Both the international ISO standard and modern soil classifi cations 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 pH1:2.5 to pH1:5 is necessary, e.g., for soil database construction or long-term soil quality monitoring, the following equations are suggested: pHH2O 1:5 = 0.14 + 0.99*pHH2O 1:2.5 and pHKCl 1:5 = 0.09 + 1.00*pHKCl 1:2.5, respectively. When the direct conversion of pHKCl 1:2.5 to pHH2O 1:5 is required, a simple logarithmic model offer precise and reliable transformation: pHH2O 1:5 = -1.95 + 11.58*log10(pHKCl 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.
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