Deep under the fields of southeastern Minnesota, Martin Larsen scrambles through narrow passageways and unexplored caverns. From this underground vantage point, he has learned how water flows from his cornfields, seeps through the earth and enters the well he drinks from.

Larsen knows the danger of consuming nitrates, which contaminate water and can lead to health problems including increased cancer risks and “blue baby syndrome”. The porous karst geology of southeastern Minnesota’s Driftless Area forms the caves that Larsen explores, but it also makes it easier for polluted water to move through the ground. Nearly 10,000 people in southeastern Minnesota drink private well water with dangerous levels of nitrates.

Swift groundwater contamination is an even bigger problem when intense rainfall occurs. “These are absolutely increasing in frequency,” Larsen said. Twice he’s been forced to leave a cave before it  flooded with sudden rain.

So Larsen and his colleagues at the Olmsted County Soil and Water Conservation District started gathering data on what was most effective for combating water quality problems and runoff.

The answer they came up with? Cover crops.

Larsen is the fifth generation of his family to farm near Byron. “Cover crops are pretty new for me,” he said. “I was an agronomist, and I was a conventional farmer. And then I started no-tilling and cover cropping following 2013, when we had, like, 17 inches of snow in May. It prevented planting, so we had these fallow acres that I could use cover crops on.” 

 Now, about a third of his farm is planted in oats, with cover crops on everything possible, except next year’s oat fields. “You have to break the crop rotation and grow a crop that’s nitrate smart, or introduce cover crops into the row cropping system,” he said. “We can tell that cover crops reduce nitrates 30 to 60 percent. We need to implement grazing systems and reverse the trend of small grains leaving the countryside.”

He and his colleagues worked with local farmers to design a cover crop incentive program that was user-friendly for farmers. “The work we did locally really made a difference,” Larsen said. It increased buy-in from producers who could see the impact that cover crops made on water quality in their county.

Olmsted County has now pioneered a streamlined cover crop incentive program that pays farmers when a cover crop reaches 12 inches. Unlike federal cover crop incentives, Olmsted County pays farmers for what they produce, not what they plant. 

Olmsted County does not require farmers to report ahead of time the exact location of acres that will be planted in cover crops or to submit seed tags. The county incentive is based on what is grown: a farmer must simply upload a photo of the 12-inch cover crop. Farmers can get more money for growing taller cover crops, with other height incentives offered to grazers. 

Last year, about a quarter of all farmers in Olmsted County participated in the incentive program, nearly doubling previous participation numbers. “To our knowledge, it’s the most participated-in cover crop program in the state,” Larsen said.

Funding for the county program is possible because the American Rescue Plan Act invested $33 million in Olmsted County, based on its population of nearly 165,000. The Rochester area is growing quickly, driven by the economic engine of Mayo Clinic.

“One thing that we’re seeing is that we have about a 2:1 payback rate,” Larsen said. The incentive maxes out at 150 acres, but participants are planting an average of about 275 acres of cover crops.

Olmsted County SWCD wants to see their program model expanded statewide, or at least regionally throughout geologically-sensitive southeastern Minnesota. Larsen estimated the program would cost about $1 million per county.

Olmsted County is one of eight counties in Minnesota in which nitrate pollution is an “imminent and substantial danger,” according to a U.S. Environmental Protection Agency request for swift action on the issue from the state of Minnesota. A January 2025 lawsuit by environmental groups allege that the state isn’t acting quickly enough.

Olmsted County-style cover crop incentives could be part of the solution.