N mistakes have never been costlier. High nitrogen (N) fertilizer prices and high corn values mean “there's more money to be made and lost on N,” says Peter Scharf, University of Missouri Extension soil scientist.

Nitrogen source, rate, timing and placement, tillage and weather all influence how efficiently N is used. Because of the diversity of Midwest soils and climate, there are no uniform N-use guidelines. But here are some general tips for fine-tuning N management, gathered from fertility experts across the Corn Belt.

1. SELECT THE MOST PROFITABLE RATE

   Nitrogen has a point of diminishing returns, says Jim Camberato, Purdue University Extension agronomist. Above the economic optimum N rate, corn yields may increase, but the extra grain doesn't pay for the additional N expense. So, you may be able to cut your N rate without hurting profits. In Camberato's Indiana trials in 2006 and 2007, for example, the most profitable N rate for corn following soybeans was about 24 lbs./acre less than the highest yielding N rate.

   Similarly, many Iowa growers who have participated in N trials through the On-Farm Network have been able to drop fertilizer N rates by 50 lbs./acre or more with no significant reduction in profits, says Tracy Blackmer, Iowa Soybean Association research director. Applied N “typically makes up only half the N source for a crop,” he notes. The rest is supplied by the soil through mineralization.

   Recent surveys show that many growers “have not changed rates of N application much over time,” says Gyles Randall, University of Minnesota soil scientist. “But with high costs of fertilizer, it's important to consider changing your rate and using all the best management practices for N.”

   Online N-rate calculators can be helpful in selecting the most profitable rate.

   For IL, IN, IA, MN, WI: http://extension.agron.iastate.edu/soilfertility/nrate.aspx

   For NE: http://cropwatch.unl.edu/focusnitrogen.htm

   For OH: http://agcrops.osu.edu/fertility

2. INVEST IN SOIL SAMPLING

   In the western and northern corn-growing regions of Kansas, Nebraska, the Dakotas and western Minnesota, it pays to use a 2-ft. soil nitrate test, says Dave Mengel, Kansas State University soil scientist.

   “There is tremendous variability, year to year, in the amount of nitrate-N available in the soil,” he says. In spring 2007, for example, residual soil nitrate values on corn ground in Kansas ranged from 8 to 86 lbs./acre, he says. Variability in wheat was even greater, ranging from 6 to 400 lbs./acre.

   With thousands of dollars' worth of N savings on the line, Mengel adds, “you can afford to invest time and money in testing.”

3. USE THE CHEAPEST SOURCE

   “Agronomically, anhydrous ammonia is the most efficient source of N in most situations,” Camberato says. All N fertilizers convert to nitrate-N in the soil — the form that's susceptible to loss from leaching or denitrification. Anhydrous ammonia converts to nitrate more slowly than other forms of N fertilizer, taking several weeks.

   Make sure the application furrow is closing well. Don't apply the fertilizer when soils are wet, because the ammonia gas may leak out of poorly closed slots, Scharf says. The same thing can happen in dry, sandy soils. Ammonia injection works best at moderate soil moisture or in dry soils of medium or heavy texture, he says.

4. APPLY N IN THE SPRING

   “The closer you can get to the time when the plant needs N, the better,” says Robert Hoeft, University of Illinois professor of soil fertility. “That often means late preplant or sidedressing.”

   Growers prefer the convenience of fall application, but the risk of N loss is greater. In Indiana, typical winter and early spring N losses from heavier soils have ranged from 20 to 50 lbs./acre in tile drainage studies at Purdue University, Camberato says.

   Likewise, “Ohio studies showed an average total N loss of 50 lbs./acre from fall-applied N at 160 lbs./acre as anhydrous ammonia, but some years as much as 90 lbs./acre was lost,” he says.

   In Illinois, you can expect to lose 15% of fall-applied N in most years, Hoeft says, and sometimes much more. Even in the more dry Nebraska climate, fall-applied N losses average 10%, Walters says.

5. WAIT TO APPLY FALL AMMONIA

   Don't apply fall anhydrous ammonia until the soil temperature at a 4-6-in. depth falls consistently below 50° F, says Carrie Laboski, University of Wisconsin Extension agronomist. “The colder, the better,” says Iowa State University's John Sawyer, Extension soil fertility specialist.

   Growers are often tempted to apply fall anhydrous according to the calendar, instead of soil temperatures, Laboski says, but that could be an expensive error. “In a winter such as 2007 when it was warm through Christmas,” significant amounts of ammonia can be converted to nitrate, raising the risk of spring loss, she says. In 2003 and 2004, an Illinois study found that 85% of ammonia applied Nov. 1 at a northern Illinois location had converted to nitrate by May 25, compared to 60% applied Dec. 1 and 35% applied April 1.

   With high N prices, more experts are recommending the use of a nitrification inhibitor. In that same Illinois study, inhibitors cut nitrification by 30 percentage points for ammonia applied Nov. 1. In Missouri, “We recommend a nitrification inhibitor for fall and spring anhydrous ammonia,” Scharf says.

6. AVOID FALL APPLICATION ON SANDY OR POORLY DRAINED SOILS

   Coarse-textured soils have a high risk for nitrate leaching. One inch of rain can move nitrate 6-8 in. deeper in sandy soils, Camberato says. “You can lose all the nitrate in one large rainfall event.”

   Poorly drained fields, by contrast, have a high risk for N loss from denitrification. In warm, waterlogged soils, nitrate-N can disappear at the rate of 5%/day, Hoeft says. Spring ammonia or sidedress N applications are the best choices in these soils, he says.

7. DON'T APPLY NITRATE FERTILIZERS EARLY

   Never apply ammonium nitrate or urea-ammonium nitrate (UAN) solution in the fall, Hoeft says. These fertilizers are susceptible to leaching or denitrification as soon as they are applied. The same goes for early spring applications, Blackmer says.

   Using UAN in an early preplant application or as a herbicide carrier in a weed-and-feed program is especially inefficient, Camberato says. “Take that N and apply it at sidedressing time, and you'll get a lot more benefit from it.”

8. INCORPORATE UREA

   Urea should not be broadcast and left on the surface because it eventually volatilizes, Camberato says. “In warm temperatures, you can lose 30% of your nitrogen to the air” before a rainfall breaks down the granules and moves the fertilizer into the soil. Dew, fog and mists can increase losses. University of Missouri research showed a 14-bu./acre yield penalty when urea was broadcast and not incorporated.

   Don't apply urea in the winter, either, Hoeft says. “We tried it in Illinois and lost 40 bu. of corn/acre.”

   To get the most out of urea, inject it into the soil or incorporate it within three days through tillage, irrigation or ¼ in. rain, the experts say. Urease inhibitors can increase urea stability, but only by about 10 days, Wisconsin's Laboski says. “You have to balance the premium against the benefits for your system.”