With nitrogen fertilizer prices approaching $1,000 a ton, farmers can’t afford to make mistakes on crop nutrient management.

But they often do, according to an Oklahoma State University nutrient management specialist. Available technology, however, takes the guesswork out of crop fertilization.

Brian Arnall, with the OSU Plant and Soil Sciences Department, says wheat farmers typically use a yield goal-based formula to determine fertility rate. “That’s not working with current prices,” he said during the Bayer CropScience Wheat Technology Meeting in Oklahoma City.

Farmers who use a five-year average yield goal to calculate fertility rates “try to hit a home run every year.” Chances of that are no better than a baseball player swinging for the fences at every at-bat, he said.

“Some years it pays to add extra fertilizer. And with wheat at $7 a bushel, producers want every bushel they can get, but they don’t need to add more fertilizer than is necessary. Over-fertilizing is not cost effective.”

Arnall said several techniques allow farmers to gauge in-season wheat nutrient needs more accurately.

“Fertilizer prices are high and farmers can’t expect to add 150 pounds of anhydrous and expect to do well every year.”

He said OSU research dating back to 1971 shows that in some years, wheat with no nitrogen yields more than wheat with 100 pounds of additional nitrogen.

“In 2003, nitrogen paid big. In 2001, we saw no benefit from nitrogen fertilizer.”

The difference is in crop and weather conditions after planting and prior to topdressing winter wheat. If conditions after planting are good, chances of a nitrogen response are good. If conditions are bad, the crop has less chance of responding to more nitrogen.

In fact, adding nitrogen when it’s not needed may result in lost money. Arnall said over the course of the long-term trial, assuming nitrogen at 70 cents a pound and the grower using a yield goal recommendation, over-fertilizing resulted in an average loss of $27 per acre.

Technology and simple observation will allow farmers to judge nutrient needs, he said.

“Sensor-based technology will predict yields at or above actual production 90 percent of the time. The ability to predict yield potential is a big advantage.”

Growers or consultants can use a Green Seeker sensor to estimate yield potential. Currently, those units cost from $3,000 to $4,000 each but can pay for themselves quickly by saving just $10 per acre with $7 wheat and near $1 a pound for fertilizer. And Arnall said OSU engineers are working on a hand-held device that will set farmers back only $200 to $300 if produced in low volumes, perhaps as little as $100 a unit if produced in high volumes. “Those are in the works,” he said.

Producers also may use N-Rich strips to gauge fertility needs. An N-Rich strip has about twice as much fertilizer as a grower would routinely add to wheat at planting. If he applies 80 pounds routinely, he’d make a second pass to one strip to boost the nitrogen level to 160 pounds per acre. “That’s enough to get the crop through a whole season,” Arnall said. “Growers use the strip as a reference. It’s a yes-or-no decision: if a farmer can drive by the field at 45 miles per hour and tell a difference, he needs to add fertilizer. If the strip is deep green, for instance, and the rest of the field is not, apply nitrogen.”

Arnall said some farmers typically apply too much fertilizer because it’s the way they have always done it. “With sensors and technology, the decision is based on knowledge. Sensors detect yield potential at topdressing time.”

Arnall said farmers may also use N-Ramp strips to determine in-season fertility needs. Ramp strips consist of a number of 10-foot wide swaths, each fertilized at a different level, from zero up to 160 pounds per acre (on a 16-swath system). Each strip gets 10 pounds more nitrogen than the last.

At topdressing time a grower walks the field, finds the greenest strip and determines fertility needs based on how much he applied to that specific swath. “If he sees little or no difference among the strips, he may apply little or no additional nitrogen,” Arnall said.

Growers may use the Green Seeker to calculate the best strips and to determine fertilization rates.

He said grower interest in using sensor technology has increased significantly over the past few years. Some dealers have added nutrient sensing to their services.

Arnall said with current wheat prices farmers should take close looks at pH levels, too. “If a grower has a field with low pH he has no reason to fertilize for 40-bushel wheat. It’s worth the cost to lime with current wheat prices. At a pH of 5, a grower can lose $40 per acre in yield.”

He said herbicide, insecticide and fungicide activity improve with proper pH.

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