Inventory and assessment of pH in cropland and grassland soils in Germany

Background Soil pH is one of the key factors affecting soil fertility. For optimal plant growth on agricultural land, suitable liming management is recommended. The basis of appropriate soil pH management in agriculture is provided by recommendations on site-specific optimum pH ranges and lime application. Aims This study determined the current variability of soil pH of cropland and grassland in Germany, analysed the factors associated with this variability, evaluated pH management on German agricultural land and developed a national map of current pH values in agricultural soils. Methods The study mainly focused on the topsoil layers of 2197 cropland sites (0–30 cm) and 812 permanent grasslands sites (0–10 cm) in the German Agricultural Soil Inventory, but reports soil pH (CaCl2) down to 50 cm. Random forest models were applied to identify environmental and management factors related to soil pH and develop a map of pH values in agricultural topsoils in Germany. The current pH status of the sampled sites was classified in accordance with German lime requirement recommendations. Results The soil pH of both land-use types showed a bimodal distribution, with peaks at pH 6.1 and 7.3 for cropland and pH 5.0 and 7.1 for grassland. Soils with pH <5 were mainly found on sandy soils in northern Germany and acidic parent material in southern Germany. Alkaline soils predominated in central and southern Germany on loess and carbonate-rich material. The carbonate content was the major factor explaining soil pH variability, but its effect was restricted to pH values >6. Only 35.4% of the cropland sites and 23.9% of the grassland sites were within the recommended optimum pH range. The percentage of sites below the recommended pH range was greater for grassland than for cropland (52.0% and 41.0%, respectively). Conclusions Soil pH of German agricultural land is influenced by natural factors and pH management. The first national inventory of soil pH showed that many agricultural soils exhibited pH values lower than those recommended. More conscientious pH management could help improve soil fertility and crop yield. This should be based on the regular topsoil pH analysis conducted by most farmers every five years, together with the determination of plant-available potassium and phosphorus in soils.