Sulfur Fertility Management
While sulfur is classified as a secondary nutrient, nutritional shortages of sulfur are more common than potassium in many areas. While sulfur deficiencies are not as prevalent as nitrogen and phosphorus deficiencies on sandy, well drained, or low organic matter soils, sulfur often limits wheat production efficiency and profitability.
Sulfur is an essential constituent of several amino acids as well as other plant constituents and processes. Often, sulfur deficiency in small plants is mistakenly identified as nitrogen deficiency. Sulfur deficient wheat will exhibit a general yellowing and stunting which is also typical for a nitrogen shortage. On older plants, the lower leaves of nitrogen deficient plants will die as N is redistributed to the younger plant parts, while the lower leaves on sulfur deficient plants will remain a pale green.
Determining S Application Rate
A routine soil test is available for sulfur, but it is of questionable value for determining fertilizer sulfur needs for wheat. High sulfur soil test values indicate additional sulfur is likely not needed, but low sulfur soil test values tell very little. The sulfur soil test may aid in determining sulfur needs, but factors such as soil texture, soil organic matter content, and yield potential are much more useful.
The form of sulfur used by plants is the sulfate ion (S04). Since sulfate is very soluble in water, it is subject to leaching, especially in sandy soils where water moves through the soil much more freely than in medium-fine textured soils. The higher the precipitation, the greater the likelihood that sulfur may limit wheat growth and development. Most of the sulfur in soils is present in soil organic matter. When soil microbes decompose soil organic matter, sulfate sulfur is released. As a result, sandy or low organic matter soils are most prone to sulfur deficiency. The higher precipitation areas of the soft red winter wheat area are also more prone to sulfur deficiencies.
Application Method / Sulfur Source
In general, sulfur sources should perform equally if they contain sulfate sulfur. Elemental sulfur sources are generally not well suited for wheat production since they require biological oxidation to convert elemental sulfur to plant available sulfate sulfur. Since wheat is a cool season crop, growth and development largely occurs when soils are cold and microbial activity is low. As a result, only minimal oxidation of elemental sulfur typically occurs. If elemental sulfur is used, it should be broadcast and incorporated early in the fall when soils are relatively warm.
Since sulfate sulfur is mobile in soils, there are several application options available. For topdress applications, ammonium sulfate and ammonium thiosulfate are commonly used for dry and liquid programs, respectively. Band applications of liquid N-P-S products have performed well for preplant and at planting, and homogeneous products are recommended for drill-row applications. Dry bulk blended materials have the possibility of segregating as the grain drill bounces across the field.