Treat Sulfur Like a Macronutrient in Corn to Boost Yields
A farmer recently asked Ken Ferrie the question, “With organic matter above 3.5%, do I need to worry about applying sulfur?”
Ten years ago, Ferrie, Farm Journal Field Agronomist, says his answer would have been different than what he recommends today.
A decade ago, 3.5% organic matter would have supplied enough sulfur and quickly enough in the growing season to meet the corn's needs. At that point in time, the biggest response from sulfur occurred in soils at 2.5% organic matter or less, with soils at 1.5% organic matter and below showing the greatest response.
“Today, we are seeing pretty substantial sulfur responses in our corn plots at and above 3.5% organic matter,” he says. “I still believe that, in total, 3.5% organic matter soils will supply enough sulfur. The issue is those soils just don't supply sulfur quickly enough in the spring.”
As proof, Ferrie says he is increasingly seeing the “yellow sulfur whorl” in soils with 3.5% organic matter between the time corn emerges and the V8 to V10 growth range. As the corn plant gets bigger, roots go deeper and the soil gets more active. Over time, the soil will release organic sulfur and that symptom of sulfur deficiency will gradually disappear.
“But under the concept of never let corn have a bad day, corn showing a sulfur deficiency is having a bad day,” he says. “This is especially true for those G and L1 hybrids that will flex down if you stress them in that window before tassel and right after emergence.”
Why More Sulfur Is Needed Today: What has occurred is less acid rain is falling today, so fields are getting less sulfur from the atmosphere. You can trace this fact back to the Clean Air Act of 1970. So, while it’s good that acid rain has been reduced, it has resulted in less sulfur being available for crops to tap into.
What that means now is that farmers in much of the Midwest must treat sulfur more like a macronutrient rather than a micronutrient.
“The continued drop in the soil's ability to supply sulfur has now reached the level that we have to start thinking about applying sulfur every year ahead of corn, no matter what the organic matter is,” Ferrie says.
Different Soils, Different Nutrient Needs. He adds that farmers have some options to consider for sulfur use as part of their 4Rs strategy.
“In soils not subject to leaching -- which most 3.5% organic matter soils are less likely to have large amounts of leaching -- the sulfate could go on in the fall, when the phosphate and potash go on,” he says.
“Sulfur could also go on in the spring if that's when you put your P and K on; it could go on at planting on the corn planter or be top-dressed with your pre-emergent nitrogen program,” he says.
Elemental sulfur must go through a biological breakdown to get to sulfate.
“This reaction doesn't get into full swing, though, until soil temperatures are above 75 degrees F, which means elemental sulfur doesn't make it to the dance until later in July and August,” he says. “That is great if you still need sulfur then, but in the 3.5% organic matter soils they usually release enough sulfur by mid-June to take care of the deficiency issues.”
The right placement and right timing of sulfur applications are important.
“Sulfate in a 2"x2” application with the planter is a good way to achieve good application timing. Sidedressing sulfate in the middle of the row with a coulter at sidedress time is usually too late, at least in central Illinois. By the time the root system and the sulfate get together, the plant has grown out of its sulfur deficiency,” Ferrie says.
He notes that Y-dropping sulfate next to the row can improve the sidedress response. “We’ll need to go early on small corn, which means we need to go slow and not splash product on the small corn plant and cause burn. Again, these are recommendations for higher organic matter soils that only have a temporary sulfur problem early in the season,” he adds.
Sandy Soils Need Another Strategy. In sandy, leachable soils, farmers need to put together a program that keeps the corn plant supplied in sulfur all season, and not just up until V10.
“When we do sulfur plots in corn, we can now see the sulfur checks show up the following year in soybeans,” Ferrie says. “These check plots from last year show up in color, and they can be seen in aerial imagery and with the naked eye.”
In the plots that got sulfur in the corn the previous year, the deficiency is a lot rarer than what Ferrie sees in high organic matter soils.
“This would suggest that we're seeing some carryover effect into the soybeans. But this, too, is something we didn't see years ago,” he says. “You could put a sulfur plot out on corn and not see anything show up the following year in soybeans. So, this is somewhat new in the last five, six years that we're starting to see in our plots.”
Soybeans Tell a Different Story. However, Ferrie says he is not seeing a positive yield response from sulfur in soybean plots following corn.
“I think this lack of response is due to the fact that soybeans set their yield much later than corn. When soybeans are setting pods and starting grain fill, these higher organic matter soils are supplying enough sulfur to meet the plants' needs,” he says.
So, back to the original question, do farmers need sulfur in 3.5% organic matter soils?
“Our plots would say yes, in corn – somewhere around 15 pounds to 20 pounds of sulfate,” Ferrie says.
However, in soybeans, Ferrie’s plot research doesn’t show a consistent response to sulfur in soils with high organic matter. At least not yet.
“If sulfur is going on the corn in these soils in a corn-soybean rotation, I think it'll be longer before we see any payback or response in the soybeans,” he says.
Ferrie adds that depending on where farmers live, the results and needs for sulfur applications can vary. For instance, growers farming downwind of a large city like Chicago may still be receiving enough acid rain to meet their crop’s sulfur needs, due to all the fossil fuel being burned on the roads and highways.
“When all the cars in Chicago are electric, then you're going to have to join the rest of us and start applying sulfur,” he says.
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