Conversion of Soil pH 1:2.5 KCl and 1:2.5 H2O to 1:5 H2O: Conclusions for Soil Management, Environmental Monitoring, and International Soil Databases

Both the international ISO standard and modern soil classifications and databases require soil pH measurement at a 1: 5 soil: solution ratio, while the ratio 1:2.5 is still the most commonly used in Poland and other European countries. The transformation of laboratory practices is necessary, but it is also necessary to establish and validate a reliable procedure for converting soil pH at soil: solution ratios of 1: 5 and 1: 2.5. Based on 200 soil samples representing typical soil types and soil properties of southwest Poland (including arable and forested areas, both in the lowlands and mountains), a general conclusion was derived that pH values measured at soil: solution ratios 1: 2.5 and 1: 5 in distilled water and KCl solution, respectively, have nearly identical values and do not require conversion in most practical applications. If precise conversion of pH 1: 2.5 to pH 1: 5 is necessary, e.g., for soil database construction or long-term soil quality monitoring, the following equations are suggested: pH(H2O1:5) = 0.14 + 0.99*pH(H2O) (1:2.5) and pH(KCl1:5) = 0.09 + 1.00*pH(KCl 1:2.5), respectively. When the direct conversion of pH(KCl 1:2.5) to pH(H2O 1:5) is required, a simple logarithmic model offer precise and reliable transformation: pH(H2O 1:5) = -1.95 + 11.58*log(10)(pH(KCl 1:2.5)). This model makes the archival records still useful, both for international soil classifi cations, background data in the long-term measurement series, and as input data for modern international soil databases.