After the primary macronutrients, nitrogen, phosphorus and potassium, come secondary macronutrients, sulphur, calcium, and magnesium. These secondary nutrients are needed in lower quantities, but just because sulphur is qualified as a secondary macronutrient does not mean it’s not as important as those primaries.
Sulphur deficiency can be diagnosed in wheat and corn for example, by identifying variations in colour throughout the field, says Ross Bender, director for new product development at The Mosaic Company. Adequate nitrogen and sulphur are the two nutrients that give a crop its nice green colour because they’re necessary for chlorophyll production.
Bender explains in this episode of the Soil School that tissue testing can confirm a sulphur deficiency ahead of correcting a problem. A soil test may also be helpful, but since sulphur is mobile in soil, it may be hard to get a representative sample throughout the problem area.
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So, what are the recommended rates for wheat and corn, and what kind of response can we expect? “It depends, and I wouldn’t be an agronomist if I didn’t say that answer,” says Bender. It depends on the expected yield level, and on the crop, as well as other factors, such as soil texture and organic matter. In corn for example, Bender says that for every 10 bushels of corn that’s produced, that crop will take up one unit of sulphur. For soybeans, however, one unit of sulphur is needed for every three to four bushels. Wheat also requires a significant amount, though less so than soybean.
Concerns about sulphur immobilization and deficiency always seem to pop up, especially in Ontario, during cooler springs. Organic matter in the soil releases nitrogen, phosphorus, and sulphur, and in order for those nutrients to be released, there needs to be adequate moisture and warmth. Ideal conditions for plant growth coincide with the ideal conditions for nutrient release from residues. If those environmental conditions aren’t there, the release of those nutrients won’t happen.
Having a well-rounded sulphur fertilizer program will help mitigate some of those less than favourable environmental conditions. One way to do that is to apply a product that includes two forms of sulphur. The first is sulphate, which is available to plants in its immediate form. The second is elemental sulphur, which has to undergo oxidation by micro-organisms in order for it to be readily available to the plant, which takes time, but provides a back-up if heavy rains push the soluble sulphate form deep in to the soil profile, away from plant roots. It also provides that backup source of sulphur during grain-fill time.
Yield responses to sulphur can be quite phenomenal, if soil pH is right, and the nitrogen, phosphorus, and potassium are there, says Bender. The complete package of nutrients needs to be in the soil.