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Corn+Soybean Digest

Defeat Denitrification

Seeing 35 units of nitrogen disappear from saturated fields takes a lot of the excitement out of $5/bu. corn for Hartwell Huddleston, Leland, MS. Timely growing-season applications are essential to prevent further cuts into the crop's marketplace allure.

Nitrogen (N) loss can occur through several methods including volatility, leaching and denitrification. Mississippi growers often experience N loss resulting from denitrification when wet weather persists after N application. This problem is more prevalent on heavy or fine-textured clay soils.

These soils are prone to remain saturated, causing denitrification when microorganisms turn nitrate N into N gas that escapes into the air, says Erick Larson, grain crops agronomist, Mississippi State University (MSU) Extension, Starkville.

“Warm soil temperatures accelerate this process,” he says. “Rates range from 2% to 3%/day at soil temperatures from 55° to 65° F, or 4-5%/ day if soil temperatures exceed 65°.”

Huddleston farms along the Mississippi River Delta. Like many growers, he has increased corn and soybean production and cut back on cotton. He strives for 220-bu. corn under irrigation in an area that often faces long, hot, dry spells in late summer.

But he can also face horrific rain episodes, like several last spring (2008) that caused the big river to swell over flood stage. “It's hard to believe, but we actually had water shooting up out of some wells (on river-bottom land),” he says.

He regularly pencils in 220-230 units of N to secure a 200-bu. yield, using the 1-lb.-N/bu. yield theory. But after an abnormal 14 in. of rain in July 2007, he figures about 35 N units escaped from the soil.

“My corn averaged 195 bu., so we may have lost at least 35 units in 2007,” says Huddleston, who farms with his brother William and father Herbert. “The ears just didn't fill out.”

At a nearly unbearable cost of about $130/acre for a 220-230-unit application, 35 lost units meant $15-20 went straight down the drain. He knows he can't do much about lost N when whopping downpours strike, creating super saturation. But, he is attempting to make applications more timely and efficient.

“I usually apply 100 units at planting,” says Huddleston. “It's knifed in. More N is applied after a good stand is established.

“I decided to ‘fly on’ more N at the tassel stage. It should provide more timely nutrients for the corn, but it's expensive.”

MSU typically recommends corn N rates based on a yield goal of 1.3 lbs. N/bu. of grain yield. However, Larson says research shows you can use 10-15% less N than the standard recommendation if you are growing corn on lighter, sandier soil (similar to most of Huddleston's production).

“NITROGEN RECOMMENDATIONS for corn in the South are based totally on corn-yield goal because our warm, wet winters keep N from carrying over from year to year,” he says. “This is different from the Midwest, where consistently cold, dry conditions effectively stop N loss during the winter.”

He says using proper N sources, timing and application method are often more important than N rate to crop utilization and corn-grain production in the Midsouth. He suggests split application methods to reduce the likelihood of considerable N loss due to wet weather before crop use.

Corn uses less than 10% of its N before rapid vegetative growth begins, usually from late April to mid-May, depending on planting date and seasonal temperatures. Therefore, growers can use N more efficiently by applying only a small portion just after plants emerge.

“Add the bulk of N fertilizer just before the growth spurt, when the plants need it most,” says Larson. “Apply no more than one-third of the total N near planting/crop emergence, then the second application about 30 days later when the corn should be about 12 in. tall or at the V6 growth stage.”

As in the plan used by Huddleston, Larson says growers who have not applied their intended N allotment may be forced to apply N by air because the corn may be too tall to permit ground equipment. “Ammonium nitrate is the preferred N source for aerial application because it is not subject to volatilize, compared to urea,” he says.

The cardinal rule is to not apply N on saturated ground. That could mean a lot of drying out for growers facing early spring drenchings. “We just have to work around it,” says Huddleston.
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