The equivalent of 70 nurse tanks of nitrogen per day and one truckload of phosphorous per hour is flowing through the confluence of the Des Moines and the Raccoon rivers in central Iowa. This represents an estimated $50 million annually in agriculturally valuable nutrients lost as water pollutants. So says the “Currents of Change” analysis recently released by Iowa’s Polk County.
“Those nutrients would be much better spent as fertilizer applied to crops that are actually helping our plants grow and producing food that feed people,” says Eliot Anderson of University of Iowa, one of the analysis’ 16 contributing scientists, who spoke at a livestreamed public event presenting the findings.
The analysis highlighted several key findings about the central-Iowa watershed that supplies drinking water to the Des Moines metro area. These included ecological issues like declining biodiversity, fish kill events and harmful algal blooms and water safety problems like antibiotic-resistant, disease-causing pathogens, PFAS and pesticides.
But excessive nutrients, namely agriculturally-sourced nitrogen and phosphorous, were a key focus.
Awash in Nutrients
According to both the analysis and the scientists who presented the findings, the watershed’s normal nitrate levels sit at around 6 to 8 milligrams per liter. While this is lower than EPA’s nitrate standards for drinking water at 10 milligrams per liter, it still ranks the rivers as having among the highest nitrate concentrations of the country. The analysis and presenters note nitrate concentrations regularly spike up to double that level, especially following rain events. Too-high nitrate concentrations in drinking water can have severe health consequences, including “blue baby syndrome” and various cancers.
Concentrations of total phosphorus in the watersheds ranged from roughly 0.2 to 0.7 milligrams per liter depending on location. These levels were above the national average for comparatively sized watersheds. High total phosphorus can result in harmful algal blooms.
Jerald Schnoor of the University of Iowa, another of the analysis’ contributing scientists who spoke at the public presentation of the findings, says stakeholders might well ask where the nutrients are coming from.
“Most of it is from agricultural land — roughly 80% — and of that, 40% of the nitrate is coming from fertilizers applied directly onto the land, about 20% is coming from manure onto the land, and another 20% from soybeans and other legumes that can fix nitrogen and make more nitrate available to run off into tile drainage,” he answers for nitrogen.
For phosphorous, roughly three-quarters of what is in the rivers came from “the state’s extensive corn and soybean production,” overwhelmingly from applied fertilizers, according to the analysis. It cites 2019 data from the U.S. Geological Survey sourced from monitoring sites across the Midwest in identifying the sources of nutrients in the waterways.
However, the problem of high concentrations of nutrients in the Des Moines and the Raccoon rivers is not new. The analysis itself shows nitrate concentration rates regularly getting into current ranges going back to the early 1980s. A 1992 essay from Iowa State University’s Leopold Center for Sustainable Agriculture notes high nitrates was a long-running problem at the time. It points out that concentrations going back to the early 1980s were quite close to those of the mid-1940s, back before commercial fertilizer use.
A History of Nutrient Control Efforts
Agricultural efforts to curb nutrient runoff in the area isn’t new either.
Greg Wandrey, director of sustainability at the Iowa Corn Growers Association, points to the 2013 Iowa Nutrient Reduction Strategy as just one example.
“The Iowa Nutrient Reduction Strategy goal for phosphorus has been met,” he says. “Farmers have done a tremendous job reducing soil erosion with different practices like cover crops and less tillage and strip-till.”
Wandrey also describes the work of the Iowa Nitrogen Initiative, a public-private partnership run by Iowa State University. The initiative does on-farm trials to try to determine the agronomically-optimum application rate. He says the results of the hundreds of trials is the realization that application rate recommendations of a couple of decades ago — roughly 1.2 lb. of nitrogen per bushel produced — are too high for modern corn production practices and needs.
“The number of pounds of nitrogen per bushel is down to about 0.85 to 0.9,” he says. “The nitrogen use efficiency of farmers has improved dramatically over the last 15 to 20 years.”
And those improvements shouldn’t be surprising, he points out.
“[Farmers] don’t just put on a bunch of nitrogen just for the fun of it because nitrogen is, if you own your land, probably the second-highest input cost behind seed. And if you rent, it’s probably the third-highest behind rent and seed,” Wandrey says.
Iowa corn growers aren’t the only ones involved with controlling nutrient runoff into the watershed.
“Iowa pig farmers care deeply about clean water and clean air, and we always have, because we live here too,” says Pat McGonegle, CEO of the Iowa Pork Producers Association, in a public statement. “We’re constantly improving how we manage nutrients and care for the environment, from using advanced manure management practices to investing in research and conservation partnerships.”
Ag retailers also have been working on fertilizer best practices for some time. In a news release celebrating its 25th year, the Iowa-based Agriculture’s Clean Water Alliance (ACWA) announced its official adoption of the 2025 Fertilizer Code of Practice. It describes the code as “a voluntary agreement between 12 ag retailers statewide to prohibit sales and application of nitrogen in early fall when conditions make nutrients more volatile.”
“Over the course of 25 years, we as an ag retail community have adopted numerous practices like nitrogen stabilizers, cover crops, reduced tillage, the 4R’s and many other practices to maximize the nutrients we are applying to be as efficient as possible,” says Dan Dix, ACWA board president. “The Code of Practice is just one tool we collectively implement to meet these nutrient goals.”
Dealing With Weather Problems
With these long-running efforts at reducing nutrients in the central-Iowa watershed, why do the levels continue to be so high? Wandrey points to weather when it comes to nitrogen.
“The single biggest factor in nitrate moving is rainfall,” he says, explaining nitrates are mobile in water, meaning that rain leads to more ending up downstream. “The last couple of years when we had literally no rainfall after June 15, there were no nitrate problems just because it wasn’t moving. It was staying in the soil and being used, and it wasn’t leaching down out of the fields.”
The inverse is also true, Wandrey notes.
“When we had all of this rainfall this year, way above average rainfall, you’re going to get more leeching,” he says. “Rainfall is the No. 1 factor.”
The analysis notes: “The greatest episodic nitrate events tend to occur in the days or weeks following heavy rain in the spring and summer.” It also highlights that, during the same time as improving farmer nutrient efficiency, climate change has meant more and more intense rain during that time, increasing 16% over the past 30 years. Not only can rainfall mobilize nitrates, the analysis notes it can increase erosion, a major contributor for phosphorous in the watershed.
Since rain cannot be controlled, farmers are working with what they can control. Wandrey points to shifting trends in nutrient application times as one example of farmers adapting to changing needs.
“We’re seeing farmers go to more in season [application] because during mid-May to July 1, that’s when that plant is rapidly growing,” he says. “Getting that nutrient, nitrogen in this case, applied when the plant is using it at that rapid rate has been found to be much more effective compared to just a fall application.”
