Here's Help with Your Toughest Nitrogen Decisions

March 2019

Nitrogen

How much nitrogen does your corn crop need each year? That question is one of your toughest, you’ve said in surveys.

Newell Kitchen, a soil scientist with the USDA-Agricultural Research Service in Missouri, is sympathetic to your quandary. The sources of plant-available nitrogen in the soil, coupled with the year-to-year variability of weather, make best nitrogen rates in any given year a very inexact science.

“In the past, the answer for many was to err on the high side,” says Kitchen. He worries that too much of that thinking still lingers. Studies indicate that up to 40% of all corn acres get 30 pounds or more of nitrogen they don’t need. The costs, both economically and environmentally, are too big to ignore, Kitchen says.

For one, he points out, numerous university research trials show the need for fertilizer nitrogen is not very related to corn yield. “Sometimes, when the right combination of weather and soil conditions comes together, you get 230-bushel yields on just 30 pounds of applied nitrogen,” he says.

The idea that today’s racehorse corn hybrids need more nitrogen is also not necessarily true, Kitchen adds. “Many modern hybrids have been selected because they use nitrogen more efficiently.”

So is it just a guessing game? No, he counters. There are good tools available that apply the economics of nitrogen and corn prices to arrive at recommendations that estimate fertilizer needs for that season. This is often called the economically optimum nitrogen rate or EONR.

One tool is the Maximum Return to Nitrogen calculator developed by university agronomists that you can access at http://cnrc.agron.iastate.edu/. It lets you plug in your nitrogen and corn prices, and it’s continually updated by the latest research. 

Kitchen further points out, “Newer tools being refined and verified now will take advantage of modern information systems and technology. They will adapt to variable soil and weather conditions.”

However, if you want to fine-tune your nitrogen management right now, Kitchen encourages you to do some on-farm strip trials. Several states offer help in setting them up, including Iowa (iasoybeans.com/onlinedb/) and Missouri (striptrial.missouri.edu). 

He suggests a rule of thumb built around ±30 pounds of nitrogen and ±5 bushels of corn yield. Give some of the trial strips an extra 30 pounds of nitrogen from your standard whole-field rate; give other strips 30 pounds under. Then compare yields.

If both the plus-or-minus strips are within 5 bushels per acre of your whole-field nitrogen rate, then you are close to EONR.

“Your nitrogen rate tool is working fairly well,” he says. 

If you don’t see a yield increase with 30 more pounds of nitrogen and you don’t see a yield decrease with 30 fewer pounds, then you’re likely over applying nitrogen. 

“Your tool for calculating nitrogen rate may be failing,” he says. 

If you see a bigger yield increase with 30 extra pounds of nitrogen, then you’re under applying.

“If you do this, I think many farmers will see opportunities to ratchet down their nitrogen, rather than ratchet up,” says Kitchen. 

While there is nothing magic in the ±30 pounds of nitrogen, when you get more than 30 pounds of nitrogen above the EONR at the end of the season, then the leftover in the soil is approximately 1 pound for every extra pound you applied. That’s a big economic loss and a greater environmental risk.

Kitchen thinks on-farm strip trials may teach some other nitrogen management lessons, too. 

“Because of soil variability, some parts of a field may respond to the extra 30 pounds of nitrogen and others might not,” he says. “It could show you the value of strategies for variable-rate applications.”

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