The rate of elemental sulfur (So) oxidation in soils may be affected by the source of So. This study was conducted to determine the effect of So source on So oxidation. Liquid culture studies with Thiobacillus thiooxidans and T. thioparus, laboratory incubations with a Holloman soil (loamy, gypsic, thermic, shallow Typic Torriorthent) and a Belen soil (clayey over loamy, montmorillonitic [calcareous], thermic Typic Torrifluvent), and a field study on the Belen soil were conducted. Periodic measurements included pH as a measure of So oxidation to H2SO4, electrical conductivity (EC) as a measure of dissolution of CaCO3 from H2SO4 production, and available Fe, Zn, P, and SO4 at the end of soil experiments as a measure of So-oxidation effects on nutrient availability. The So sources included pure So (Sp), Wettable S (Sw), flowable So (Sf), and Disper-Sul So (Sd). In liquid culture, Sw, Sf, and Sd inhibited or slowed So oxidation, compared with Sp, suggesting an ingredient of the commercial So sources was responsible for oxidation inhibition. Oxidation of Sw, Sf, and Sd were similar to Sp oxidation in laboratory soil incubations and in the field. Results indicate that the So source is not the cause of slow So oxidation in some New Mexico soils. Chemoautotrophic So oxidizers were present in unamended soils and their numbers increased in So-amended soils. Although So amendment (25 g So kg−1) in the field decreased pH approximately 0.5 unit and increased SO4 tenfold, So amendment did not increase available Fe, Zn, or P. The slow rate of So oxidation in some New Mexico soils and failure of So amendment to increase nutrient levels are attributed to the buffering capacity of the calcareous soils.