spread over 33 European countries and covering an area of 5.6 million km2. The pH of all samples was determined by one single laboratory applying a 0.01 M CaCl2 extraction, and following a strict quality-control procedure. The resulting pH-value distributions for European Ap and Gr soil are both bimodal. Broad acidic modes, with pH between 4 and 6, and sharp alkaline modes, with pH between 7 and 8 due to the Ca2+ buffer system, are clearly separated. The European median pH is 5.8 for the GEMAS Ap soil samples and 5.5 for the GEMAS Gr soil samples. According to the pH distribution maps, Europe is separated into two main zones: northern Europe with generally lower pH values (Ap: 5.2, Gr: 4.8), dominated by acidic soils occurring in Fennoscandia, and southern Europe with higher pH values (Ap: 6.3, Gr: 5.9), dominated by carbonate rich soils. The separation line coincides with the southern border of the sediments of the last glaciation. The dominant factors controlling pH at the European scale are thus geology (crystalline bedrock) in combination with climate (temperature and precipitation). The GEMAS pH maps mainly reflect the natural site conditions on the European scale, whilst anthropogenic impact is hardly detectable. The GEMAS results provide a unique set of homogenous and spatially representative soil pH data for the continent. The data set defines a dependable continental-scale background, and offers the possibility to calibrate studies on more detailed scales.