Controlled environment studies were conducted to characterize the effects of cropping treatment and degree of particle dispersion on S oxidation rate. In two soils (a Chernozem and a Luvisol), S oxidation rates were not greatly affected by cropping treatment (barley, beans, canola, or fallow). In a second experiment, S oxidation was shown to approach maximum rates at a dispersion level of 1000 g soil g−1 S. Key words: Sulfur, placement, rhizosphere, fertilizer, elementals