The University of Nebraska-Lincoln has revamped its nitrogen (N) fertilizer application rate recommendations to help farmers better account for different application times and varying corn and nitrogen prices.

Prices for anhydrous ammonia, the most common N fertilizer for corn and the base material for other N fertilizers, continue to rise, says Charles Shapiro, soils specialist at UNL's Haskell Agricultural Laboratory near Concord. Higher fertilizer costs combined with low corn prices are changing the price ratio between what farmers earn for corn and what they pay for fertilizer.

Through years of on-farm testing, Institute of Agriculture and Natural Resources scientists developed a formula for determining optimum N rates for corn. The formula predicts the amount of N needed to achieve an expected yield. It also takes into account soil organic matter, nitrate content in the soil profile and other N credits, such as from the previous crop, manure and irrigation, Shapiro says.

To update this formula, scientists created a way to adjust the recommendations when fertilizer costs change relative to corn's value.

"What’s happened over the past two to three years is corn isn't getting more valuable, but the cost of nitrogen fertilizer is going up," Shapiro says. "We are using the old recommendation procedure and adjusting that based on changes in the cost of nitrogen. Essentially, we calculate the agronomic rate and then adjust for economic conditions."

As part of broader fertility research, the team analyzed economically optimal fertilizer rates under different corn-to-nitrogen price ratios. This ratio is used to calculate an economically optimal fertilizer rate.

This analysis revealed, for example, that when the corn to N ratio is 10-to-1, the economically optimal N rate averages 155 lbs. of N/acre for corn following corn or dry beans and 120 lbs. of N/acre for corn following soybeans, says Achim Dobermann, UNL soil fertility/nutrient management specialist.

When the ratio drops to 8-to-1, the economically optimal N rate averages 143 lbs./acre for corn following corn or dry beans and 112 lbs./acre for corn following soybeans.

"As the price ratio decreases, the optimal nitrogen rate also needs to decrease," Dobermann says. "This economically optimal nitrogen rate is the amount of nitrogen that leads to maximum net return from nitrogen application for a given price scenario."

These adjustments should make it easier for producers to use UNL's recommendations to calculate the most profitable N levels under different price scenarios.

Adjustments for fertilizer application timing take into consideration applying N in the fall or after crops emerge. Scientists found fall N application is less efficient than spring, so farmers should increase N rates by 5 percent. Those applying N after the crop emerges, such as by side-dressing or fertigation, should decrease their application rates by 5 percent because the crop will better use the nitrogen.

These new recommendations were added to a user-friendly spreadsheet software program available at UNL's Soil Fertility and Nutrient Management Web page at http://soilfertility.unl.edu/.

The changes in recommendations are a result of the Nebraska Soil Fertility Project, which was started in response to farmers' questions about corn and N price ratios and higher corn yields. Three years of field research were conducted beginning in 2002. Analysis of the research began last year.