NOEL KING, HOST:
Farming has destroyed a lot of the rich soil of America's Midwestern prairie. Some scientists have come up with an estimate of how much soil, and it is staggering. Here's NPR's Dan Charles.
DAN CHARLES, BYLINE: If you fly over the Midwest in spring and look down at the bare ground, you can see slight differences in color. There's a lot of dark earth, what people call topsoil. Scientists like Evan Thaler, a PhD student at the University of Massachusetts Amherst, call it the A horizon.
EVAN THALER: Black, organic rich soil that's really good for growing crops.
CHARLES: And it's a storehouse of carbon in the form of living microorganisms and decaying plant roots. When settlers first arrived in the Midwest, it was everywhere.
THALER: And the roots in the prairie go down, you know, six to eight feet sometimes, creating this really deep, dark soils.
CHARLES: But plowing destroyed a lot of it, releasing huge amounts of carbon dioxide, contributing to climate change. Erosion also carried it away. And now you'll also see areas that are a different color - light brown. Thaler started comparing that color as seen from satellites with direct soil measurements, and he found that the light brown soil had so little organic material, you really couldn't call it A horizon soil. That topsoil layer was gone consistently on particular parts of the landscape.
THALER: The A horizon is almost always gone on hilltops.
CHARLES: Thaler says most of it just gradually fell down the hillsides as farmers tilled the soil year after year. So he estimated topsoil loss across much of the upper Midwest based on how much crop land is located on these erosion-prone hills and just published the result in the journal PNAS. He says a third of all the cropland in this whole area has completely lost its layer of topsoil. That's way more than official estimates from the Department of Agriculture.
THALER: I think the USDA is dramatically underestimating the amount of loss.
CHARLES: Some soil scientists in the Midwest are not convinced. This method, they say, relies on a lot of assumptions. But they agree that soil loss is a big problem; Anna Cates, for instance, who's Minnesota's state soil health specialist.
ANNA CATES: To me, it's not important whether it's exactly a third. Maybe it's 20%, maybe it's 40%. There's a lot of topsoil gone from the hills.
CHARLES: Farmers know that those eroded hilltops are less productive, she says, and many of them are looking for solutions.
CATES: We're essentially trying to make up for many years of fairly thoughtless practices.
CHARLES: There's no quick fix, she says, but there are ways to farm and also rebuild topsoil, starting with don't keep tilling those fields. Maybe even grow grass there. Harvest hay instead of corn. Dan Charles, NPR News.
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