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Wet spring putting off your fertilizer plans? Try sidedressing

With rain delays widespread across the Corn Belt, more growers this year are expected to catch up on fertilizer applications by sidedressing corn.

"If you didn't get any nitrogen on in the fall, then there's a good chance you won't have time to apply any until after corn planting," says Dean Collamer, agronomist and certified crop advisor with Honeywell. Sidedressing is also an option where growers suspect loss of fall or early spring N due to denitrification in warm, saturated soils.

Ideally, you want to make sidedress nitrogen applications about a week before the crop needs it, Collamer says in a company report.

"Six weeks after planting is when the N use curve for corn really starts moving upward and that trend will continue at a pretty steep climb all the way past tasseling," he explains. "The tougher question to answer is: When will your crop run out of residual N and reserves from organic matter? That's when you want your sidedress fertilizer to kick in."

If you're growing corn-on-corn in a no-till, total-post Roundup Ready system and you haven't applied any starter fertilizer, then you’re going to want to get into the field just as soon as possible, says Collamer. On the other hand, if you're growing corn after soybeans and you applied some nitrogen with your planter or with your pre-emergence herbicide, then you’ve got a little more leeway.

A paler shade of green in early-season growth is a tell-tale sign of nitrogen deficiency. If corn is short on nitrogen, it may also be short on sulfur. The symptoms are hard to distinguish so if in doubt, submit a plant sample for lab diagnosis.

"Sulfur deficiency is a fairly new problem in parts of the Midwest and it's largely due to reductions in sulfur dioxide emissions from coal-burning industries," says Collamer. Reductions were mandated in the 1990s, but actual compliance is more recent.

"We're currently getting 30% to 40% less sulfur from the atmosphere and by 2015, the goal of new mandates is a 70% reduction," Collamer says.

That's starting to catch up with some Midwest soils that used to get plenty of sulfur from the air and from organic matter reserves.

"The most dramatic turnaround we've seen is in northeast Iowa," says Collamer. University testing at 20 sites in 2007 demonstrated a 25-bushel yield increase from a 24-pound-per-acre sulfur application to corn grown in loamy sand and sandy loam soils and a 15-bushel increase using 14 pounds per acre in corn grown in loam and silt loam soils. With $5.00-per-bushel corn and $0.50-per-pound sulfur applied, Collamer calculates that satisfying the sulfur need under these Iowa conditions would increase net return by $112 and $68 per acre, respectively.

University research has also documented crop response to sulfur in areas of Kansas, Minnesota Wisconsin and South Dakota in soil types that have traditionally been considered non-responsive to sulfur.

In Wisconsin, the sulfur work was on alfalfa, says Collamer. "This crop is a very high user of sulfur and it's usually a good indicator that the sulfur status of area soils is beginning to change. If you're seeing sulfur deficiencies in alfalfa, it could be an indicator that you'll need to supplement corn with sulfur fertilizer in subsequent years."

Sulfur acts a lot like nitrogen in the plant and in the soil. Sulfur plays multiple roles in photosynthesis and is used in protein formation. And like nitrogen, the predominant source of sulfur in soils is the organic matter. If cool soils are delaying nitrogen mineralization (from organic matter), they are probably also delaying sulfur mineralization. Likewise, if residues are tying up nitrogen, they are probably also tying up sulfur. And where nitrogen has leached, it's likely that some sulfur has also leached.

"In corn, yield loss from sulfur deficiency typically begins at 21 days after emergence, with a one to two bushel yield loss for every day that follows," says Collamer. "Supplemental sulfur fertilizer is not needed everywhere, but where it is needed, it can make a big difference."

As is the case with nitrogen, soil tests are not the best indicators of sulfur need. "There's just too much variability that can affect the results," says Collamer. Tissue testing is a much more accurate indicator of sulfur need.

"You want to see one part sulfur for every 10 to 15 parts nitrogen in the plant tissue," Collamer says. When the N:S ratio in the plant is 15:1 or greater, sulfur fertilization is generally recommended for most crops.

Where needed, 10 to 30 pounds per acre is generally enough sulfur to satisfy crop needs. For sidedressing, Collamer says blending in the range of one part ammonium sulfate (21-0-0-24S) with one part urea (46-0-0) or one part ammonium sulfate to two parts urea are popular options. These produce 33.5-0-0-12S and 37-0-0-8S blends. Ammonium sulfate also can be sidedressed alone or dissolved with water to make an 8-0-0-9S solution suitable for mixing with UAN solution.

With rain delays widespread across the Corn Belt, more growers this year are expected to catch up on fertilizer applications by sidedressing corn.

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