Identification of potential acid sulfate soils at the Reda River mouth (northern Poland) using pH measurements

The definition of sulfidic material (in Polish: material siarczkowy) was first introduced in the 6th edition of the Polish Soil Classification in 2019, but the methods of identifying potential acid sulfate soils (PASS) using pH measurements had not earlier been applied in Poland. Therefore, the aim of this paper is to compare two simple tests approved as international standards and recommended in the Polish Soil Classification - incubation method and hydrogen peroxide tests - to identify PASS in three soil profiles in the Baltic coastal area (Reda River mouth, northern Poland). In the first test, soil pH (1:1 soil-to-water suspension) was determined in newly taken soil samples and after incubation for 8 weeks at room temperature. Initial pH values varied from 5.5 to 7.0, and after 8 weeks of incubation dropped below pH 4 in two of the three soil profiles. In the second test, freshly collected samples, after initial pH measurement, were oxidised in 30% hydrogen peroxide. Application of this strong oxidising agent drastically decreased pH values to below 2.5 in all three soil profiles. The pH measurements both after incubation and after application of hydrogen peroxide allow for an unequivocal identification of the sulfidic material, thus confirming the presence of PASS in the Reda River mouth area. Both tests based on pH measurements are easy to perform and low-cost, which implies their common application in PASS recognition. However, careful attention should be paid to some limitations of the hydrogen peroxide test - mainly, the possible overestimation of acidity in organic samples. This procedure should therefore be considered only as complementary. Because of the dominance of organic PASS materials in Polish soils, a modification of the pH criterion for the sulfidic material was proposed in accordance with the approach used in the Finnish-Swedish classification.