But that nitrogen double whammy is causing dangerous overloads in parts of rural Minnesota, particularly in regions with intensive animal agriculture, according to a national environmental group's report out Thursday. Nitrate, a byproduct of nitrogen, taints the groundwater many Minnesotans drink and pollutes the state's lakes and streams despite decades of work to address the problem.

The Environmental Working Group (EWG) report links nitrogen overload to the proliferation and concentration of large-scale livestock farms in Minnesota. Enforcement is hampered by regulation splintered among three state agencies, and a dearth of staff to check application rates and safeguards, the group says.

Minnesota is now home to more large, concentrated animal feeding operations than any other state except Iowa, according to the U.S. Environmental Protection Agency.

"There's just too much manure and the landscape can't handle it," said EWG analyst Sarah Porter, who co-authored the study.

The report is EWG's latest look at Minnesota's growing nitrate problem. Perhaps best known for its database of farm subsidies, the advocacy group opened a Minneapolis office in 2018.

State officials did not dispute the group's latest analysis. One farm group that looked at the analysis said it distorted the picture, and that the environmental group is opposed to large-scale farming and focused on regulating agriculture.

Porter used mapping techniques to show where the nearly 50 million tons of manure that Minnesota's cattle, pigs, turkeys and chickens produce each year get spread on farm fields. The volume is about what 95 million people would produce, the report notes.

Manure is not trucked far. In the case of hogs it's typically spread on fields in a 5-mile radius.

Porter then calculated the average amount of nitrogen in the manure, as well as the amount of nitrogen in the commercial fertilizer sold, and compared it to the available acres of cropland and the state's recommended levels of nitrogen. Calculations were based on a six-year crop rotation, 2012 to 2017, and fertilizer sales in 2016.

What she found was a major disconnect. Too much nitrogen is being applied in nearly all Minnesota farm counties, she found, but particularly in those with an intense concentration of livestock operations. In 13 counties EWG deems hot spots at greatest risk of nitrogen overload, the average amount of nitrogen applied exceeded state guidelines by more than half. That resulted in more than 80,000 extra tons of nitrogen on the soil.

Topping the list is Martin County. Bordering Iowa, the county is home to more livestock facilities than any county in the state, many of them hog farms. The average level of nitrogen applied to crops in Martin County is nearly twice what the state recommends, the report said, resulting in 14,000 tons a year of extra nitrogen, on average. The city of Fairmont has been battling high nitrate levels in its drinking water in recent years.

Porter's group says the mapping illustrates, for the first time, exactly where the nitrogen overloading is worst and why Minnesota's water quality "is declining at an alarming rate."

The state limits nitrate levels in drinking water because it's particularly dangerous for pregnant women and infants to drink. Infants can develop a potentially fatal condition called blue baby syndrome that deprives them of oxygen. Nitrates have also been linked to non-Hodgkin's lymphoma and other health effects.

Extra nitrogen also wreaks havoc on the environment. It stimulates algae growth, which can block light and suck up oxygen in water, threatening fish and other aquatic life. The nitrates washing into the Mississippi River are a major contributor to the dead zone in the Gulf of Mexico.

"The question is why is there so much commercial fertilizer going in?" said Craig Cox, who directs EWG's Minneapolis office. "Clearly, it seems like there's a really promising opportunity to probably save farmers some money and protect the water if there was just much more careful crediting of the nitrogen going on in manure."

Larger livestock operators are required to have a manure-management plan, calculate how much can be spread and track applications. However, only the largest must file them with state or county regulators. Many are required only to have a plan to show regulators; inspections are infrequent.

Warren Formo, executive director of the Minnesota Agricultural Water Resource Center, which represents about two dozen major farm groups, said he thinks EWG "has a mission" and overstated the problem. He also said that "there's a continued need to help farmers do the nitrogen balance on their farm and fine-tune their practices."

"I would acknowledge that some farmers have areas where they could make progress," Formo said. "But to broadly characterize it a problem for the whole county misses the fact that most farmers in that county, I think, are spot on."

State regulators, provided a copy of the report, said they continue to work diligently to address nitrate pollution.

Darin Broton, spokesman for the Minnesota Pollution Control Agency (MPCA), which regulates feedlots, noted that within the next five years, all 80 watersheds in the state will have comprehensive plans to protect water. The agency doesn't single out farmers as the cause, he said, and "brings all stakeholders to the table to find common-sense solutions."

Department of Agriculture spokesman Allen Sommerfeld said his agency regulates commercial fertilizer but not manure, which the MPCA handles. The agency's historic Groundwater Protection Rule taking effect this year recommends farmers follow best practices for nitrogen fertilizer, he said.

The new state rule aims to curb commercial fertilizer use. It prohibits the application in the fall or on frozen fields in parts of the state with leach-prone soils such as karst, and in designated drinking water supply management areas with elevated nitrate levels.

By omitting restrictions on manure, however, the rule is "missing a tremendous source of additional nitrate," Cox said.