Good topsoil does not accumulate quickly. Less than a tenth of a millimeter of soil forms per year in some places, though the amount can vary depending on the environment. Compare that to the rate of topsoil erosion in agricultural regions of the United States: around half a millimeter per year, or 5 times as much, according to a recent study in the journal Catena. That imbalance is imperiling our ability to grow food in large swaths of America’s breadbasket.
“It’s really important that soil erosion should be mitigated,” said Shahab Shojaeezadeh, a hydrologist at the University of Kassel in Germany and Sultan Qaboos University in Oman and the study’s first author. “If soil is not protected, we are losing a valuable resource.”
Croplands see far higher rates of soil erosion than other places, often because tilling leaves soil exposed. It’s in croplands that erosion is most impactful, however. Past studies estimated that erosion costs the United States about $8 billion each year and that globally, it reduces agricultural food production by 33.7 million metric tons per year. Rates of soil erosion have likely worsened over the past decade, and climate change will probably make that trend worse.
To better understand the future of our soils, Shojaeezadeh and his coauthors took precipitation data from 3,200 weather stations maintained by NOAA, along with land use and land cover data from the Landsat and Sentinel-2 satellites, and estimated how much erosion is caused by rainfall, the main cause of soil loss.
They paired those estimates with a deep learning model that estimated future rainfall changes and a machine learning model that showed how land use might change in the future under three different climate scenarios.
Their results indicated that the United States currently sees 4.7 metric tons of topsoil washed from every hectare of cropland per year, on average. By 2050, that could increase anywhere from 8% to 21%, depending on the level of greenhouse gas emissions. Much of those increases would happen in the South and East, the authors estimated, where erosion could increase by more than 50% in some places.
Real Problem, Difficult Predictions
Soil erosion is a real and challenging issue, said Bruno Basso, a soil scientist at Michigan State University who was not affiliated with the research.
“We lose about a pound of soil for every bushel of corn that is produced,” he said.
The study’s estimates of erosion in agricultural areas are stark and line up with the results of previous studies, Basso said. But estimating exactly how much soil erosion is likely to happen in the future is exceedingly difficult, he noted.
Part of the issue is that changes to agricultural practices, land use, and more now will affect soil erosion in the future. For example, increasing practices such as alley cropping and cover cropping would reduce erosion. But predicting to what extent those practices will be adopted by farmers isn’t really possible.
In addition, multiple factors affect soil erosion, sometimes in unpredictable ways. A trend toward monoculture crops today has hindered soil conservation, said Rick Cruse, a soil scientist at Iowa State University who wasn’t involved with the study. Those same monocultures also leave more biomass sitting in fields when the season is over, “kind of like a carpet,” which can help protect soil, he said. The balance between those competing factors could change in the future as weather grows more unpredictable, though in what ways is unclear.
Most soil erosion doesn’t lead to soil loss either, Cruse noted. Soil that is eroded from one location usually ends up deposited somewhere nearby, instead of being washed away into rivers and oceans. So much of our topsoil is being reshuffled, rather than simply disappearing. On top of that, the amount of land used for agriculture will likely expand in the future, and the ways we use that land will also change. The study found that 4.6%–7.8% of future erosion could come simply from establishing new agricultural and urban areas.
Holding On to Our Soil
These factors make accurately estimating how much topsoil we’ll have left by 2050, or any date in the future, reasonably difficult. Nevertheless, it’s clear that topsoil erosion is a serious issue, Basso and Cruse said, one that climate change will almost certainly worsen.
Increases in the intensity of rainfall, paired with droughts that leave soil dry and loose, would leach more topsoil from agricultural fields each year. For example, the study showed that the erosive effects of rainfall will likely increase on average in the future.
Some of that eroded soil makes its way into rivers such as the Mississippi and down to the Gulf of Mexico. Agricultural soil carries nutrients from fertilizers that can lead to toxic algal blooms in the ocean, threatening sea life. Back in the fields, less topsoil means plants find it harder to grow, lowering productivity and straining food supplies.
Researchers have suggested potential strategies for reducing soil loss. Increasing the use of cover crops, which Basso said act as “an umbrella” for soil by reducing the impact of raindrops, is one. Alley cropping, or planting rows of trees and shrubs in fields to shield soil and hold it in place, is another strategy.
For those things to work, Cruse said government policy would be most helpful. “Not all farmers can grow cover crops successfully. Not all farmers can do alley cropping or do some of the things we’d really like to see,” he said. “And then policy moves them, or acts to fail to move them, in one direction or the other.”
Cruse said it’s likely we already have the tools to prevent large-scale soil erosion and hold on to the precious centimeters of topsoil that feed hundreds of millions of people in the country. Whether we’ll do it is another question.
“I’m optimistic it can be done,” he said. “I’m not optimistic that it will be done.”
