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Corn+Soybean Digest

Using R To Optimize N

Right product, right rate, right time, right place. A concept used by the International Plan Nutrition Institute, those are the four keys to optimizing nitrogen (N) use and stretching your dollars.


You have choices for N fertilizer. There's the usual fall anhydrous ammonia. Then there's manure. It can be a great source of fertilizer, but the trick is management.

“Manure N management is more important now, but estimating optimal manure N rates is very difficult,” says Tracy Blackmer, research director of the On-Farm Network (a program of the Iowa Soybean Association). “Manured fields come up short (of N) more often than fields with commercial sources of N. That's because leaching and denitrification are huge issues with manure.”

Understanding leaching is key in manure management. “Environmentally, nitrate leaching is the major cause of N loss: nitrate is negatively charged, and soil is negatively charged,” Blackmer says. “When water moves through the soil, nitrate will move with it.”

Some manure converts to nitrate a lot faster than anhydrous ammonia, says Blackmer. “Don't be out chasing the combine to apply manure. From an N management view, your losses stand to be a lot higher when applied that far from planting.”

There are products available to help prevent or minimize N loss. Studies by On-Farm Network showed nitrification inhibitors can be profitable, but only when conditions are right and N application rates aren't excessive.

The bottom line when it comes to using these types of products is to determine if it will be useful to your operation. “In the last three to four years, there's been a tremendous push of N stabilizers into market, allowing you to change how you place fertilizer,” says Greg Binford, University of Delaware Extension specialist. “Understand the mechanism; what is it that stabilizer is supposed to do? You may not need an available mechanism in the situation you're using or in your environment.”

The On-Farm Network also did some research on a new product designed to make the N in sidedressed UAN more effective. These trials suggested its use didn't increase profits in 2008.


Once you've decided on the right product, apply it at the right rate. The most current method of determining the optimum N rate in the Corn Belt is based on the economic optimum rate. Referred to as the maximum return to N, this rate will continually change.

“The economic optimum does not equal maximum yield,” says Binford. “The economic optimum is the point where the last increment of N applied is paid for by the increment of increased yield. If you apply less than the economic optimum, you're not maximizing your return. If you apply more, you're wasting money.”


Once the right product is ready to be applied at the right rate, it's important to figure out the right time for application.

“Applying N as close to plant uptake as possible will minimize the risk of N loss,” says Binford.

Research by On-Farm Network has shown there's not a huge difference in fall vs. spring application, as long as the right product is used.

“Anhydrous ammonia is the only commercial N fertilizer to put out in fall because it'll be there in the spring,” says Blackmer. “It's also important to remember the 50° rule, although that won't eliminate nitrification.”

Sometimes Mother Nature will have her own plans, and your right time could turn into the wrong time. Enter: rescue fertilizer treatments.

“The dilemma is always should we apply extra N or should we not?” says Peter Kyveryga, senior research associate, On-Farm Network. “Last year much N was lost due to heavy rainfall before corn could take it up. It's difficult to estimate how much yield, in dollars, was lost due to the problem.”

On-Farm Network studies in Iowa in 2008 showed that in a very rainy spring/summer, a rescue N treatment was only profitable in three of seven studies. Those trials had lower initial N application rates and above-average spring rainfall.

“For N loss, the major concern is in spring: March, April and May,” says Kyveryga. “This is the time when the majority of N applied in the fall is converting to nitrate.”

To determine if it's time for a rescue treatment, a late-spring soil nitrate test can be used, as well as canopy sensors. “Using sensors allows us to adjust N rate on the go, and apply it to the most responsive parts of the fields,” says Kyveryga.

In On-Farm Network research in 2007, sensors detected N stress and corrected it. However, in 2008, in all but one of seven trials, the sensor was unable to correct the N stress, as N deficiency appeared later in the growing season.


To determine the right place to put that right product, there are different methods.

“Diagnostic tools have greater value as the cost of N increases,” says Binford. “They provide site-specific knowledge, which improves the ability to make N decisions.”

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Some of those tools include:

  • Late-spring soil nitrate test to quantify N fertilizer loss and N mineralization

  • Guided corn stalk nitrate test to evaluate N status during the growing season and determine when N was deficient or excessive in selected areas of a field

  • Leaf chlorophyll meter to determine in-season N status.

“A pound of N is not just a pound of N. If it's lost or tied up in organic matter, the plant won't get it,” Blackmer says. “We can no longer use yield goals alone. When it comes to N, we have to look at how much is applied, how much is lost and what's left for the plant to take up.”


It's important to remember that the same fertilizer plan likely won't work year after year. Instead of following the same old plan, you should use an adaptive management strategy: “plan, execute, evaluate and adjust,” says Tracy Blackmer, Iowa Soybean Association.


Take information from soil tests, stalk samples and other tests and information for your area and farm to determine what your needs are.


Follow your plan, and put the right product at the right rate in the right place at the right time.


How did your crops do? Were your crops stressed? Did you have your best yields? Your worst? Use diagnostic tools including late-spring soil nitrate test, guided corn stalk nitrate test, leaf chlorophyll meters and strip trials.


Refine your plan for the next growing season. Take into consideration the growing conditions from the previous year, analyze test results and determine what the best rates are for your next season. Be sure to gather and save multiple years of data to create your best plan each year. “Adjustment is an ongoing process,” says Greg Binford, University of Delaware Extension specialist.

Following the plan, execute, evaluate and adjust mantra, you should be able to stretch your N dollar.

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