The determination of adsorbed sulfate with isotopic dilution of sulfur (S-35) compared with calcium dihydrogen phospate extraction

A method using isotopic dilution of carrier-free sulfur (S-35) for determining adsorbed sulfate (SO42-) in soil samples from B horizons of podsolized soils was compared with an extraction of SO42- by 10 mM calcium dihydrogen phosphate [Ca(H2PO4)2]. The two methods gave comparable results for flesh soil samples while the extraction method resulted in significantly higher amounts of adsorbed SO42- when using air-dried samples. Since the S-35 method assumes equilibrium between adsorbed SO42- and SO42- in solution, these results are evidence of complete exchangeability of adsorbed SO42- extracted with Ca(H2PO4)2 in fresh soil samples. It was thus concluded that the extraction method is a useful method when determining adsorbed SO42- in equilibrium with the soil solution in these soils. The S-35 reached equilibrium with the soil solution within 2 h. The difference between the methods when using air-dried samples indicates that part of the adsorbed SO42- is irreversibly bound to the soil during air-drying. Furthermore, it is recommended that the amount of SO42- determined in an extraction with water should be expressed as an intensity variable, i.e., SO42- activity, rather than a capacity variable.