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.
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
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.”
“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.
“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.
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.
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.
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.”
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.”
Also avoid broadcasting UAN solution in no-till corn, because there's a high risk of losing N to the air and tying up N in the residue, Scharf says. Missouri research has shown a 25-bu./acre yield loss from that practice. Injecting UAN is more efficient, he says.
CONSIDER SIDEDRESSING N
“Sidedressing supplies the N at a time when there is maximum need,” lowering the potential for leaching, says George Rehm, retired University of Minnesota soil scientist. The maximum N-uptake rate by corn occurs when the plants are about knee-high to shoulder-high. On sandy soils and irrigated corn, Rehm recommends split sidedress.
Trials in southern Minnesota from 1998 to 2006 showed no yield penalty for sidedressing N, Rehm says. “Lower yields with sidedressing are almost always associated with late application,” Illinois' Hoeft says. He recommends starting “as soon as you can see the row.”
SKIP ROWSSkip-row sidedressing cuts fuel and power costs, and allows you to use a smaller tractor, Hoeft says. And the injector positions can be adjusted to avoid applying anhydrous ammonia in wheel tracks, where it's harder to get a good closing seal. “Be sure to set up the applicator correctly,” Hoeft says, “with every hose the same length.”
EVEN UP APPLICATIONPotential for uneven N application is highest for spinner spreaders that broadcast ammonium nitrate or urea, followed by anhydrous ammonia and boom-type spreaders that broadcast ammonium nitrate or urea, he says. Broadcast or injected UAN solution has the lowest potential for uneven application.
Spinner spreaders should be regularly checked and adjusted, Scharf notes. Wear on parts, humidity and poor-quality fertilizer all cause uneven application.
On anhydrous ammonia applicators, older manifolds often distribute ammonia to the knives unevenly, Scharf says. “When an older manifold is used, the most important management practice is to randomize the hoses. This means that a row getting a low rate is more likely to be next to a row getting a high rate, which will minimize yield loss.”
On boom-type spreaders, Scharf says, check the distribution pattern at least once a year for hot spots with higher output.
KILL WEEDS EARLY
You might be surprised to know exactly how much N large weeds steal. In 2006 and 2007 trials at the University of Wisconsin, Laboski found that “when weed control was postponed until weeds were 12 in. tall, an extra 100 lbs. N/acre was needed to achieve the same yield as preemergent control. An additional 40 lbs. N/acre was needed when weeds were controlled at a 4-in. height.”
COMPARE COLOR
Plant color sensors can help you gauge N deficiency and fine-tune in-season fertilization. “I think this is the wave of the future,” Scharf says.
Chlorophyll meters, aerial images and other sensing instruments measure leaf color late in the vegetative growth period, when N stress is expressed through reduced leaf greenness. The measurements must be compared to an adequately fertilized reference strip in the field. You'll need a reference strip for every field and every variety.
Nitrogen-deficiency sensing can be used as early as V-6 until pollination, Scharf says. If N deficiency is detected, apply supplemental N as soon as possible — and before silk emergence — says Dan Walters, University of Nebraska fertility expert says. You'll need high-clearance application equipment or irrigation systems equipped for fertigation.
“This is still a developing technology,” says Iowa's Sawyer. “I see it as a good tool to assess N deficiency when things don't go right and there is considerable uncertainty about N losses and additional N application needs,” as in 2008's wet spring conditions.
“There's a lot of interest in this from irrigators,” Mengel of Kansas adds. “We really like it for wheat and sorghum. It's just getting started in corn.”
TAKE ADVANTAGE OF N CREDITSAnd remember, Camberato adds, that “corn grown in rotation with soybeans requires less N, and the more expensive N gets, the more valuable that rotation is.”
In Iowa, for example, “research over many years has shown that the N rate needed for corn rotated with soybeans is 50 lbs. N/acre less than continuous corn,” says Sawyer.
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