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Corn+Soybean Digest

UNL Soil Scientists Caution Against Excessive Nitrogen Application

As corn prices rise and nitrogen fertilizer prices are lower in some cases than in recent years, corn growers may want to increase the amount of nitrogen (N) they apply to their corn fields. However, University of Nebraska-Lincoln soil specialists caution against excessive nitrogen application.

"With corn prices currently in the $3.60-3.80/bu. range and N in the 25 cent to 45 cent/lb. range, the corn-to-N price ratio can range as high as 13-to-1 to 15-to-1," says Richard Ferguson, UNL soils specialist. "However, the incremental yield increase with more fertilizer is relatively small and the risk of leaving substantial residual nitrate-nitrogen in the soil becomes much higher."

To discourage over-application of fertilizer, UNL soil scientists are recommending a maximum 10-to-1 corn-to-N ratio and a minimum 5-to-1 corn-to-N ratio when calculating economic N rates for corn.

"We believe these limits will not significantly impact profit potential for Nebraska's corn growers and will allow for good stewardship and minimize excessive nitrogen applications from seeping into and contaminating groundwater," says Achim Dobermann, soil fertility/nutrient management specialist.

The demand for corn is being driven mostly by ethanol production and Nebraska corn growers anticipate a much more positive economic outlook for 2007 than in recent years.

From 2005-2006, producers were facing expensive N fertilizer prices and low corn prices. This, combined with recent UNL research evaluating optimal fertilization rates for irrigated corn, led UNL researchers to set N recommendations at a base price ratio of 8-to-1, where the value of corn is eight times the value of a pound of N fertilizer, Doberman says. With lower price ratios of 5-to-1 or 6-to-1 in 2006, the economic adjustment to N fertilizer recommendations reduced rates from this base by 20-40 lbs./acre.

"These lower rates slightly reduced yield potential, but more importantly maximized profit," Doberman says.

When applying N this spring, corn growers also must take into account all N sources, Ferguson says.

"In Nebraska, the UNL recommendation procedure for corn is based on accurate soil sampling every three to five years for organic matter, annually for residual nitrate-nitrogen and accurate yield goal estimation using at least five years of yield history," Ferguson says.

It also is important to account for other N credits that may be available, such as legumes as a preceding crop, nitrate in irrigation water and N derived from manure.

For more information about N rates, the most current N rate calculator for corn is available as a downloadable Excel spreadsheet from UNL's soil fertility Web site at http://soilfertility.unl.edu. The most recent version is called UNL N calculator 2007.xls.

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