Image 1. Sulfur deficiency symptoms in corn (Source:Purdue University).
- Sulfur is a plant-essential element, often classified as a ‘secondary’ nutrient.
- Sulfur deficiency is becoming more common across corn and soybean production areas.
- Soil sulfur tests are unreliable and should be supplemented by tissue testing.
- Several different fertilizer products can be used to correct sulfur deficiency.
The Role of Sulfur in Crop Production.
Sulfur is a plant-essential element, often classified as a ‘secondary’ nutrient because it is needed by the plant in lesser quantities than primary nutrients. Sulfur is a structural component of two major amino acids, cysteine, and methionine, which are essential for the formation of proteins that have numerous roles in the plant. Examples include chlorophyll production, growth and development of cells, carbon and nitrogen metabolism, and energy production.
The primary source of sulfur in the soil is organic matter, with each 1% of organic matter containing about 100 lbs. of sulfur per acre. Organic sulfur must be mineralized via microbial activity to sulfate (SO42-) anions to become plant available. Only about 10% of the sulfur in the soil profile is present as sulfate at any given time. Sulfur mineralization rates are appreciably higher when soil temperatures warm up in late spring and early summer, while very little sulfur is available for crop uptake early in the season. Like nitrate, sulfate anions are repelled by the negatively charged cation exchange sites in the soil and are subject to leaching losses, particularly in sandy soils.
Crop removal rates of sulfur on a per-bushel basis for corn grain and stover are 0.1 lbs. S/ bu. Soybean crop removal rates of sulfur is higher for soybeans because they contain a much higher protein and oil content on a per bushel basis compared to corn. Soybeans require 0.2 lbs. S/bu, while stover needs are an additional 0.2 lbs. S/bu of yield. Unlike corn, soybeans require early season sulfur uptake to form root nodules for nitrogen fixation. Research by Shaun Casteel and others at Purdue University found significant increases in the size, number, and longevity of nodules formed where sulfate-sulfur is applied at 0.4 lbs. S/bu yield goal, two weeks prior to or after planting soybeans. MiField trials conducted from 2018 to 2023, using similar rates of sulfur, resulted in an average yield increase of 6.6 bu/A in corn and 2.6 bu/A in soybeans across 100 locations.
Sulfur Testing and Deficiency.
Higher crop yield trends, fewer sulfur fertilizer applications, and lower atmospheric sulfur deposition rates (due to clean air regulations), have all contributed to a growing occurrence of sulfur deficiency in cropping systems in the United States. Sulfur deficiencies are usually variable within a field and may be associated with areas of sandy texture, low organic matter, and hill tops or side-slopes, where organic matter may be depleted due to erosion.
Sulfur deficiency symptoms resemble nitrogen deficiency in that foliage becomes yellow with interveinal chlorosis (Image 1). However, unlike nitrogen, which is mobile in the plant and deficiencies are observed in older tissues, sulfur deficiency is observed in developing tissues. Tissue testing is recommended for confirmation. If sulfur deficiency is misdiagnosed as nitrogen deficiency, application of nitrogen will make the deficiency symptoms worse.
Soil testing for sulfur is unreliable and has a weak relationship to sulfur needs for crops due to variable organic sulfur mineralization rates, sulfate mobility in the soil, and sulfate accumulation in the subsoil. For this reason, many universities do not recommend sulfur fertilization based on soil testing results.
Tissue testing for total sulfur concentration can be used to determine sulfur needs in a growing crop when used in conjunction with the N:S ratio. This ratio serves as a guideline for determining sulfur sufficiency because nitrogen occurs in corn plant proteins at a ratio of 15:1 relative to sulfur. As an example, for corn, tissue S concentrations less than 0.12% with a N:S ratio greater than 20:1, are considered deficient, while levels above 0.20% and a N:S ratio less than 12:1, are considered sufficient, and values in between are inconclusive.
Sulfur Fertilization.
Several different forms of sulfur fertilizer are suitable for use in field crops. These fertilizers can be divided into two classes: those containing elemental sulfur (S0) and those containing sulfate (SO42-). Application rates of 15-25 lbs. of S/A will be adequate to meet crop needs in deficient situations, depending on soil type and crop being grown.
Elemental sulfur is not immediately plant-available and must be biologically oxidized to sulfate. However, this transformation takes considerable time and is best facilitated by adequate moisture, aeration, and warm temperatures. Oxidation rates are highest at temperatures greater than 85 °F in the late spring and early summer, conveniently coinciding with plant need. Applications of elemental sulfur blended into dry fertilizer applications in the fall are economical and provide adequate time and exposure for sulfur to gradually be made available.
Due to the dynamic seasonal availability and mobility of sulfate, the application of fertilizers containing sulfates should be timed as close as possible to match the sulfur need of the growing crop. Two convenient ways to add sulfates include: blending dry ammonium sulfate (AMS) or gypsum with urea pre-plant incorporated or top-dressed, and blending liquid ammonium thiosulfate (ATS) with urea-ammonium nitrate (UAN) pre-plant incorporated or side dress. Sul-Po-Mag (K-Mag) also provides a cost-effective pre-plant option when blended with dry fertilizer.
In-furrow sulfur applications are not recommended but sulfur can be used in a 2”x2” system. ATS should never be used with starter fertilizers in-furrow and with caution in a 2”x2” placement in coarse textured soils. Sulfates can be added in adequate quantities to starters in 2x2 placements, which provide physical separation from the germinating seed. ATS can potentially be added to pre-emergent herbicide applications but mixing compatibility should be checked and label restrictions followed.
Summary of Management Recommendations for Sulfur.
- Consider the need for sulfur fertilizer in soils with low organic matter and sandy texture.
- Test plant tissue to evaluate sulfur sufficiency in growing plants.
- Apply about 0.1 lbs./bu of yield for corn and 0.4 lbs./bu of yield for soybeans as sulfate-sulfur.
- Apply elemental sulfur with dry fertilizer in the fall to allow adequate time for sulfur to oxidize to sulfate and become plant available.
- Apply sulfate fertilizers pre-plant, side-dress, or top-dress to closely match timing to crop needs.
- The risk of crop injury increases with sulfur placements close to the seed or directly to foliage.