Several methods for measuring sulfite oxidases (EC 126.96.36.199 and EC 188.8.131.52) have been developed for microbial systems, but none have been systematically adopted for soils. We describe an assay method to measure sulfite oxidase in soil that is based on the use of potassium sulfite (K(2)SO(3)) serving as the substrate and electron donor, and potassium ferricyanide [K(3)Fe(CN(6))] serving as the electron acceptor. Sulfite oxidase activity is expressed as micromoles of K(3)Fe(CN)(6) reduced g(-1) soil 4 h(-1). Sulfite oxidase activity was linearly related to amount of soil in the assay (up to 3 g) and incubation time (up to 5 h). The average K (m) value for K(2)SO(3) serving as the substrate for the sulfite oxidase reaction in four diverse soils was 3.48 mM. The activation energy values calculated for soil sulfite oxidase ranged from 40.4 to 61.4 kJ mol(-1). The sulfite oxidase assay does not require any specialized equipment and is expected to be useful in studying biological sulfite oxidation in soils treated with sulfites or in naturally occurring soils where sulfur cycles between reduced (i.e., sulfite) and oxidized (i.e., sulfate) states due to flooding and drying.