Sulfur (S) is one of the 17 elements essential for plant growth, and the fourth most important after nitrogen (N), phosphorus (P) and potassium (K) in terms of amount required by crops. When considering nutrient sources, elemental S is the most concentrated form of S, making it appealing due to lower transportation costs (per lb. of nutrient) when compared to sulfate-based fertilizers. Elemental S is not taken up by crops in the applied form, and must be converted in the soil to sulfate S before uptake by plants or microorganisms.

Oxidation of elemental S in soil is a microbial process requiring the presence of both water and oxygen. The converted sulfate can be taken up by crops, taken up by microorganisms in the soil, or leached below the root zone in coarse-textured soils or high-rainfall areas. There is a wide variety of microorganisms in soil that are capable of oxidizing elemental S, including both bacteria and fungi; oxidation is not solely dependent on the presence of specific S-oxidizing organisms.

Occurring more rapidly in warm, moist soils with high organic matter contents, oxidation reactions of elemental S are also faster in alkaline soils than in acidic soils. Of the soil and environmental factors affecting oxidation rate, temperature and soil pH have the greatest effect.

Elemental S oxidation is also affected by the characteristics of the fertilizer, including the particle size of elemental S and/or its concentration in the fertilizer.

Oxidation is a surface-based process, and surface area increases dramatically as particle size decreases; therefore, particle size is one of the most important attributes affecting oxidation. When elemental S is dispersed throughout the soil, the oxidation of elemental S is faster as the particle size of the elemental S decreases. In co-granulated elemental S fertilizers (i.e., in which elemental S particles are co-granulated with macronutrients [N, P, K]), the oxidation is reduced compared to the elemental S particles of the same size dispersed through soil. This is not because the macronutrients reduce the oxidation rate, but because of the reduction in surface area of elemental S available for oxidation when dispersed in soil. Therefore, the lower oxidation rate of co-granulated elemental S can be explained by a reduction in the surface area of S in contact with the soil.

MicroEssentials^® fertilizer ensures season-long sulfur availability by supplying both elemental and sulfate sulfur in the same granule. When MicroEssentials is applied to the soil, the sulfate sulfur is already in the proper form to feed the plant. Fertilizer granules rapidly dissolve and move sulfate-sulfur into the root zone for early-season development. Throughout the growing season, soil microbes convert the elemental sulfur into sulfate-sulfur for season-long feeding.

As mentioned earlier, the smaller the size of elemental sulfur particles, the easier it is for the microbial population to oxidize it into sulfate-sulfur. With other sulfur products that have larger particles, it may take multiple years for that elemental sulfur to break down and become available to the plant. However, MicroEssentials’ Fusion^® technology provides small, evenly distributed pieces of elemental sulfur that allow for mineralization throughout the growing season. This creates the best of both worlds — the sulfur the plant needs early from the sulfate is immediately available, and the microbes oxidize the elemental sulfur throughout the season, to provide the gradual feeding.

For more information about elemental and sulfate sulfur, visit CropNutrition.com.

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