Matt Darr, ISU

Matt Darr, professor of agriculture and biosystems engineering at Iowa State University

Aerial drones in 2020 are what yield monitors were 25 years ago – great sources of information leading to new/refined practices.

Farmers have sent up drones to photograph fields and can see inconsistent corn vegetation color across some fields.

It’s easy for farmers to see that light green corn rows sometimes match up with an applicator boom-length. The light green areas may mark the ends of booms, or row skips, or plugs.

Now drone photographs are leading farm or retail applicators to double check the precision of their equipment and the fertilizer itself.

“We encourage retailers and producers that are using dry N to be really cognizant if they are comfortable with their spread calibration and their applicator calibration before they hit the field, or their crop is likely to show it,” said Matt Darr, professor of agriculture and biosystems engineering at Iowa State University.

Darr spoke during an April webinar hosted by University of Minnesota and Iowa State University Extension and gave a number of tips for improving dry fertilizer application accuracy:

Physics comes first

The ag community will continue to push the bounds for spreading material. Applicators with 105-foot booms are out there. There is also an uptick in how much dry fertilizer is top dressed or replaces a UAN application, Darr said, but physics still applies.

“Ultimately, there are things that limit how far we can throw fertilizer that are going to govern any application, no matter what we’re trying to spread,” he said.

Four concepts of physics always determine fertilizer spread: the speed the particle is coming off the applicator; the height when it is thrown; the mass of the particle; and the shape of it, he said.

Fertilizer quality is a major driver of spread pattern quality

Darr recommends installing a particle size tester in every dry fertilizer spreader cab. A shake test will determine the percentage of material by particle size, and will allow the user to assess if the material is similar to what they’ve used in the past.

“Based on the particle size and the ballistics, our ability to throw a heavy, dense product like DAP fertilizer (diammonium phosphate) or MAP (monoammonium phosphate) is going to be different than a softer product like urea,” he said. “We have to be thoughtful about what we are trying to spread.”

Dry fertilizer applicators require annual calibration for every product

Darr reminds farmers that fertilizer spreaders need calibration to achieve good accuracy. The COV (coefficient of variation) is about 20 percent on a well-calibrated dry fertilizer applicator vs. a COV of less than 3 percent for a typical liquid applicator.

Anything that can improve the accuracy of applicators is helpful. Adjusting for changes in the weather and the product is also needed.

“Plus or minus 20 percent on a 100-pound N application is a big deal and one that the crop will show response to if not done well,” he said.

Wider spread patterns require higher spinner speeds

Iowa State University has conducted research on the differences in spreading between a spinner spreader, the more traditional high throughput machine or a pneumatic boom.

The pneumatic applicator often operates similar to a sprayer with a boom. The fertilizer is pushed down through the air stream on the boom and should have a higher consistency in delivery than a spinner spreader.

With a spinner spreader, the spread pattern will change based on the material type and flow rate. All machines can do a good job of applying consistent fertilizer, but they need to be set up for the specific material and flow rate of the field, he said.

“When we take a machine to the field, we have to consider how much it’s applying while still providing a reasonably consistent, reasonably high-quality spread pattern off the back end,” he said.

Change guidance lines for fertilizer application to avoid consistent over- or under-application

“This gets really important if you are spreading N or urea and you can only spread it 80 feet, and you drive your guidance lines at 90 feet in the field – there is going to be nutrient deficiency,” he said. “There’s going to be a yield reduction due to that.”

Darr also encourages farmers to keep variable-rate fertilizer application simple. If a variable rate map has a wide range of application rates, it would be unrealistic for a machine to accurately make those changes.

“The take-home message is technology is changing all the time in this industry,” Darr said. “It is certainly making the job easier as a fertilizer applicator, but at the end of the day, we can’t cheat physics. If we don’t have high quality products, they are not going to spread evenly.”