Soil pH is probably the most important, single, parameter characterizing its fertility. For theoretical and practical reasons the soil pH is usually measured in salt solutions of different strength. The most common solution used in Europe is 1M potassium chloride (KCl) in a soil/solution ratio of 1:2.5 (W/V). The ultimate goal of the international projectCopernicusof the European Community is to develop a uniform soil testing program based on the universal 0.01M calcium chloride (CaCl2)‐extraction procedure. In the Czech Republic, Hungary, The Netherlands, and Poland over 2,500 samples of representative soils have been collected and analyzed for soil pH in 1M KC1 and 0.01M CaCl2. Statistical analysis of these data shows a close correlation between pH values in both extracts. In very acid and acid soils, the pH in 0.01M CaCl2is higher than in 1M KCl. In neutral and alkaline soils, there is no significant difference in soil pH measured by both methods. Soil samples from all countries present the same population in respect to the relation between soil pH in KCl and CaCl2.