Fertilizer Over-Application Cuts Into Your Bottom Line

As the cost of nitrogen or the price of corn changes, the optimum rate of nitrogen also changes. Compiled by staff

Published on: Oct 21, 2005

Nitrogen application recommendations for Midwest farmers will soon be changing, mainly driven by a need to be more cost efficient as fertilizer prices continue to rise.

Historically, fertilizer recommendations for field crops have offered optimum nitrogen rates based on the maximum yield potential for a particular area. Such a system, however, relies on nitrogen being inexpensive and over-application not too costly, says Robert Mullen, an Ohio State University soil scientist. Both are no longer the case.

"This approach has served agriculture well. The economic detriment due to over-application has historically been small from an economic standpoint," Mullen says. "But as nitrogen prices have risen over the past several years, the economic penalty for over-application has reached a point where economic considerations need to be made. Producers -- especially those managing large acreages -- are beginning to look at fertilizer nitrogen application from an economic standpoint."

Fertility specialists throughout the Corn Belt have devised a new system basing optimum nitrogen rates on the current price of fertilizer and the average price of the crop. For example, if nitrogen is 25 cents per pound and the price of corn is $2.50 a bushel, to achieve 175 bushels per acre of corn in northwest Ohio the best nitrogen rate would be 156 pounds, at an application range of 150-180 pounds. As the cost of nitrogen or the price of corn changes, the optimum rate of nitrogen also changes.

"It boils down to an exercise in risk management," Mullen says. "The old system uses a single value, while this new system gives farmers a range to work with. If farmers are risk averse, they can use the high side of the rate range. If they are more willing to accept risk, they can use a lower side of the rate range, increasing their potential for economic reward."

Current fertilizer recommendations needed to be updated for several reasons, Mullen says. "One reason is that the system assumes the soil is a blank medium and devoid of natural nitrogen. We know that's not true," he says. "And the problem we run into is that we don't know exactly how much nitrogen is in the soil and how much will be available to the crop. The release of nitrogen is dependent on the weather, so there's always a possibility of adding more or less nitrogen to the soil than is needed."

Also, nitrogen applied to the soil always reaches a point of saturation, and yield eventually levels off no matter how much more nitrogen is added, Mullen says. As a result, farmers could be wasting money on unneeded nitrogen using current nitrogen recommendations.

"Is it always economical to shoot for maximum yield? Research has shown that it's not," Mullen says. "It may take the same amount of nitrogen to reach 179 bushels per acre as it does to only reach 170 bushels per acre. It's impossible to determine at what point the nitrogen level is reached to where it is no longer a benefit to gain more yield without a nitrogen rate trial in every field."