Metric analysis

The US Midwest has lost 57.6 billion metric tons of soil due to agricultural practices

A new study in the journal Earth’s future conducted by the University of Massachusetts at Amherst shows that since Euro-American colonization around 160 years ago, agricultural fields in the American Midwest have lost an average of two millimeters of soil per year. This is almost double the rate of erosion that the USDA considers sustainable. Additionally, USDA estimates of erosion are between three and eight times lower than the figures reported in the study. Finally, the study authors conclude that plowing, rather than wind and water work, is the primary culprit.

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Isaac Larsen, professor of geosciences at UMass Amherst, standing on the eroding escarpment of Stinson Prairie, Iowa. Credit: UMass Amherst

“A few years ago, my wife and I were at a wedding at a pioneering Norwegian church in Minnesota,” says isaac larsen, professor of geosciences at UMass Amherst and one of the paper’s co-authors. “After the ceremony, I walked to the edge of the cemetery, which was surrounded by cornfields, and I was shocked to see that the surface of the field was a few feet lower than the surface of the never-ploughed cemetery. I started to wonder why.

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The team’s GPS unit marking the escarpment in Willis Prairie, Iowa. Credit: UMass Amherst

Fast forward a few years, and Larsen, with the paper’s co-lead authors, Evan Thaler, who completed the research as part of his Ph.D. at UMass Amherst, and Jeffrey Kwangpostdoctoral researcher at UMass Amherst at the time of the study, found himself in central Iowa on the “escarpment,” or drop, separating a native prairie from a soybean field.

Thaler had worked extensively with the Iowa Natural Heritage Foundation and other organizations to identify the few remaining pockets of original, never-cultivated prairie. He then reached out to farmers whose land adjoins the grasslands, asking permission to survey their fields. Thaler ended up with twenty sites, the majority of them in central Iowa, with a few in Illinois, Minnesota, South Dakota, Kansas, and Nebraska. “I spent days driving around the Midwest, knocking on doors,” Thaler said. “People want to see your face and have a conversation before they let you onto their land. No one turned me down when I showed up in person.

Once Thaler got clearance from the landowner, the team got to work. Using an extremely sensitive GPS unit that looks more like a lamppost than a hand-held device, the team walked dozens of transects or perpendicular routes across the escarpment, from untouched grassland to field eroded agricultural, stopping every few centimeters to measure elevation change. They did it hundreds of times over the summers of 2017, 2018 and 2019.

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Jeffrey Kwang
Jeffrey Kwang walking a transect across the eroding escarpment marking the boundary between unploughed land and farmland. Credit: UMass Amherst

Once they had their raw data, the team used historical land use records and state-of-the-art computer models to reconstruct erosion rates across the Midwest. What they discovered is that topsoil in the Midwest is eroding at an average rate of 1.9 millimeters per year. Put another way, the authors estimate that the Midwest has lost about 57.6 billion metric tons of topsoil since farmers began tilling the soil 160 years ago. This is despite the conservation practices put in place following the Dust Bowl in the 1930s.

It is also clear that a large part of the erosion is due to tillage, plowing. “The modeling I’m doing shows that plowing has a ‘diffusive’ effect,” says Kwang. “It melts the landscape, flattening the highest points of a field and filling in the hollows.” But because the USDA does not explicitly include such “tillage erosion” in its own analysis, it has “significantly underestimated the rate of erosion” currently at work in the heartland, says Thaler.

“As erosion degrades our soils, it reduces our ability to grow food,” says Larsen. “Combine that with the increasing world population and climate stress, and we have a real problem.” The team suggests that more sustainable practices, such as no-till agriculture and soil regeneration, “will likely be needed to reduce soil erosion rates in the Midwest to levels capable of maintaining soil productivity, ecosystem services and long-term prosperity”.