Where Has All the Soil Gone?
Isaac Larsen grew up in rural Iowa surrounded by cornfields. A geologist and geochemist by training, his PhD research took him to the other side of the world to study soil erosion in the Himalayas, one of the world’s most dramatic landscapes. Then one day, while Larsen was wandering around the yard of a pioneer Norwegian church in southeastern Minnesota after a friend’s wedding, the relevance of his life’s work for his own homeland truly dawned on him.
“I was standing at the edge of the church cemetery and there was a fence separating it from the adjacent cornfield. I saw corn growing about a foot or two below where I was standing,” recalled Larsen, associate professor in UMass Amherst’s Department of Earth, Geographic, and Climate Sciences (pictured above in Stinson Prairie near Algona, IA). “It was one of those lightbulb moments where I realized, ‘Wow, there has been a lot of soil erosion here.’”
Specifically, Larsen’s “lightbulb” idea was to use remnants of native prairie—like the churchyard—as a basis to measure the amount of erosion in adjacent farmlands since the beginning of European settlement and farming in the area. The more he thought about it, the more ideas he had for important research questions to investigate related to soil erosion in the American heartland.
“Here I was using all these new techniques to study landscape evolution in places like the Himalayas, and it occurred to me that no one was really using these tools to study erosion in farmlands,” said Larsen.
Today, Larsen and members of his Geomorphology Research Group are still among only a handful of researchers studying this consequential phenomenon. They use a variety of research methods—from taking precise measurements in fields to viewing satellite images from space—to understand the scope of the problem and its causes.
Soil Loss = Food Loss
According to Larsen, soil erosion is caused by traditional plowing of farmlands. "Every pass of a plow on any kind of hillside will move soil downslope,” he said. “Over the timescale of agriculture—which in the midwestern United States is about 160 years—that all adds up.”
Plowing removes decaying vegetation that protects soil from natural erosion by water and wind. Rainfall that would normally soak into the ground instead runs across the surface, picking up soil particles with it and exacerbating erosion.
All this erosion is detrimental to the soil’s productivity because topsoil—also known as the soil A horizon—contains most of the carbon that is necessary for the microbiotic interactions that help plants to grow. The nutrient-rich topsoil layer is also the most effective at holding onto water and preventing drought.
“Long term, this soil loss makes it more difficult and more expensive to grow food,” Larsen said. “This could have a huge economic impact on the agriculture industry, and could lead to increased food insecurity.”
The use of synthetic fertilizers can help to replace the soil function, but these are expensive—adding to the cost of food—and can leach into waterways.
Erosion also caries soil out of fields and into bodies of water, causing sediment to fill up lakes and riverbeds. This leads to problems like excessive algal blooms, such as the “Dead Zone” that develops in the Gulf of Mexico every year.
Quantifying the Problem
In Larsen’s research, everything starts out in the field. Larsen and his students use high-precision GPS instruments to measure, within a centimeter or less, the differences in soil height between native prairies—like the Norwegian churchyard he visited—and nearby farm fields.
“In some cases, we observe up to half a meter of soil erosion,” Larsen said.
The researchers also use high-resolution lidar to relate this survey data to topography. An airplane equipped with a scanner flies over the landscape shooting laser pulses down to the Earth’s surface and measures how long it takes for them to return.
“We use this data to try to understand the topographic influences on the erosion rates we measure,” Larsen explained. “We can use this information to scale up our local measurements and create regional estimates of soil loss in the Midwest over time.”
Above, from left, Caroline Lauth (MS student) and Evan Thaler (PhD student) stand in an agricultural field that has experienced soil erosion, while Jeffrey Kwang (postdoc) and Brendon Quirk (postdoc) stand in a native prairie. (Photo taken at Stinson Prairie, near Algona, Iowa, by Isaac Larsen.)
This research suggests that, since agriculture began in the region, the Midwest has lost two millimeters of topsoil a year—nearly double the rate of erosion that the U.S. Department of Agriculture (USDA) considers sustainable. Put another way, the Midwest has lost approximately 57.6 billion metric tons of topsoil since agriculture began in the region.
To understand how much agriculture has accelerated erosion, Larsen’s group has worked to quantify the thickness of topsoil, the rate at which new soil is produced, and the rate at which erosion is occurring with support from a National Science Foundation (NSF) early career development (CAREER) grant. Key to this is measuring the presence of a rare isotope, Berrylium-10, in UMass Amherst’s newly constructed Cosmogenic Nuclide Laboratory. Berrylium-10 is produced when stars exploding in the Milky Way emit high energy particles that come into contact with Earth’s atmosphere. Some of the high-energy particles make their way down to Earth, and smash into atoms in rock and soil on or near the surface, producing Berrylium-10. Larsen’s group collects soil samples, separates out pure quartz from the soil, and measures the concentration of Berrylium-10 nearly down to individual atoms. This technique, which has only been in use since the 1990s, allows them to calculate the erosion rate in the Midwest prior to the initiation of agriculture.
Larsen has also received funding from NASA’s Earth Science Division to pursue an innovative use of high-resolution satellite imagery to scale up local measurements of topsoil loss. This complete loss of topsoil—the A Horizon—can be seen by the naked eye as light tan-colored soil topping many hills around the Midwest. In research published in the Proceedings of the National Academy of Sciences, Larsen and colleagues combined satellite imagery with data from a USDA nationwide soil carbon assessment to produce an estimate of total topsoil loss across the entire Cornbelt: around 30 percent.
A Better Way
Taken altogether, Larsen’s research paints a worrisome picture of soil health in the American heartland—especially compared to the official position of the USDA.
In the mid-20th century, the USDA developed T-values—the allowable rate of soil erosion that would still maintain agricultural productivity in perpetuity—which have been used to guide agricultural policy ever since. But Larsen’s research has found a significantly lower rate of naturally occurring soil formation, while also estimating erosion rates about two times higher than the USDA’s estimates. In addition, Larsen’s research calculates that about one-third of the Midwest has completely lost its topsoil, while the USDA places this number at zero. Larsen noted that the scientific tools used by the USDA in developing its T-values are now outdated, and the values have been critiqued for some time, but have not yet been updated.
According to Larsen, the solution is for farmers to switch to no-till farming or regenerative agriculture. He and his students have developed models to simulate what would happen if all farms in the Midwest converted from agriculture to no-till farming. “We predict that erosion would be reduced by 98 percent,” he said.
Yet, such a change is unlikely to happen without policy incentives that make it easier on farmers to transition—which involves a few initial years of lower crop yields.
Once they do make a transition to no-till farming, though, Larsen said, “The benefits are huge. It’s more economical for farmers—they don’t need to buy fertilizers, they’re using less diesel fuel and chemical herbicides—and their yields are comparable. It also puts more carbon from the atmosphere back into the soil, which is an important piece of the climate change mitigation puzzle.”
This story was originally published in February 2023.