RELEASE OF AVAILABLE SULFUR FROM FERTILIZERS

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Abstract
The rate of release of plant-available sulfur (S) from two soluble sources (thiosulfate, and sulfate) and four elemental S forms (S powder, S-coated urea, S suspension, and S-impregnated urea) was determined in a growth chamber study, using five successive harvests of rapeseed as the test crop. The soluble S sources were plant-available shortly after application. Thiosulfate was rapidly oxidized, and was comparable to sulfate in its short-term release pattern. The elemental S forms, however, showed widely different rates of release, apparently related to the initial particle size of the product and the dispersion of the particles within the soil. Finely-divided S suspension was rapidly oxidized, and the product was completely converted to sulfate by the third harvest. Relatively coarse S powder was oxidized more slowly and in some cases was not completely converted to sulfate even after 124 days. S-impregnated urea, which yields a pocket of finely divided S particles upon urea dissolution, showed negligible initial oxidation but rapid oxidation after the soil was mixed. S-coated urea produced minimal amounts of sulfate over the five harvests and could not be considered a viable fertilizer S source. Key words: Sodium sulfate, sodium thiosulfate, S-coated urea, S-impregnated urea, S oxidation, elemental S