Considering Sulfur in a Winter Wheat Fertility Program

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Considering Sulfur in a Winter Wheat Fertility Program

Figure 1. Symptoms of S deficiency in wheat include yellowing of young foliage, while the older foliage remains green. This is the opposite of what is expressed in N deficient plants. Photo UNL Cropwatch https://cropwatch.unl.edu/soils/nutrient-deficiency-wheat
  • Sulfur deficiency is becoming more problematic in high-yielding wheat production systems 
  • Sulfur issues are often misdiagnosed as nitrogen deficiency 
  • Proper tissue sampling is the best way to detect sulfur issues in your wheat crop
  • Sulfur applications in the Spring can improve yields in sulfur-deficient wheat fields
     

Wheat coming out of dormancy in spring, sometimes has a “yellow” appearance.  A frequent assumption by growers is this chlorosis is due to a lack of nitrogen.  However, another vital nutrient may be the issue: sulfur (S).  This article will discuss S deficiency in wheat, symptomology, causes, and potential mitigation strategies.  

Why is S important?
Sulfur plays an important role in building structural proteins, enzymes, and chlorophyll.  Milling analyses also show that wheat with sufficient S contain greater wheat grain protein levels.  Pollution mitigation has substantially reduced S emitted from factories and automobiles.  This means that less S is present in the atmosphere, and as a result, the nutrient can occasionally be a limiting factor in wheat production systems.   

When and where would we expect to observe S deficiency?  
Wet conditions and sandy soils are often where S deficiency is observed in wheat fields.  This is because S is mobile in water and these conditions favor movement of the nutrient deeper into the soil profile.  In addition, symptoms typically are observed shortly after green-up because organic S has yet to be adequately mineralized and root systems are not yet large enough to access S located deeper in the soil profile.   

Symptoms of S deficiency in wheat
From the road, S deficiency will resemble nitrogen (N) deficiency, but closer inspection can help you determine if S is the issue.  Unlike N, S is not mobile in the plant, so deficiencies (yellow color) tend to appear in the newer leaves, while nitrogen deficiencies would be in the lower (older) leaves (Figure 1).  Sulfur deficiency in wheat typically first observed as patches of yellow plants within a field (Figure 2).  Wheat deficient in S can also be stunted and thin and may show delayed maturity.  The reduced growth and thinned stands often result in inadequate canopy closure and therefore more issues with annual weeds.  

Testing for S in the wheat crop
Soil test values for S are not considered as reliable as some other nutrients, because mobile S often accumulates lower in the soil profile and may be missed by traditional sampling methods.  Sulfur availability from organic matter and manures is not accurately predictable from typical soil tests taken at a depth of 4-6”.  Tissue analysis is the most accurate method of possible S deficiencies within the crop.  When collecting wheat tissue samples in early spring, be sure to follow proper protocols as noted by your soil/nutrient testing service.  Clip and send as much of the small wheat plants as possible, avoiding any soil and dust contamination.  Most labs require enough plant material to equal the size of a softball.


Application of S
Sulfate forms of sulfur are preferred in spring applications, because it is the most readily available form of sulfur.  There are several sulfate sulfur fertilizer products commonly used in dry blend or liquid formulations, which can be utilized to supply the sulfur.  Elemental sulfur is not readily available to the plant, but could be used in the fall, allowing time for conversion to plant available sulfate form.  

Without specific guidance from tissue testing or other reliable indicator of sulfur needs, an application of 20-25  lbs. sulfate sulfur per acre should provide adequate sulfur to a wheat crop and would normally be applied in the tillering stages to jointing phase, along with the topdress nitrogen applications.  There are multiple sources of sulfur in both dry and liquid forms.  Ammonium sulfate, a 21-0-0-24S analysis, is one of the more common dry granular products used and is often diluted with water for liquid application.  Ammonium thiosulfate (12-0-0-26S) is a popular liquid source, as it blends well with most liquid fertilizers.
 

Figure 2. Patches of S deficient wheat.  Photo D. Mengal – KSU. 

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