Monitoring system evaluates nutrient losses through field tiles

WALNUT, Ill. — On-farm tile monitoring provides data to researchers that shows losses of nutrients is not simply an overapplication of fertilizer.

“This is pretty novel research on Chris’ farm and the watershed that is trying to provide good-quality evidence where nutrient losses are coming from,” said Andrew Margenot, associate professor of crop sciences at the University of Illinois.

“We hear about nitrate nitrogen and dissolved phosphorus as major nutrients that are of concern to water quality,” said Margenot during a presentation at the Nutrient Stewardship Field Day, at the Chris Von Holten farm, organized by Illinois Farm Bureau.

The program in the Walnut Creek watershed includes farmers in Bureau, Henry and Lee counties in northwestern Illinois.

“We are trying to understand and quantify the nutrient amounts being lost from fields into streams and where it is coming from,” Margenot said.

“We lose about 178 million pounds of nitrates across the state, which is about 15 pounds per acre of corn and soybeans,” he said.

For the program, the researchers are monitoring drainage tiles from 23 fields that total over 1,200 acres of drained land.

“We can measure the amount of water exiting every five minutes and we take water samples to measure nitrate and phosphate concentrations,” Margenot said.

The same is done for the Walnut Creek watershed, which is about 12,000 acres.

“We are trying to understand how many pounds per acre are being lost at the field scale and also the stream scale,” Margenot said.

Some people assume that every pound of nitrate loss through a field tile is from fertilizer.

“That is not the case because soils can mineralize organic nitrogen into inorganic nitrogen which becomes nitrates,” Margenot said. “We are finding as much as half of the tile nitrate loads are coming from organic matter sources.”

By measuring the flow, of a tile main of a known drain field area, that gives the researchers a good idea of how many gallons of water per acre are being discharged annually.

“When it rains a lot, you see the tiles flowing pretty high, but not always,” Margenot said. “In November 2025, we had a large rain event, but the tiles barely flowed because it was pretty dry going into that water year which starts on Oct. 1.”

But by springtime, he added, a lot more rain events led to more tile flow.

“Tiles are not simply one to one, it depends on how dry the soil is, and the tiles also help with the groundwater level coming up to reach the crop root zone,” he said.

“Here are three sites all within 10 miles of each other that had pretty much the same rainfall patterns,” said Luke Bergschneider, U of I on-farm research specialist.

These sites are all patterned-tiled and have the same crop rotation.

“But the flow peaks are drastically different — each site exhibits different relationships with rainfall and flow,” Bergschneider said. “Not all tile systems behave the same. They interact differently based on rainfall, so uniform regulation is not going to fix the problem.”

Part of the project is aimed at understanding the nonlinear relationships of precipitation, tile flow and streams.

Researchers use a drainage coefficient which is the percent of rainfall that fell and how much came out the tiles in inches per acre.

“In 2024, this field had a drainage coefficient of 0.32, which is 32% of the 34 inches of rain that fell that year,” Margenot said. “In all likelihood, that was a mix of groundwater and precipitation.”

In 2025, this field received 5 inches less precipitation.

“So, there was 14% less precipitation, but 45% less tile drainage,” the associate professor said.

Margenot talked about a nitrate load graph that showed the pounds of nitrates that were lost on average across 23 fields over two seasons.

“There were 14 pounds of nitrates per acre under corn and about 7 pounds of nitrate per acre under soybeans,” he reported.

“Soybeans don’t get nitrogen, so that means of the 14 pounds that we are losing under corn, roughly half of that — the 7 pounds is from organic matter because that is the amount of losses underneath soybeans,” he said. “About half of our nitrate losses are not from fertilizer.”

There is room for improvement with management practices such as planting cover crops or double cropping wheat to help reduce these losses.

“You are going to lose nitrates from these high-fertility soils, even with good practices,” Margenot said. “The question is how much less you lose, but you will always lose some because the system leaks.”

Studies evaluated the loss of nitrates on fields with a cover crop of cereal rye versus fields with no cover crops.

“We lost about 10 pounds of nitrates with the cover crop and 15 pounds without a cover crop,” Margenot said. “That’s about a 36% reduction.”

In addition, there was almost half as much water flow from tiles under the cover crop than without the cover crop.

Margenot noted that median loss of nitrates is a lot lower than the average losses.

“The median is when you list the values from lowest to highest and you pick the center of the list,” he said.

There are a few fields that have more tile flow and they are losing high amounts of nitrates per acre.

“They are losing 50 to 80 pounds because of more flow, not because they are over applying nitrogen,” Margenot said.

“If you take out the high-flow fields, the median goes to 3 to 7 pounds of nitrate per acre,” he said. “The Midwest Corn Belt average loss per acre is 25 to 35 pounds of nitrates per acre.”

Most corn-soybean rotations are in a negative N balance, Margenot said, which means more is coming out of the system than is going in.

“We have been incorrect in research to assume soybeans are self-sufficient for nitrogen,” he said. “They are not — soybeans need soil nitrogen.”

There is more nitrogen leaving with the harvest of grain than is going on either from nitrogen fixing by soybeans or by fertilizer, Margenot said.

“We are still seeing losses of nitrates under negative N balance,” he noted.

“We often have the assumption that a positive N balance or overfertilizing leads to more nitrate losses, but this demonstrates that is not the case,” he said. “Organic matter is releasing nitrates, as well, so the losses that we are seeing don’t relate to the balances.”