116 3rd St SE
Cedar Rapids, Iowa 52401
It makes sense that farmers would want to apply only as much fertilizer as they need to maximize their yield. Any more and it’s money wasted on fertilizer that likely will run off into a nearby stream or lake.
Nearly 20 years ago, a group of Midwestern soil scientists, including Iowa State University Professor Emeritus John Sawyer, developed the Maximum Return to Nitrogen, a practice that calls for weighing the prices of corn and fertilizer to find the sweet spot of productivity and profit without waste.
They also developed the Corn Nitrogen Rate Calculator, an online tool that lets farmers in six Midwest states plug in local prices and get a fertilizer rate recommendation based on scientific field trials in their area.
But lately, some in Iowa’s agricultural community have questioned ISU’s nitrogen fertilizer recommendations.
“Why is that particular tool and that logic in general, why is that flawed and what’s a better, maybe more informed way of looking at this issue of nitrogen that in our soil, both naturally and what’s being applied?” Wheeler asks about five minutes into the Spokesman Speaks podcast.
Robinson responds: “Many soil scientists say that the long-standing MRTN rate calculator, which estimates the economic return to nitrogen application rates with different nitrogen sources and corn prices, really doesn’t account for weather and soil variability or changes and improvements in genetics and management that we’ve seen over time. That kind of information hasn’t been updated in the MRTN.”
Wheeler declined to answer The Gazette questions, including a request to know which soil scientists think ISU’s recommendations are outdated.
ISU actually has updated its recommendations regarding nitrogen fertilizer application in recent years, said Antonio Mallarino, an ISU professor of nutrient management research and extension.
After using the N-rate calculator to determine how much nitrogen to apply, farmers may tweak that amount after taking soil samples in the late spring when the corn plants are 6 to 12 inches tall, Mallarino said. Another technique is using remote sensing, either through a drone or satellite, to determine based on the color of corn leaves whether the farmer needs to apply more nitrogen.
“We recognize there are other tools that could adjust that rate,” Mallarino said.
Mallarino, who grew up on a dairy farm in Uruguay and has worked as a soil scientist in Iowa for more than 30 years, said he is open to other ideas for nitrogen fertilizer recommendations — once these ideas are proven through years of research, like the MRTN.
“When they show something else works better, we will adopt it,” he said. “Until then, we will use this.”
Too much fertilizer in some parts of state
Many farmers don’t follow ISU’s fertilizer recommendations, with some using too little and others too much, Mallarino said.
Farmers surveyed in 2017, 2018 and 2019 by the Iowa Nutrient Research and Education Council, a group that includes commodity groups, fertilizer companies and crop advisers, reported their nitrogen fertilizer application at rates that were more than 30 pounds higher than rates recommended by ISU.
A 2019 University of Iowa study showed farmers in two Western Iowa watersheds — Floyd and Rock — were applying nitrogen fertilizer, in the forms of commercial fertilizer and livestock manure, at more than double the ISU rate.
The Floyd and Rock watersheds, thousands of square miles in northwest Iowa ultimately draining into the Missouri River, have the highest density of animal feeding operations in the state.
Nitrate levels on the Floyd and Rock rivers in 2017 consistently were over the 10 milligram-per-liter standard for drinking water, with the Floyd River averaging 16 milligrams per liter and the Rock at 11.5 milligrams per liter, the research paper reported.
Nitrate in drinking water has been linked to health problems, including colorectal cancer, thyroid disease and infant methemoglobinemia — often called blue baby syndrome — a life-threatening condition reducing the blood's ability to carry oxygen.
Whose recommendation to follow?
The purpose of the Maximum Return to Nitrogen approach isn’t to protect water quality. It was designed as an economic tool to let farmers know how to get the best bang for their buck on fertilizer.
“It’s predicated on being profitable for farms,” said Carrie Laboski, a soil science professor at the University of Wisconsin-Madison who helped develop the MRTN strategy in 2004.
“But what we know from other research we’ve done, if there’s over-application of N (nitrogen) to the point you’re not making money off that application anymore, that’s detrimental to water resources and to the environment in general.”
Mallarino said ISU has been educating farmers about the Maximum Return to Nitrogen approach since 2004 through field days and ISU Extension. But farmers may choose to listen to other voices.
“What some of them want, especially those interested in selling fertilizer, is for recommended rates to be higher,” Mallarino said. “If we recommend higher rates, some farmers may be happy, but they will get less profits and we’ll contaminate the water even more.”
Farmers who want to get federal subsidies to defray the cost of developing nutrient management plans must follow ISU’s nitrogen fertilizer recommendations. The U.S. Department of Agriculture’s Natural Resources Conservation Service is revising its standard for nutrient management and will have public comment this fall.
Kevin McCall, state resource conservationist, expects the nitrogen rate calculator to continue to be part of the requirements.
“It’s about as good of tool as we’ve got,” he said. “It’s a very reasonable number to allow producers to use to maximize the environmental impacts and profitability.”
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