Heavy rains raising nitrogen questions

Be careful what you wish for, as the old saying goes, and that might include early season rainfall. Rain has fallen to the extreme in some areas of the lower Southeast during the 2003 growing season, especially in north Alabama, where some cotton and corn fields have been deluged.

Following two weeks and more than 10 inches of rainfall in most areas during May, cotton growers in north Alabama’s Tennessee Valley were asking how much of their at-planting nitrogen was still available and how much nitrogen should they use as a sidedress treatment, says Charlie Burmester, agronomist with the Alabama Cooperative Extension System.

"As with many biological processes, there isn’t a simple answer," says Burmester. "We know that we’ve lost a great deal of nitrogen in the soil due to denitrification. The ground is waterlogged, and bacteria use the oxygen from nitrate fertilizer as their oxygen source. This causes a conversion to nitrogen gas, which is lost in the atmosphere."

The more organic material in the surface soil and the higher the temperature, the more rapid this process occurs, he adds. Most nitrogen fertilizers are quickly converted to the nitrate form under spring planting temperatures.

"Since nitrogen soil testing generally is unreliable in our soils, we’re left to use the results of past experiences and some assumptions. My experience with flooded fields in north Alabama indicates that most nitrogen fertilizer is lost rapidly. Where cotton was flooded two to three days, and the cotton survives, it often develops deficiency symptoms very quickly. Nitrogen loss also has occurred in fields that have stayed saturated for many days, but probably to a lesser extent," says Burmester.

Most cotton fields probably will benefit from additional nitrogen fertilizer, he says, especially if growing conditions are good in June and July.

"I would recommend increasing sidedress nitrogen treatments by at least 10 to 20 pounds per acre over original plans. Areas replanted due to flooding — or where water stood for two or more days — should receive at least 40 to 60 additional pounds of nitrogen. Since cotton’s peak demand for nitrogen isn’t until bloom, this additional nitrogen could be delayed until the field has a good stand and the cotton is growing normally."

As cotton approaches first bloom, leaf tissue sampling may be done to determine nitrogen levels, says Burmester. Leaf tissue sampling often does a better job of indicating the nitrogen status of dryland cotton plants than petiole sampling, he adds.

In north Alabama’s Jackson County, rain has been too much of a good thing for corn producers. Some fields already lined with young, pale green corn plants were under water for more than four days. Other fields were waterlogged, taking days to dry out. Jackson County Extension Agent Lewis Tapley says this is the worst looking corn crop he can remember.

"Every field has spots that are just drowned out," said Tapley in late May. "We had some fields where as little as 10 percent of the crop was lost while others were a total loss. It’ll be several weeks before some fields in creek and river bottoms dry out."

Jackson County is the leading corn producer in Alabama, with more than 20,000 acres planted annually. While corn is planted in almost every county in Alabama, the greatest concentration of acreage is in the northeast corner of the state, in Jackson, DeKalb, Madison, Colbert and Lawrence counties.

Extension agents in DeKalb County were predicting that excessive rainfall could delay the corn crop there by as many four weeks.

While the situation statewide isn’t as critical as in Alabama’s northern counties, most corn producers are facing a number of challenges with this year’s crop, says Paul Mask, a corn and small grains agronomist with the Alabama Cooperative Extension System.

"In general, the crop is looking good for the most part and that's with more than 80 percent of the crop out of the ground," said Mask in late May. "However, producers across Alabama have some degree of problems."

Mask says while some are the normal problems associated with production, other problems are directly related to the widespread, heavy rains that swept across the state during the early part of the 2003 growing season.

Corn growers are worried about the loss of nitrogen fertilizer applied before or at planting time, adds Mask.

"While farmers are concerned about leaching, I think the bigger concern is denitrification," he says. "When soil is under water or waterlogged, it becomes an anaerobic environment (or an environment without oxygen). Anaerobic bacteria begin to break down the nitrogen, making it unusable by the corn plants." Fertilization with adequate levels of nitrogen is required for good corn production on all Alabama soils. Mask says no other element produces such large and consistent increases in corn yields. For economical returns, larger quantities of nitrogen normally are required than are needed of any other element.

"While we don't recommend it, some farmers will have applied all of the recommended nitrogen fertilizer at planting," says Mask. "They may have lost all of those nutrients because of denitrification in waterlogged soils."

Current research indicates that as much as 5 percent of the total amount of nitrogen is lost for every day the soil remains flooded or waterlogged. Flooded corn fields are under substantial stress, which will increase the possibility of disease and decrease the yield potential.

Mask and Tapley agree that farmers will have to evaluate each of their fields and determine which are in the best shape and which will benefit most from additional applications of nitrogen.

"It's just a waste of money to apply more fertilizer to a field that has been waterlogged for 10 days or so," says Mask. "As expensive as fertilizer is, a farmer would be better served to only apply additional fertilizer to those fields in the best condition."

Weed control is another challenge for Alabama corn growers who have been affected by excessive rainfall, says John Everest, Extension weed scientist. There are no blanket recommendations, he says, and farmers should consider their individual field situations, including the amount of rainfall they’ve received over an extended period of time.

"If they have fields that have been flooded and have been under water, their at-planting or early postemergence herbicide treatment could be halfway to the Gulf by now. It’s difficult to say, depending on factors such as soil movement," says Everest.

On the other hand, if a field has been saturated by the rainfall and not flooded, the herbicide probably is still there, he says. "This just means that the herbicide probably will break sooner than we would like, and we’ll have to be on guard to address any problems in a timely fashion."

Growers should not "double-up" on their herbicide treatments, for obvious economic reasons and because it may damage the crop, he says.

For producers with herbicide-resistant corn varieties, Everest recommends getting back into the field as soon as possible to make timely applications.

After several consecutive years of drought, some Alabama corn producers might see a resurgence of weeds due to this year’s rainfall, he says. "After so many dry seasons, they’ll finally see what weeds can do given decent rainfall. They might find out for sure what they have in some of their fields."

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