A few years ago, Isaac Larsen attended a wedding at a pioneer church in Minnesota. After the ceremony, he wandered around a cemetery by the church.
He noticed the cemetery, which had never been tilled, was at least a foot higher than a corn field just on the other side of a fence.
“That was one of those ‘lightbulb’ moments that told me that a lot of soil had been eroded from that field since the founding of the church,” Larsen said.
The University of Massachusetts Amherst geosciences professor and his co-researchers have released a new study that found topsoil in the Midwest is eroding at an average rate of 1.9 millimeters per year. They measured elevation differences between native prairie and farm fields at about 20 sites, the majority in central Iowa, with some in Illinois, Minnesota, South Dakota, Kansas and Nebraska.
The researchers estimate the Midwest has lost 57.6 billion metric tons of topsoil since farmers began tilling 160 years ago. This erosion, Larsen said, makes it more difficult and more expensive to grow crops.
“We’re going to need to feed more people in the future,” he said, “and degraded soils that have lost their organic rich horizons just aren’t as productive.”
The solution, Larsen said, is to adopt no-till farming. “It’s not some unsolvable problem.”
In Iowa, never-farmed prairie is one of the rarest ecosystems besides oak savanna, said Emily Martin, the conservation programs coordinator for the Iowa Natural Heritage Foundation. The group helped the researchers identify prairie sites throughout the state where they could take measurements.
Martin said the study’s findings that topsoil is eroding at an average rate of 1.9 millimeters per year is “a shocking number. But it’s not surprising either.” She said conservation groups like the Iowa Natural Heritage Foundation need to work with farmers to protect land and further integrate conservation with agriculture.
As topsoil thins, soil productivity declines. Iowa State University agronomy professor Richard Cruse said farmers can lose 50%-70% of their yield potential because of the loss of topsoil.
“When you’re talking about that, you’re also talking about sustainability or resilience, the potential for an area to produce under good times and bad,” Cruse said. “The loss of soil reduces that capacity.”
Cruse, who said he thought the study was “done pretty well,” added that topsoil isn’t gone. Most of the topsoil that moves because of erosion is deposited in the same field downslope. But that still affects production potential, he said.
“The loss from the hillslope, where it is progressively thinning, has a much bigger negative impact on production than does the accumulation of that on the soil downslope which already has more than adequate topsoil to produce,” he said.
Cruse added a little less than a third of Iowa’s soils are no-tilled, so “it is realistic” to stop tillage, he said.
“By and large, we have the technology now to make no-till work or something that approximates it, maybe strip till,” Cruse said. “So it’s realistic. It’s more challenging with some soils than others.”
