Sulfur is an essential plant nutrient and is considered a macro-nutrient along with nitrogen, phosphorous, potassium, magnesium and calcium (and carbon, hydrogen and oxygen, which are supplied by air and water). Most soil sulfur is contained in soil organic matter and becomes available to plants as soil organic matter decomposes. Sulfur is taken up by plants primarily as sulfate ions (SO42-). As an anion, sulfur in the soil water solution is vulnerable to leaching, like soil nitrate (NO3-). Plant available sulfur doesn’t stick around long. It is either taken up by plants or leaches below the root zone. Plants can also absorb sulfur directly from the atmosphere.
In the past, deposition from industrial “smokestack” emissions contributed significant amounts of sulfur to Michigan soils, plenty for normal plant growth and development. With the increased regulation of emissions, sulfur deposition from the air has been greatly reduced. Soils that receive applications of livestock manure are generally richer in sulfur than those that don’t. Sulfur content of livestock manures varies based on species, age and feeding practices.
Sulfur fertilization studies from 1957-1984 were summarized by MSU professor Don Christianson. At that time, results indicated that sulfur fertilization was not needed on Michigan crops in most situations. Coarse-textured soils low in organic matter were an exception.
Studies in Wisconsin have resulted in data showing significant yield responses to sulfur fertilizer on alfalfa grown on well-drained, low organic matter (1.1 to 2.8 percent) soils. Sulfur fertilization trials on canola in the Upper Peninsula during the late 1990s indicated a significant yield response resulting from applications of 10 pounds sulfur per acre applied as ammonium sulfate. These results suggest that soil sulfur levels can be limiting to plant performance on certain soils. Although a sulfur soil test is available, the results are difficult to interpret. Tissue analysis is a much more dependable test for sulfur deficiency. A table of nutrient sufficiency ranges based on plant tissue concentrations for plant nutrients including sulfur can be found in MSU Extension publication E-486, Secondary and Micronutrients for Vegetables and Field Crops.
Michigan crops vary widely in their sensitivity to sulfur deficiency. Crops more vulnerable to sulfur deficiency include alfalfa, potatoes, dry beans, canola and field peas. However, any crop can become sulfur deficient if inadequate soil sulfur is available. Sulfur deficiency symptoms include light green color, similar to nitrogen deficiency. Application of 20 to 40 pounds of sulfur per acre will correct sulfur deficiency. Soluble sources of sulfur include potassium sulfate, potassium magnesium sulfate, Epsom salts, ammonium sulfate and gypsum. For most soils, a sulfur application will be sufficient for two to three years. On very sandy soils low in organic matter, more frequent application may be needed.