Potential Sources of Exchangeable Acidity in Strongly Acid Soils (pH_KCl < 3.3) and Validation of Its Determination

Exchangeable acidity in samples from organic and mineral horizons of taiga and bog soils with strongly acid reaction (pH_KCl < 3.3 and \({\text{p}}{{{\text{H}}}_{{{{{\text{H}}}_{2}}{\text{O}}}}}\) ≤ 4.4) was determined by the Sokolov method of potentiometric titration of KCl extract before and after the addition of a sodium fluoride solution. In the same extracts, the contents of Al, Fe, and Mn ions were determined by the atomic emission method. NaF was added in an amount sufficient for binding all metal ions into fluoride complex compounds. It was found that the content of metals determined by atomic emission is lower than that calculated from titration results in 1.5–2 times in organic horizons and by 20–25% in mineral horizons. Even a higher difference between the two methods was observed for exchangeable protons: from the titration data, their content in organic horizons was 10–12% of the content determined as the difference between total exchangeable acidity and total metal ions determined by atomic emission, and 3–15% in mineral horizons. The revealed differences were attributed to the underestimated results of determining exchangeable protons in the presence of NaF, which was confirmed by the potentiometric titration of model solutions of strong and weak acids prepared from a KCl solution. It was found that the addition of NaF during the titration with a strong acid results in reaching the point of equivalence after the addition of a smaller amount of base. Reasons for the shift of the point of equivalence during titration with a strong acid in the presence of NaF require special investigations. It is recommended to estimate the exchangeable acidity of strongly acid soils due to metal ions by atomic emission or other adequate methods.