Elemental sulfur (S⁰) is an important source of S to produce fertilizers containing high concentrations of N, P and S. This form of reduced S can only become available to plants once oxidized by soil microorganisms to sulfate (SO₄^-2). Four Brazilian soils varying in clay content, responsive to S and limed to pH 5.0 (1:2.5 soil/water), were used in a greenhouse experiment to study the effectiveness of different S sources in supplying SO₄^-2 to corn crops along the time. The experiment utilized a control for each soil, three sources of granular (2 to 4 mm) S (ammonium sulfate - AS, S⁰ and micro essential 15 - MES15), one rate of sulfur (25 mg Kg^-1) for three soils, and three rates of sulfur (12.5, 25 and 50 mg Kg^-1) for one soil. The MES15 is a sulfur source containing 15% of S (7.5% as S-SO₄^-2 and 7.5% as S-S⁰). The above ground plant tissue was harvested in five consecutive crops of about 45 days each, dried at 40^oC and weighted for dry-matter yield. Sub samples were analyzed for total P and S. The effectiveness of the S⁰ source was very low at the first crop but tended to increase with time, suggesting the oxidation of S⁰. The MES15 showed to be a good source of S to plants, due to the likely uptake of readily available S-SO₄^-2 and oxidized S-S⁰ in initial and final crops, respectively. Our results suggest that whereas the use of the S⁰ source may be a problem due to low initial oxidation, MES15 can present high effectiveness with time, due to a more adequate release of S to plants. The oxidation varied with soil type and a preliminary test maybe necessary to predict S⁰ oxidation potential.