Dr. Dan Talks Agronomy
By Daniel Davidson
DTN Contributing Agronomist
I am a fan of gypsum and use it mainly to improve the soil. However many producers apply gypsum as an economical source of sulfate for crop production. Even 100 pounds of gypsum per acre will supply 17 to 18 pounds of sulfur (S), which will meet the needs of a crop in addition to what the soil can supply.
Most crops require 15 to 30 pounds of S per acre. Oilseed crops, legumes and forages (alfalfa in particular) require more sulfur than phosphorus for optimal yield and quality compared to cereal grains. When making a sulfur recommendation, agronomists recommend applying 10 to 30 pounds per acre -- lower end for grain crops and higher end for oilseed and forage crops.
Don't rely on soil tests as they aren't reliable measures of sulfur levels in the soil. Sulfate in the soil solution is vulnerable to leaching. Most of the sulfur in the soil is tied up in organic matter and has to be mineralized to be available. Tissue tests are a more accurate measure of sulfur concentrations, which should range from 0.20% to 0.25%. The most important number is the nitrogen-to-sulfur ratio. Both nitrogen and sulfur are important components in amino acids. The ratio should range from 10:1 to 15:1. Ratios of 18:1 to 20:1 or greater suggest a sulfur deficiency.
John Sawyer, soil fertility specialist at Iowa State University has been studying sulfur responses in Iowa over the past decade. He has discovered that sulfur can be limiting and crops can respond to supplemental sulfur application. He considers alfalfa very responsive, corn responsive and soybeans less responsive.
A 2009 Iowa State University sulfur study resulted in significant yield increases in six out of 11 on-farm corn trials in central and northeast Iowa. The Iowa Soybean Association's On-Farm Network ran a statewide nutrient benchmarking survey in 2011 and learned that at least 20% to 30% of corn and soybean fields might be sulfur deficient based on in-season tissue testing.
An On-Farm Network study in northeast Iowa with pelletized calcium sulfate showed statistically significant corn yield responses in one-fourth of the 25 trials conducted in northeast Iowa. These responses were attributed to fields having a sandy texture or soils with a high degree of slope with low soil organic matter. These types of soils are known to be at risk of S deficiency.
For the past several years, farmers have seen more symptoms of sulfur deficiency -- a yellow striping on younger plant leaves. The occurrence of sulfur shortages are not surprising because deposits from the atmosphere have declined significantly during the last couple decades as manufacturing, coal-fired power plants, and automotive industry have cleaned up emissions.
Today's higher crop yields also require more sulfur and commercial fertilizers no longer contain it. This leaves crops to rely solely on sulfur mineralization from organic matter. Light soils (sandy) or soils low in organic matter (1.5% or less) can't supply enough sulfur to the crop.
Sawyer has been studying sulfur responses in alfalfa, corn and soybeans in Iowa. He told DTN that alfalfa is the biggest responder to supplemental sulfur. "It has the highest demand and is the most sensitive to shortages. We know sulfur works, but don't how often to apply during the life of a stand," Sawyer said.
"We now have a reliable test for measuring sulfur deficiency in alfalfa at early bud stage," Sawyer said. "Sample the top 6 inches of growth at early bud stage and send in for tissue analysis. If sulfur concentration is less than 0.24%, it's deficient."
He recommended applying 30 pounds per acre after the first cutting and twice over four years. He also suggested applying elemental sulfur, gypsum or potassium sulfate. "Ammonium sulfate is popular source of sulfur, but alfalfa doesn't need nitrogen," he said.
Sawyer says soil tests aren't reliable for corn, and tissue tests are slightly more reliable, for example, in corn when sulfur concentration is extremely low (0.12% or less indicating deficiency) or high (0.20% or greater indicating adequate to high). Between 0.14 and 0.20%, Sawyer said there may or may not be a yield response in corn to sulfur application, you do not always get a response, so applying sulfur may or may not be of benefit.
Still, he considers corn responsive to sulfur. Sawyer conducted sulfur trials from 2006 to 2013 in corn and 47% of the trials showed a positive response to sulfur. "The response rate was greater in Northeast Iowa and smaller elsewhere, but we still saw responses across the state," Sawyer said. "Our sulfur rate experiments for corn showed the optimum rate to be 15 pounds [per acre] on fine textured soils and 25 pounds on coarse textured soils." Fertilizer options for corn include elemental S, ammonium thiosulfate, ammonium sulfate, gypsum and starter fertilizers that contain sulfur.
Soybeans are an oilseed crop and produce oil and protein meal and sulfur is an essential nutrient in several amino acids that are components of proteins and enzymes, and is important for nitrogen fixation in legumes. However Sawyer hasn't been able to identify a clear deficiency pattern. "I have seen a lower frequency of response compared to alfalfa and corn in Iowa. And if farmers are applying sulfur for corn, soybeans can remove any residual the following season that remains in the soil."
Sawyer recommends considering soil type, crop and yield history before applying sulfur. "Is the soil a light or heavy texture, is it productive and are you pulling off high yields that require a lot of nutrients," he asked.
Monitoring organic matter in the soil is an important part of this decision. Soils testing 2.5% to 3% or greater probably have adequate sulfur levels. Growers who apply gypsum, animal manures or compost on a regular basis probably have adequate sulfur present.
The first step is to run soil and tissue tests to see if sulfur could be limiting. If you do apply sulfur, Sawyer recommends using strip trials to see if you get a response.
Dan Davidson can be reached at AskDrDan@dtn.com
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