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Fine-Tune N Rates

With today's fertilizer prices, you need to make the most of your nitrogen applications to maximize the profitability of your corn crop. If you're willing and able to take a little extra time for crop management, you can use one of several tests available to help you pinpoint the best nitrogen rates for each field.

Pre-sidedress soil nitrate test (PSNT)

This soil test allows you to adjust your banded corn nitrogen applications based on what is currently available in the soil. To use the PSNT in fields where corn is 6 to 12 in. tall (four- to six-leaf stage), you need to collect soil samples to a depth of 1 ft. Labs that analyze this test provide fast turnaround to allow you to make the necessary N application before corn gets too tall.

The biggest benefit of this test is that it measures the amount of N currently available to the crop from a variety of sources, including the previous crops, manure applications, soil organic matter and N carried over from the previous growing season. It's ideal for confirming N credits from manure or legumes, which was its original purpose.

Time is the biggest drawback of this test, says Al Bertelsen, Agriliance agronomist. “You've really got to be on top of your corn crop and not be busy with other farmwork to have the time for a PSNT,” he says. “But it can provide very useful information for adjusting your nitrogen rates and is a good way to make sure you're not overapplying.”

Most major soil test labs conduct PSNTs and offer one-day turnaround of results, which are either e-mailed to you or made available online. Plan to take at least 10 sets of core samples per field, and as many as 25, if manure was applied to the field within the last six months. Each sample set should consist of three cores: one taken 3 in. to the right of the corn row, one taken 15 in. from the corn row, and one taken 3 in. to the left of the corn row. You should bag these samples and send them to the lab as soon as possible.

The PSNT is not recommended for use on sandy soils, because nitrate N is more likely to leach through them. Cost is usually $7 to $8/sample.

Chlorophyll meter

This handheld tool measures the greenness of plant leaves and can indicate if a plant is under stress due to inadequate nutrients, such as N. Results from four years of university field tests in seven states (Illinois, Kansas, Michigan, Minnesota, Missouri, Nebraska and Wisconsin) show with reasonable consistency that chlorophyll meters can track nitrogen stress in corn.

“You simply clamp the meter onto a corn leaf and within seconds get a chlorophyll level reading,” explains Doug Keiffer, soil and water product manager for Spectrum Technologies, a main distributor of the SPAD meter. “You then simply compare your field readings to those of a reference area in the field to determine if more nitrogen should be applied to all or parts of the field.”

Farmers who are concerned about laying down too much nitrogen early, for fear of N losses, can use this tool to choose N rates for sidedress applications, says Peter Scharf, University of Missouri plant scientist. “Most producers would want to sidedress as early as possible, but you can't get a good recommendation from the chlorophyll meter until the corn is at least 12 in. tall. This will limit when and where the meter can be used,” he says. How late the meter readings and N applications can wait is open to debate.

“In a field with less than 50 lbs. of preplant N applied, I'd be most comfortable with diagnosis and application by the time the corn is knee high,” Scharf says. “But it's surprising to me how often later applications gave full yield in our Missouri studies. Our longer growing season may be more forgiving than states to the north of us.”

Nitrogen rates based on the meter became more accurate as the season progressed in the seven-state study. This may provide a good management option for producers who can economically apply late N — irrigators who can inject N into their pivot systems, for example. Scharf suggests that a moderate preplant N application (80 to 100 lbs. N/acre) is advisable to get the corn to waist high or taller before diagnosing N need and applying additional fertilizer.

He says the key to getting good recommendations with this type of meter is to overapply N to a small portion of the field to provide a reference reading that measures ultimate greenness of that hybrid. “The reference area can be as small as 30 by 50 ft., and you need to apply enough N there so the corn hits a saturation point,” he says. “After that, more N has no effect on color or meter reading; it's as much as the corn plant can use. We applied N at a 200-lb./acre rate in our reference areas.”

This meter could help you cut back on preplant N to between one-quarter to one-third of the normal rate, Scharf says. “Then by checking fields with a chlorophyll meter, you could more precisely adjust your N applications to just what the plant needs, while reducing N loss, especially on sandy soils. The environmental benefits of this approach could be sizeable,” he says. In the University of Missouri study, the consistency of the results across the seven states was encouraging, Scharf says, adding, “It looks like the same color means the same thing no matter where you are.”

The most commonly used model is the SPAD 502 chlorophyll meter made by Minolta, which is available with or without data logger and GPS compatibility. Price for the basic unit is $1,495, and the unit with built-in data logger is $1,895. Contact Spectrum Technologies, Dept FIN, 12360 S. Industrial Dr., East Plainfield, IL 60585, 800/248-8873, visit or, or circle 102.

Illinois soil nitrogen test (ISNT)

A fairly new and rather controversial test, this soil screening is intended to help determine which soils will respond to additional nitrogen and which won't. Developed by researchers at the University of Illinois several years ago, the ISNT isolates and measures the amount of amino sugar nitrogen in the soil. According to Richard Mulvaney, one of the soil scientists who created the test, fields with a high level of amino sugar N (more than 240 ppm) do not respond to N fertilizer, and those with low levels (less than 240 ppm) do.

“Traditionally, higher-yielding areas of a field get more fertilizer N, but the actual response from the additional N is often lower in these areas than in poor soils,” Mulvaney says. “We've found that what matters most is how much nitrogen comes from the soil. Rich soils need less N from fertilizer, while poor soils need more.”

Making adjustments for plant populations and crop residue levels is critical to the test's accuracy, he adds. “If you've got more plants, you need larger soil reserves to feed them. And more plants lead to more crop residue, both on and below the soil. That extra residue impacts soil nitrogen cycling. With higher planting rates, more nitrogen is tied up initially, but some of this will be released or mineralized as the microbes die and decay,” he says.

The ISNT won't help you determine specific rates for each field, but it could provide insights into why certain fields respond more than others. Midwest soil scientists continue to debate the value of the ISNT in helping to predict optimum N rates, mainly because tests outside of Illinois have not yet proven reliable, says Carrie Labaski, extension soil scientist at the University of Wisconsin. “We just haven't been able to get consistent results from year to year,” she says.

Several midwestern soil labs offer this test, which costs $25 to $27/sample.

Cornstalk nitrate test (CSNT)

Although this test needs to be done toward the end of the season, it can help you assess how well you estimated the nitrogen needs of your crop. By using the pre-sidedress soil nitrate test early in the season and the cornstalk nitrate test toward the end of the season, you can better identify the most profitable N rates for your field, says Giles Randall, University of Minnesota Extension soil scientist.

“It won't help you with the current year's crop, since it's taken after the grain is physiologically mature, but it will help you evaluate whether your crop had enough nitrogen,” he says.

Based on the fact that corn plants deficient in N will draw it from the stalk and leaves to supply the needs of the developing grain, this lab test analyzes the lower half of the stalk. Typically, 15 or so 8-in.-long stalk samples are taken at least 6 in. above the ground. The time for sampling must be between one and three weeks after black layers have formed on about 80% of the kernels of most ears. Cost of the CSNT is about $14/sample.

“This test is gaining validity because we've been able to get more reproducible results in recent years, and I'm seeing more agronomists and crop consultants using it,” Randall says. “It's just one more tool to help you evaluate your overall N management.”

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