Fall Fertilizer Economics

Consider Data, Soil Tests Before Making Fall Fertilizer Decisions

Russ Quinn
By  Russ Quinn , DTN Staff Reporter
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While farmers need to consider soil temperature when applying nitrogen this fall, they should also consider using an inhibitor to keep it in the field over the winter. (DTN File Photo by Pam Smith)

OMAHA (DTN) -- Farmers should consider the economics of fall fertilizer application before hitting the fields after harvest, and experts say reducing fertilizer costs while still maintaining yield potential for the next growing season is an important consideration.

In an Oct. 15 University of Minnesota (UMN) Extension Nutrient Podcast titled "Fall Fertilizer Economics," three members of the University's nutrient management team discussed practices farmers can employ on their operation. The panel included Dan Kaiser, UMN nutrient management specialist; Fabian Fernandez, UMN nutrient management specialist focused on nitrogen for corn cropping systems; and Brad Carlson, UMN Extension Educator based in Mankato.


Kaiser hopes farmers use the best information they can when making decisions on fall fertilizer applications and shouldn't just apply what they took off the previous year.

"I think fertilizer is one area we are so ingrained with, particularly with phosphorus (P) and potassium (K), of what has been done historically when we look at applications," Kaiser said. "So we are looking at maybe not eliminating applications but at least reducing them."

He said it's important to look at the overall cost of your fertilizer application plan, and that using technology, such as variable rate application, is a convenient way to examine and reduce fertilizer costs. While switching technology comes with a cost, Kaiser said farmers make it back by saving money on fertilizer applications.

Carlson said applying enough fertilizer for what you think you removed from a field is not a good way to apply fertilizer because nature is too dynamic, he said.

Data from UMN Extension's farm management program show the most profitable 20% of farms in the state spent 30% less on fertilizer per acre than farms in the bottom 20% of profitability, he said. No other input had that kind of spread, with seed and herbicide at only 10%.

Carlson said that statistic shows some people are engaging in practices that do not provide a return on investment.

"And this crop removal strategy is one of them. You need to base fertility recommendation on a soil test," Carlson said. "And if you're in that very high range and above, skip it; you just don't need it."

Kaiser pointed out that applying a maintenance level of fertilizer may not be the same thing as a crop removal application. One could be applying downward of 60% of crop removal while still maintaining levels at zero- to 6-inch soil tests, he said.


Farmers should look at their soil tests and see what levels their soil has of each nutrient, he said. If your soils are in the very high category you can cut back.

The prudent thing to do is perhaps trim back to the point at which you are not spending upward of what may be needed for removal of about 80 units of P, for instance. If you have a soil test of a 30 parts per million (ppm), there is no way you are going to get a return on the cost of application, he said.

"In most cases, what we see in that circumstance is maybe going with a small starter rate being ideal, about 20 pounds," he said.

If farmers are utilizing variable rate technology, they need to identify areas of the fields that have been grossly over fertilized, Kaiser said. Then you save some money on these areas and increase yields in other spots by applying some additional fertilizer there.

Fernandez said it's important to read the soil test closely. In some areas you might need to apply a good amount of K and very little to no P, he said.

"That would be the most profitable thing that you can do instead of just applying the typical rate that you've been applying," Fernandez said.

Carlson added that soils in the southeastern part of Minnesota have higher levels of P, and thus, have a smaller response when this nutrient is applied. Then as you move northwest across the state, the soil generally has higher levels of K, he said.


Nitrogen application should not be based on date, but a soil temperature of under 50 degrees Fahrenheit 6 inches below the surface. Carlson said UMN research has shown the average date the soil temperature reaches 50 degrees in southern Minnesota would be around Oct. 25.

Those who apply nitrogen in the fall also need to consider the weather on the days following application and soil moisture.

"And the way I look at it, when you realize it's too wet you can't go, and if it's too warm you can't go either," Carlson said. "It just simplifies a weather-based situation."

Fernandez said the magic number is 32 degrees, this is where nitrification will stop, but since that's impractical in most cases, the 50-degree point is kind of a nice compromise where bacterial activity is diminished sufficiently, he said.

Anything that minimizes nitrification is useful, and nitrogen inhibitors were created for this situation.

Inhibitors provide the best return when used in the fall, because there's a long window for nitrogen to transform to nitrate, and then be subject to loss, Fernandez said.

"I would say using a nitrification inhibitor is important," Fernandez said.

Carlson said the fall application of anhydrous ammonia with an inhibitor is a best management practice in south-central Minnesota, and there is no environmental risk for using an inhibitor.

The question is, does using an inhibitor pay for itself? Carlson said UMN data shows that it does in most years.

Both a recording and a transcript of the podcast is available at: https://blog-crop-news.extension.umn.edu/…

Russ Quinn can be reached at russ.quinn@dtn.com

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Russ Quinn