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Saving our soil: How to extend US breadbasket fertility
for centuries
May 25,
2023
By University of Massachusetts Amherst
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UMass Amherst geosciences professor Isaac Larsen standing on the
erosional escarpment at Stinson Prairie
The Midwestern United States has lost 57.6 billion tons of topsoil due
to farming practices over the past 160 years, and the rate of erosion,
even following the U.S. Department of Agriculture's guidelines, is
still 25 times higher than the rate at which topsoil forms.
Yet, we need not despair: researchers from the University of
Massachusetts Amherst recently reported in the journal Earth's
Future that no-till farming, which is currently practiced on 40%
of cropland acres in the Midwest, can extend our current level of soil
fertility for the next several centuries. This has implications for
everything from food
security to climate-change mitigation.
The vast majority of the food we all eat is grown in topsoil, that
carbon-rich, black earth that nurtures everything from watermelons to
brussels sprouts. What most of us call topsoil, scientists call
A-horizon soil, and these A-horizon soils, whose fertility has
developed over eons, are susceptible to erosion.
"When most people think of erosion, they think wind or water," says
Jeffrey Kwang, currently a postdoctoral fellow at the University of
Minnesota who completed this research as part of his postdoctoral
studies in Isaac Larsen's Geomorphology Research Group at UMass
Amherst and is lead author of the paper. "It turns out that the far
greater driver of soil
erosion in the midwestern U.S. has been conventional agriculture."
But what that current rate of erosion is has been very difficult to
pin down precisely, though, as the Geomorphology Research Group has
shown over the past few years, soil erosion in the U.S.'s breadbasket
is far greater, and occurring at a far faster rate, than had
previously been suspected.
![](images/images_06_06_2023/pic3.jpg)
Erosion on the Willis farm since European
settlement. Dark brown represents rich topsoil. Credit: Jeffrey Kwang
A brief history of soil loss in the Midwest
Since 2021, members of Larsen's research group, including Kwang,
Evan Thaler, Caroline Quarrier and others, have been breaking new
ground in the world of soil science.
The group's initial study showed that more than one-third of
the Corn Belt in the Midwest—nearly 30 million acres—has completely
lost its carbon-rich topsoil, that rich A-horizon layer. Furthermore,
the team showed that the erosion was likely due to contemporary
tillage practices, in which plows are dragged through fields, moving
topsoil from higher to lower elevations. Unfortunately, the USDA's own
assessments don't include erosion due to tillage, and so the agency
has missed a major driver of erosion.
A year later, the team discovered that the Midwest has lost
57.6 billion metric tons of soil since Euro-American cultivation of
the region began, approximately 160 years ago. This historical rate of
loss, which is mostly due to tillage, is nearly double the rate which
the USDA considers sustainable.
Finally, the team recently showed that Midwestern soil is
eroding between 10 and 1,000 times faster than it did in the
pre-agricultural era, and that the USDA's current upper-limit of
sustainable erosion—1 mm per year—is an average of 25 times more than
what is actually sustainable.
![](images/images_06_06_2023/pic4.jpg)
Cerro Gordo County, Iowa and detail of individual farms
(inset) soil loss over next 160 years under a conventional
plowing scenerio. Dark red areas show soil loss of 32.5 cm (12.8
inches); blue show gain of 32.5 centimeters. Credit: Jeffrey Kwang
Modeling the future
"We already discovered how the history of erosion in the U.S. has
shaped our present reality," says Isaac Larsen, associate professor of
earth, geographic and climate sciences at UMass Amherst and the
paper's senior author. "But what's going to happen in the future?"
For this latest research, Kwang, Larsen and the Geomorphology Research
Group relied on the insights of their earlier work into historical
rates of erosion to predict future scenarios. Their first breakthrough
was to finally determine the current rate of tillage-driven soil
erosion. It turns out that the Midwest loses 1.1 kilograms of soil and
12 grams of soil organic carbon (SOC) per square meter every year,
which far outpaces the rate at which new topsoil is created.
But no one knows what the future will look like. "Since we don't know
how farming practices and policy will change," Larsen says, "we've
used the current erosion rate to model a few different future
scenarios."
"We looked at the current business-as-usual method, under which
approximately 40% of the midwestern U.S."s acres are no-till farmed,
all the way up to 100% adoption of no-till methods. We then modeled
the erosional rates under each scenario for the next century," says
Kwang.
Their initial finding was that, if the U.S.'s current agricultural
practices remain largely unchanged, approximately 8.8 billion metric
tons of soil and 170 million metric tons of soil organic carbon will
be lost over the next century alone.
When the team modeled the impact of a 100% no till scenario, the
picture turned rosier. Much rosier.
"Approximately 95% of the erosion we see under the business-as-usual
scenario over the next century would be prevented," Kwang says.
Put another way, the soil savings are so significant that if the U.S.
adopts no-till practices now, it would take 10,000 years to see the
same level of soil and SOC loss that would occur in only a century if
our agricultural practices do not change.
Furthermore, the rate of loss decreases over time: the more soil and
SOC there is, the faster we lose it, and the rates of loss taper off
as there's less to lose. "This means there's real incentive to act
now," says Kwang, "when we'll see the most long-term benefit."
Soil and climate
It's no surprise that topsoil is crucial for agriculture; but most
forecasts for greenhouse gas emission and plans for climate mitigation
also need to account for topsoil loss, because soil is the largest
pool of terrestrial carbon. Scientists hypothesize that accelerated
soil erosion alters this carbon pool enough to influence the global
carbon cycle.
However, Kwang says, "most models that look at soil and its effect on
climate don't account for erosion rates slowing down over time. We
need to get this right if we're to prepare effectively for the
future—and know we have a rate that can help inform predictions of
what the future climate might be."
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