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The Secrets of Sulfur

August 27, 2011
By: Darrell Smith, Farm Journal Conservation and Machinery Editor
sulfur cycle2
Understanding the sulfur life cycle is key. It is driven by microbial action to mineralize soil sulfur forms and applied elemental sulfur into sulfate that is usable by the plant. Anything that impacts soil microbial activity impacts sulfur availability.  
 
 

Here’s how to manage a crucial, but often overlooked, building block of high corn yield

Nitrogen is the foundation of high corn yields. But the nitrogen you apply might be wasted if your soil lacks sulfur, a critical micronutrient that helps plants metabolize nitrogen.

"Sulfur deficiencies are becoming more common," reports Farm Journal Field Agronomist Ken Ferrie. "It used to be one element we seldom had to worry about. But, during the past 40 years, things have changed, and we now are removing more sulfur than we put into the soil."

Acid rain, containing sulfur emitted by industry, used to be farmers’ main source of sulfur—and it was free. But the Clean Air Act of 1972 has drastically reduced the amount of acid rain falling on fields.

"Also, today’s fertilizers contain fewer impurities," Ferrie points out. "The old single superphosphate your grandpa applied contained 12% sulfur. Modern pesticides contain less sulfur and we seldom burn crop residue anymore, which released sulfur into the atmosphere. Today’s higher yields remove more sulfur."

Fast Facts


Sulfur’s role in plants:
  • Produces lignin and pectin
  • Produces chlorophyll
  • Metabolizes nitrogen

     

Sulfur removal per bushel of corn:

  • 0.08 lb. by the grain; 0.09 lb. by the stalk

     

Sulfur removal in 200 bu.:

  • 34 lb. of elemental sulfur, or 102 lb. of sulfate

A bushel of corn contains 0.08 lb. of sulfur in the grain and 0.09 lb. in the stalk, for a total of 0.17 lb. "So 200-bu. corn removes 34 lb. of sulfur, or 102 lb. of sulfate [the form taken up by plants] per acre," Ferrie says. "Your challenge is to figure out how much sulfur the soil will provide and how to supplement the rest."

Sulfur is required to transfer sunlight energy into plant growth. It is a big factor in the production of chlorophyll, which is why sulfur-deficient plants turn yellow.

"Most importantly, sulfur is required to metabolize nitrogen. If a tissue test shows plants are low in sulfur, they also will be low in nitrogen. In other words, the absence of sulfur can create a nitrogen deficiency in the plant—you can apply 300 lb. of nitrogen to a field, but, without sulfur, the plants won’t be able to use it," Ferrie says.

Deficiency symptoms. Severe sulfur deficiency stunts plants and delays maturity. Besides retarded growth, the most obvious symptom reflects reduced photosynthetic activity: The younger leaves, at the top of the plant, will have a light green or yellow stripe.

"Because sulfur is not mobile in the plant, it can’t be moved upward from the bottom to correct the deficiency," Ferrie says. "So the symptoms show up in the new growth rather than at the bottom of the plant, as they do with nitrogen deficiency."

Sulfur in the soil. In the soil, sulfur exists as mineral sulfates (such as calcium sulfate, magnesium sulfate and potassium sulfate), as sulfide gas and as elemental sulfur.

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FEATURED IN: Farm Journal - September 2011

 
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