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Serving: OH
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JUST RIGHT: The ultimate amount of nitrogen required is determined by three things — crop demand, nitrogen release from the soil and nitrogen loss from the system.

Should you change your nitrogen application rate?

Ask a CCA: Weather poses the biggest challenge in deciding on rate.

An often-asked question, not just from farmers but policymakers as well, is this: What is the right rate of nitrogen to apply? While the ultimate answer is too much for me to tackle completely in this short communication, I want to discuss an ideal system, the realities and some opportunities to make changes.

So, what does an idealistic system — one that would generate the highest efficiency — look like? The ultimate amount of nitrogen required is determined by three things — crop demand (dictated by achievable yield); nitrogen release from the soil (mineralization); and nitrogen loss from the system, which is dictated by rainfall and soil drainage. If we had the ability to accurately predict these three, we could do a much better job at making nitrogen rate decisions. It would still not be perfect, but we could make substantial improvements.

The reality is, we do not know what the achievable yield is in the spring when most nitrogen is being supplied. We do not have a reliable method of measuring and predicting nitrogen mineralization, although we do have some tools that have specific applicability. And we do not have a reliable method of measuring and predicting nitrogen lost from the system.

There are different models that exist to predict these, but just how robust are they? The biggest challenge we have in accurately predicting yield, nitrogen release and loss is our inability to accurately predict weather. So, what are we left with?

MRTN guidelines

Ohio State University, like a lot of universities in the Midwest, employs the maximum return to nitrogen (MRTN) approach to nitrogen recommendations. This approach relies upon an average response to nitrogen (determined from field experimentation); and from that average response, an economic recommendation is developed.

On average, the MRTN approach does a good job of providing nitrogen rate guidelines. It is not perfect though; it can over-recommend nitrogen in some environments, and under-recommend nitrogen in others  An evaluation of data from more than 228 field experiments conducted in Ohio reveals that approximately 40% of the time, MRTN recommendations over-recommend nitrogen, and 30% of the time, MRTN recommendations under-recommend nitrogen. Do not assume that these “misses” (specifically, the under-recommended ones) represent dramatic yield loss, because they generally do not.

How do you evaluate your current approach to make improvements in rate decisions?  Here are couple of things to think about:

Guidelines. How does your typical rate compare to the MRTN guidelines? If you are on the high side, could you decrease your rate slightly? Conversely, if you are on the low side, could you increase your rate slightly?

Additives. Are you using a nitrogen additive (urease inhibitor or nitrification inhibitor) or an enhanced efficiency fertilizer that could result in decreased overall application rate?

Timing. Can you slightly alter your timing and/or placement of nitrogen to change rate? Sidedressing is an improvement (at least theoretically) over preplant applications, and there is some evidence that it can be advantageous in certain environments.

The ability to determine the right decision for your farming operation is dictated by your level of understanding the complexities of the system you manage, and finding opportunities to make tweaks that lead to improvements. The value of collaborating with a CCA is accessing that individual’s knowledge of the system and exposure to scientific information to help lead you to a better decision.

Reach Mullen, a certified crop adviser and Nutrien director of agronomy sales, at 330-601-0396 or 847-830-1657 or

TAGS: Crops
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