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Determining nitrogen rate at sidedress proves challenging

TAGS: Fertilizer
corn field
HOW MUCH NITROGEN? Can you tell how much nitrogen is needed here? Obviously some N was lost in spots. Soil tests or sensors may help only in certain situations.
A tried-and-true method for assessing available N still hasn’t been found.

If you knew exactly how much nitrogen to apply on corn at sidedressing every time, you likely wouldn’t be reading this article. You would have more money than you can count from whatever machine or method you invented!

People have tried to solve this riddle for more than three decades, and there are strategies and tools out there designed to help you zero in on an estimate for a sidedress rate. However, Bob Nielsen, Purdue University Extension corn specialist, says all come with challenges. There are no silver bullets.

Nielsen and Jim Camberato, Purdue Extension soil fertility specialist, have experimented with various soil testing and sensor methods to gauge nitrogen need at sidedressing. They’ve yet to find one that is consistent and reliable at predicting accurate rates every time.

Soil test
One option is the pre-sidedress nitrogen test (PSNT). Pull cores of soil a few days before sidedressing and send them to a lab for analysis. The goal is to determine how much N is still in the soil and available for plants.

“If you’re using the PSNT where you’ve already applied commercial nitrogen fertilizer, it gets very tricky to get a good sample,” Nielsen says. That’s especially true if the nitrogen was applied in a band, such as with anhydrous ammonia.

“Some people will take a core every few inches across the width from one row to another,” he explains. “They figure they’re hitting the band with some cores and not others, and that it balances out.

“The truth is it’s difficult to get meaningful results in situations where commercial fertilizer has been applied. In cases where manure was applied, or following cover crops, or in other high organic-matter situations, the results seem more reliable. If PSNT has a fit, it’s likely in those situations.”

If you’re going to prepare a PSNT sample, collect soil cores to a depth of at least 1 foot, Camberato advises. Especially in sandy soils where N can be lost through leaching, consider collecting a deeper sample, say from between 1 and 2 feet deep, to see if N moved deeper in the soil profile.

Either dry or refrigerate samples and handle them carefully on their way to the lab, Camberato says. Request analyses for exchangeable ammonium as well as nitrate, particularly if anhydrous was applied recently or an N inhibitor was used with starter, he says. Otherwise, low levels of soil nitrate may just indicate that little conversion had occurred, not that N was lost.

Sensors and other methods
Commercial sensors are available that are designed to estimate nitrogen needs by determining how much is currently in the plant. A few consultants use chlorophyll meters, an expensive tool, for this purpose. Some cheaper methods are also available.

They all have one thing in common, Nielsen says. “They need to be calibrated in that field,” he notes. “Otherwise the numbers themselves are meaningless.

“You need a strip where more N was applied, or a very green strip where you know N is not limiting, to calibrate most of these devices. Without a comparison strip, it’s difficult to interpret results.”

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