Almost one-third of crop-growing land in the upper Midwest is now devoid of its most fertile topsoil, says a controversial new study. Evan Thaler, a PhD student at the University of Massachusetts, Amherst who worked on the study, acknowledged that their estimates are at odds with those published by the U.S. Department of Agriculture.

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“I think the USDA is dramatically underestimating the amount of loss,” said Thaler.

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As any home gardener knows, soil varies in color and quality. Even if you don’t garden, you’ve probably seen the different soil colors when flying over agricultural land. The darkest, richest soil is known as topsoil, the “black, organic, rich soil that’s really good for growing crops,” Thaler explained. When farmers first settled the Midwest, there was no shortage of this soil, which is full of organic carbon, made by stuff like decaying plant roots and living microorganisms. The topsoil layer is known by soil scientists as the “A-horizon.” But a century or two of plowing released this trapped carbon. Water erosion and wind scattered the topsoil. The remaining, depleted soil is much lighter in color.

Thaler and his team used satellite images and the USDA’s direct measurements of soil quality in their study. They concluded that the light brown soil is so lacking in organic carbon that it can’t be considered A-horizon soil.

Not all soil scientists are convinced by Thaler’s new study. Some question his methodology and say there’s not enough data to prove the extent of topsoil loss that he’s claiming. According to Michelle Wander of the University of Illinois, some topsoil might also be mixed into underlying soil layers, rather than entirely gone.

However, everyone agrees that topsoil is in trouble. “To me, it’s not important whether it’s exactly a third,” said Anna Cates, Minnesota’s state soil health specialist, as reported by NPR. “Maybe it’s twenty percent, maybe it’s forty percent. There’s a lot of topsoil gone from the hills.” Unless farmers are willing to till the land less and perhaps change crops to slowly rebuild topsoil, the A-horizon will continue to recede.


Image via Pixabay