• Sulfur is a plant-essential element, often classified as a 'secondary' nutrient.
  
• Sulfur deficiency is becoming more common across the Midwest.
  
• Soil sulfur tests can be 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 in lesser quantities than primary nutrients.  Sulfur is a structural component of two major amino acids, cysteine and methionine, which are essential in plant primary and secondary metabolism.  Sulfur functions in a wide array of physiological processes, including growth and development of cells, photosynthesis, carbon and nitrogen metabolism, and protein synthesis. 

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 (SO₄^2-) 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.  Similar to 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 are presented in Table 1.  A 200-bushel corn crop would remove about 14 lbs of S per acre in harvested grain and another 7 lbs S per acre as stover to be recycled.  By comparison, a 60-bushel soybean crop would remove about 6 lbs of S per acre in harvested grain and about 3 lbs S per acre as stover to be recycled. Sulfur needs in a high yield corn-soy rotation would be 20 lbs S per acre grain removal and about 30 lbs S per acre total need.

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 Midwestern US.  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, a yellowish appearance to the foliage and interveinal chlorosis (see Image 1).  These symptoms are easily confused with other deficiencies, so tissue testing is recommended for confirmation.  Sulfate is less mobile in the plant than nitrogen, so sulfur deficiency symptoms will be more uniform over the plant.  If misdiagnosed as nitrogen deficiency, application of nitrogen will make the deficiency symptoms worse.

Soil testing for sulfur in the plow layer is generally considered to be unreliable and has a weak relationship to sulfur sufficiency for crops, largely due to variable organic sulfur mineralization rates, sulfate mobility, 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 sufficiency in a growing crop, when used in conjunction with the N:S ratio.  Because nitrogen occurs in corn plant proteins at a ratio of 15:1 relative to sulfur, this ratio serves as an additional guideline for determining sulfur sufficiency.  As an example for corn, tissue S concentrations less than 0.12% and N:S ratio greater than 20:1 are considered deficient, while levels above 0.20% and 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 (see Table 2).  These fertilizers can be divided into two classes:  those containing elemental sulfur (S⁰) and those containing sulfate (SO₄^2-).  Generally, application rates of 15-25 lbs of S per acre will be adequate to meet crop needs in deficient situations, depending on soil type and crop grown.

Elemental S is not immediately plant-available and must be biologically oxidized to sulfate.  This transformation takes time and is best facilitated by adequate moisture, aeration, and warm temperatures.  Oxidation rates are highest at temperatures in excess of 85 degrees 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.  Elemental sulfur increases soil acidity, necessitating careful monitoring of soil pH.

Due to the dynamic seasonal availability and mobility of sulfate, 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) with urea pre-plant incorporated, or blending liquid ammonium thiosulfate (ATS) with urea-ammonium nitrate (UAN) pre-plant incorporated or side dress.  Sulpo-mag and gypsum also provide cost-effective pre-plant options when blended with dry fertilizer in the spring.  

In-furrow sulfur applications are restricted to a few pounds due to the cumulative risks of ammonium and salt index, which may damage seedlings.  Sulfates can be added in adequate quantities to starters in 2x2 placements, which provide physical separation from the germinating seed.  AMS or ATS can potentially be added to pre-emergent herbicide applications, but mixing compatibility should be checked and label restrictions followed.  Caution should be taken to limit sulfur in either dry or liquid post-emergent applications, as sulfur products will increase risk for crop injury.

Summary on 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 15-25 lbs of sulfur per acre to meet crop need in deficient environments. 
  
• 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 or side-dress to closely match timing to crop needs. 
  
• Risk of crop injury increases with sulfur placements close to the seed or directly to foliage.  

Contact your FS Crop Specialist for your agronomic needs.

Table 1

Table 3

References:

Image 1.  Sulfur deficiency symptoms in corn (Source:  Purdue University).
Table 1.  Sulfur removal by crop.  (Source:  Iowa State University.) 
Table 2.  Common fertilizer sources of sulfur (Source:  Purdue University).