Fate of Seed-Row Placed Sulfur Fertilizers in Saskatchewan Soils.

Sulfur (S) fertilizers are typically recommended for canola crops and occasionally recommended for cereal and pulse crops on the Canadian prairies. Sulfur sources include soluble ammonium sulfate (AS) and potassium sulfate (PS); slightly soluble calcium sulfate (gypsum); and reduced forms including elemental sulfur (ES) and liquid ammonium thiosulfate (ATS) that undergo oxidation in the soil. Our objective was to compare S fertilizer forms, and in combination with monoammonium phosphate (MAP) fertilizer, for their effects on residual soil S in three Saskatchewan prairie soils. Soil extractable sulfate (SO42-) was evaluated in cores extracted from the seed row at 7, 28 and 56 days after being seeded to canola (Brassica napus) and yellow pea (Pisum sativum) in a Brown and Gray-Black Chernozem and a Gray Luvisol soil in Saskatchewan, Canada. The five S fertilizer forms, with and without the addition of MAP fertilizer, were applied in the seed row at rates of 20 kg S ha-1 and 20 kg P2O5 ha-1. All soils were considered marginally deficient in plant available SO42-. Slightly soluble gypsum form of S resulted in significantly (p?0.10) higher soil extractable SO42- on day 7 in the Brown and Gray-Black Chernozem and Gray Luvisol soils, seeded to canola and peas compared to the soluble AS form. Soil extractable SO42- in the slightly soluble and liquid ATS in canola crop at the Brown Chernozem was 3 and 5 times higher, respectively, than the soluble AS form on both days 7 and 56. Addition of MAP to all the S forms in canola cropped soil resulted in a decrease in soil extractable SO42- on day 7, compared to S alone treatments in the all three soils. Ammonium thiosulfate and gypsum were most effective in substantially increasing soil extractable S in canola and peas at all three soils.