Setting nitrogen rates the same across a field of corn is much like the school lunchroom where beefy football players get the same amount of food as the skinny scholars. Because of soil type and fertility differences, some plants need more food, others less. But each year, growers mimic the school cafeteria lady and dish out the same rate of nitrogen (N) for each of their fields.

“There are areas of fields that have low yields and areas that have high yields,” says Jeff Nesbitt, president of Precision Partners, Fergus Falls, MN. “Some areas of a corn field can yield 160 bu./acre while others yield 120 bu., and still other areas yield 200 bu. No one who can grow 200-bu./acre corn would go out and spread N for 160 bu. But that's what farmers often do.”

Savings with high-value crops

That's why variable rate N makes so much sense. A number of controllers — which range in price from $2,000 to $4,000 — can vary N on the go. And on high-value crops such as sugar beets, variable N application pays.

Precision Partners sponsors a program that uses a combination of satellite imagery and N sampling in 2½-acre grids 4 ft. deep. The program, which costs $13 to $15/acre, saves little in N costs compared with uniform applications. However, variable N application to sugar beets adds approximately $60/acre in revenues because it boosts sugar content.

“If you add too much N to sugar beets, the plant won't make sugar,” Nesbitt says. “Variable rates will result in more sugar from an acre than a straight N application.”

Nesbitt adds that variable rate N can benefit corn and wheat as well. Precision Partners uses satellite imagery to detect field N levels in sugar beet leaves at the end of the growing season. The next spring, participating farmers vary N rates on corn or wheat based on these readings. For an average $5/acre, farmers save about $11.50/acre in N costs, Nesbitt claims.

Yet others aren't so sure that variable rate N works on lower-value crops such as corn. Because sugar beets are typically a high-value crop, variable rates magnify N savings, says David Bullock, a University of Illinois agricultural economist. But with corn, N savings often don't cover the investment.

“There was a lot of enthusiasm a few years ago from farmers because the engineering was so cool,” Bullock says. “But a lot of the enthusiasm has waned. Farmers can program equipment so it drops 100 lbs./acre of N here and 120 lbs./acre of N there. But they don't know what amount of N one area needs versus what another needs.”

Nitrogen requirements change

Pinpointing plant N needs is like catching minnows in a bait bucket. It's possible to sample the amount of N in soils at a particular time, but much can change during the growing season. Heavy rainfall after sampling can subtract N content via leaching or denitrification.

Meanwhile, soil mineralization can yield additional N. “Farmers often don't give appropriate N credit to mineralization in the soil,” says Jim Schepers, a USDA-ARS soil scientist in Lincoln, NE.

That's because N derived from mineralization is difficult to track. In tests from 1994 to 1999, University of Nebraska researchers found no advantage to variable N applications. That's largely because they could not predict the amount of mineralized residual N in the soil.

“We used a mineralization equation that is used on a statewide basis,” says Richard Ferguson, a University of Nebraska extension soil scientist. “That's too generalized an equation.” Now the researchers will examine more localized mineralization equations.

Tools such as yield maps also help little. “Farmers looked at yield maps and could tell that part x yields 140 bu. [per acre] and part y yields 170 bu.,” Bullock says. “But it didn't take too long for a farmer to say, ‘So what? Should I put more or less N on that 170-bu. spot?’ Lots of farmers accepted the old paradigm that high-yielding fields needed more inputs, and that's not necessarily true.”

University of Kentucky research shows that high-producing soils benefit least from high N rates. In 2000, researchers divided cornfields into high-producing areas of 200 bu./acre, medium-producing regions of 115 bu./acre and low-producing areas of 50 bu./acre. They varied N rates at 100 lbs./acre and 170 lbs./acre.

In the high- and medium-yielding zones, yields rose little when the researchers applied the high N rate. But in the low-yielding zone, yields jumped from 80 to 90 bu./acre when they bumped the N rate by 70 lbs./acre.

Current soil-sampling procedures also fail to track soil N content. Grid sampling, which was the belle of the soil fertility ball in the early '90s, soon proved futile. Researchers found that soils can vary as much in an acre as they can in a 160-acre field.

“Yield maps are as imprecise as grid sampling,” says Bill Raun, an Oklahoma State University (OSU) soil scientist. “Their resolution is 900 sq. ft. Even in a 30- × 30-ft. area, there still can be lots of variability.”

Farmers who use these methods incur costs with no guarantee of additional profits. For corn, research shows that the maximum potential benefit of variable N versus a conventional rate is $5 to $15/acre. This net return often dwindles down to zero, due to sampling, controller and execution costs, says Tom Doerge, Pioneer Hi-Bred International agronomy research manager.

Help is on the way

Yet new technology may make variable rate application of N pay. At OSU, researchers are testing a variable applicator that adds N according to yield predictions made in a 1-sq.-m region.

To predict yields, sensors decipher red and infrared colors of the light spectra on wheat plants. After the sensor takes readings, the applicator immediately topdresses dry N at the Feekes 4 and 5 (tillering to joint) stages. With instant reading and application, the sensor prevents overapplication and underapplication of N.

“Why put down N for a crop that's not there?” Raun asks. “We want to increase the yield and decrease the rate. We want to have our cake and eat it too.”

The OSU trials show that the sensors do just that. In two field tests, the light sensors increased wheat yields by an average of 15% and decreased N rates by 20% when compared with uniform N applications.

NTech Industries, a California-based company, is marketing the applicator and sensors as an add-on system called the GreenSeeker. Installing it on an existing sprayer will cost approximately $60,000 for a 60-ft. boom, or about $1,000 per boom foot. Despite the high price, the company claims its product will save enough money on fertilizer to pay for itself in two years on a 2,500-acre farm. For more information, contact NTech Industries Inc., 740 S. State St., Ukiah, CA 95428, 888/728-2436, www.ntechindustries.com or circle 202.

In Nebraska, USDA-ARS researchers are testing a similar light-based sensor to apply N on corn, when it is 12 to 18 in. tall.

Because corn is larger than wheat, the sensors take light readings in a 3-sq.-m area. Sensors focus upon the green color band rather than red bands.

“Our work shows that N use in corn is highly related to greenness,” Schepers says. “When plants are measured when they are smaller, as they are in wheat, red color bands work better.”

Though individual farmers may not have the cash to use this technology, they may be able to partner with cooperatives or fertilizer dealers that have the expertise and financial resources, Schepers adds.

Raun says that yield increases and N rate decreases will easily cover costs. Retrofitting a 60- to 80-ft. A-frame applicator costs $50,000. If commercial applicators spray 50,000 to 80,000 acres, growers will only see a $1/acre application increase from their fertilizer dealer.

“The cost of the fertilizer savings will only pay for 20 to 25% of this amount,” Schepers says. “The real savings come from increased yields. If you can increase the yield in low-yielding areas, it will pay for itself.”

In time, such developments may make variable rate N economical, Doerge says. In many ways, the progress of variable rate N mimics that of variable rate seeding.

“The technology available to vary these inputs likely exceeds the knowledge of how to best use it,” Doerge points out. “When finally successful, variable rate N strategies will need to be carefully customized to fit local soil, climatic and agronomic conditions.”