There's less brimstone dropping from American skies.

Sulfur dioxide emissions - think acid rain - have declined sharply in the last three decades, so less sulfur (S) is falling on cropland. Does this mean farmers need to add S fertilizer for maximum corn yields?

That question is prompting Corn Belt researchers to pinpoint exactly where S pays today.

Research from the 1970s found that S rarely increased corn yields, says University of Minnesota Extension Agronomist emeritus George Rehm, “and the conclusions were consistent across the Corn Belt.”

Since then, there have been important changes in Midwest corn production, says Brian Lang, an Iowa State University Extension agronomist who is running S trials in Iowa. Progressively higher corn yields have increased S removal. One is the shift to conservation tillage, which slows decomposition of organic matter, the main source of soil sulfate. And livestock production has consolidated, so many farmers no longer apply manure, another prime S source. Of course that is changing again with changing fertilizer economics.

More corn production on eroded soils is also spurring S research, adds Howard Woodard, South Dakota State University (SDSU) fertility specialist.

Most important, America is cleaning up its industrial smokestacks. Across the Corn Belt, the amount of sulfur reaching the soil in rain has dropped 30-60% since the 1980s, according to the National Atmospheric Deposition program.

All of these changes suggest that crop responses to S could become more common, says John Sawyer, Iowa State University Extension fertility expert.

One region where S need has been documented recently is northeast Iowa, an area of windblown loess soils low in organic matter. In 2007, trials at 20 sites found that calcium sulfate boosted average corn yields by 18 bu./acre across a variety of soil textures, Sawyer says.

Sulfur response was most dramatic on coarse-textured soils, where corn yields rose by 25 bu./acre. On fine-textured soils, the yield bump was 15 bu./acre. The economic optimum S rate was 24 lbs./acre in loamy-sand and sandy-loam soils, and 14 lbs./acre in loam and silt-loam soils, Sawyer says.

Kory Jacobsen, agronomy manager of Farmers Co-op Produce Company in Fort Atkinson, IA, says that most of his customers have started adding ammonium sulfate to their corn fertilizer programs. Despite higher S prices in 2008, “S sales on corn acres were up 30%,” he says.

Adding S also pays in reduced-tillage corn grown on soils with less than 2.5% organic matter, and on eroded knolls and slopes, says Woodard, fertility research leader at SDSU, which has been doing S trials since 2000.

In a 2001 study at three sites in eastern South Dakota, S raised yields in hillside test plots by 36 bu./acre for no-till and 17 bu./acre for conventionally tilled plots. Likewise, a 2004 study found that S raised corn yields by 8 bu./acre in hillside no-till plots near Brookings, SD. A similar response to S was seen in 2005, when ammonium sulfate increased yields by 12 bu./acre in no-till corn plots.

It only takes about 25 lbs. of S to correct S deficiency, Woodard says, “and the earlier it's applied, the better.” Liquid ammonium sulfate is convenient to apply between the rows, and in hilly terrain, “if you plant along slopes, you can pick and choose which rows” to fertilize.

Another strategy for S management in no-till corn is to apply a combination of ammonium sulfate - for the current crop - and cheaper elemental S, which is slowly converted by soil microbes to plant-available sulfate. “This is a long-term approach,” Woodard says.

Meanwhile, the latest research on heavy-textured soils confirms that most Midwest farmers don't need to worry about adding S, Rehm says. Trials at six locations across southern Minnesota in 2007 - all with over 4% organic matter - showed no corn yield increase from S.

Likewise, S didn't increase corn yields in field trials across Iowa in 2000 and 2001, or in southwest Iowa from 2000 to 2005, Sawyer says. In Ohio, current trials “show that the probability of S response by corn and soybeans is still quite low, with a few exceptions,” says Ohio State University Extension Soil Fertility Specialist Robert Mullen.

In 2008, University of Minnesota Extension Agronomist Dan Kaiser looked at corn and soybean response to S in starter fertilizers. In soybeans, there were some differences in early plant growth, but “no yield advantage for S, even on the low-organic-matter sandy soils,” Kaiser says.

However, corn grown on a low-organic-matter hillside did respond to S applied with the planter in a band beside and below the seed, he says. Other Minnesota trials are evaluating S response in different tillage systems. “We're seeing more interest and questions from farmers about this,” Kaiser says.

PUTTING S TO THE TEST

After hearing how sulfur (S) boosted corn yields in northeast Iowa in 2007, Nick Leibold decided to put S to the test on his own farm.

The New Hampton, IA, farmer raises 1,100 acres of corn and soybeans on glacial-till soils with 2-4% organic matter. He no-tills beans into corn stalks and field-cultivates soybean stubble before planting corn.

“I'd heard about S in alfalfa,” Leibold says, “but this was the first I'd heard of S for corn here.”

Sulfur management in corn is tricky, says Iowa State University Extension Agronomist Brian Lang, because there's no soil test that accurately predicts need, and not every field responds. “One way you can tell if you'll get a response is to run your own strip trials,” he says.

In 2008, Leibold set up strip trials on two corn varieties, with five replications per trial. Just before planting, his co-op dealer spread 125 lbs./acre of gypsum, or calcium sulfate, on the test strips, recording the locations with GPS. Cost, including the application fee, was $20/acre. The 60-ft. strips extended the length of the fields.

Leibold saw a modest yield bump from S, ranging from 2-7 bu./acre — but not enough to make it profitable, he says. “At best, I broke even with $4 corn.”

He'll probably do another S trial in 2009, though. “2008 was a strange weather year,” excessively wet early in the season and dry in July and August. His corn yields were below normal, “but not as bad as I expected,” averaging 160 bu./acre. “Most of the time, I test new things for three years.” 7