Early in the growing season, Richard Wurtzberger began to notice a strange occurrence in his southern Minnesota corn fields: The leaves were yellowing at the tips.
The fields had ample nitrogen, so Wurtzberger was confused. He called in his independent crop consultant, Steve Commerford of New Ulm, Minn., who diagnosed the problem as sulfur deficiency.
"I was surprised because no one ever talks about sulfur problems around here,” Wurtzberger says.
It's true that for decades, soil fertility research in the Corn Belt rarely noted sulfur deficiency in soils. Agronomists credit years of rainfall that deposited sulfuric acid into the soil for helping keep sulfur levels high.
Now as pollution controls reduce the acid rain problem and higher yields demand more organic matter, sulfur is becoming more limited, says Dan Froehlich, agronomy manager for Mosaic Crop Nutrition.
"When farmers were content getting 150-bu. corn, organic matter could supply the sulfur readily,” Froehlich says. "Now with 180-bu. corn, it takes 8 lb. to 10 lb. more sulfur to produce those yields.” An 180-bu.-per-acre corn crop needs about 25 lb. of sulfur, he adds.
Know what to look for. Unless a farmer really knows what to look for in sulfur deficiency, it can easily be misdiagnosed as nitrogen
deficiency, Commerford says.
Both nitrogen and sulfur deficiency cause the plant to yellow, he says. The primary difference with sulfur deficiency is that yellowing shows up after the crop is initially very green.
"Sulfur does not readily translocate in the plant, so older leaves will remain green and the newer leaves will become yellow,” he adds.
Commerford realized sulfur was becoming a problem when he saw unusual symptoms in high-yield environments where nitrogen was not a concern, such as fields with manure applications. He noticed green streaking in these fields along wheel tracks, and he began doing side-by-sides of sulfur and nitrogen in these fields.
"I couldn't get the corn to green up with nitrogen, but I always could with sulfur,” he says. Commerford also saw a 15-bu. to 20-bu. yield improvement with the sulfur applications in small-plot replicated trials.
Sulfur deficiency is also difficult to diagnose because there isn't a good soil test available for the micronutrient, Froehlich adds. "There are a lot of factors that can throw off a sulfur test on any given day,” he says.
Since there is no good diagnostic tool, the only way a farmer is going to know if there is a sulfur deficiency present within corn fields is by setting up a field trial and monitoring at harvest for an economic response, Commerford says.
Wurtzberger established replicated trials on his farm using 40-acre fields in a corn/soybean rotation for six years. The treatments were split the final year: One side of the treatment
received a starter application with 10% sulfur and 1% zinc and the other side received no sulfur.
Overall, the treatment with sulfur applications saw an average 15-bu.-per-acre increase, Wurtzberger says. "When we got up to the highest rate, we got a 20-bu. increase in yield,” he adds.
What to do. The most common method of applying sulfur is using a combination of monoammonium phosphate and ammonium sulfate. Sulfur can be applied as starter, which is helpful if you have a lot of organic matter that may not become available right away.
Ammonium sulfate is also a common choice in the northern Corn Belt.
"As we push yields up, sulfur and zinc impact nitrogen and phosphate uptake efficiencies,” Froehlich adds. "To get that synergy, these nutrients need to be close together in the soil.”
It's important to note, Commerford says, that sulfur responds best when all other nutrients in a field are supplied. "That's the thing about sulfur—it will only respond when the rest of the plant's needs are met,” he says.
For More Information
Iowa State University recently evaluated corn response to sulfur fertilization in central and northeast Iowa and found an average 13-bu.-per-acre yield increase in corn with increased sulfur fertilization. For more information on this study and others, visit www.agronext.iastate.edu/soilfertility and search for "sulfur.”
You can e-mail Jeanne Bernick at firstname.lastname@example.org.