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NPR: New Evidence Shows Fertile Soil Gone From Midwestern Farms

The following is excerpted from an article published in 2021 at fertile-soil-gone-from-midwestern-farms. Extrapolating soil organic matter levels by comparing soil color is an interesting idea. GROW BIOINTENSIVE can build topsoil up to 60 times faster than in nature, and can be an important tool for small farmers to restore soil fertility.

Farming has destroyed a lot of the rich soil of America's Midwestern prairie. A team of scientists just came up with a staggering new estimate... [that] the most fertile topsoil is entirely gone from a third of all the land devoted to growing crops across the upper Midwest... Some of their colleagues, however, remain skeptical about the methods that produced this result. ... Even the study's critics, though, agree that topsoil is endangered.

The new study emerged from a simple observation[that the] color of bare soil varies, and that variation is related to soil quality. The soil that's darkest in color is widely known as topsoil. Soil scientists call this layer the "A-horizon." It's the "black, organic, rich soil that's really good for growing crops," says Evan Thaler, a Ph.D. student at the University of Massachusetts, Amherst. It's full of living microorganisms and decaying plant roots, also called organic carbon. When settlers first arrived in the Midwest, it was everywhere, created from centuries of accumulated prairie grass. Plowing, though, released much of the trapped carbon, and topsoil was also lost to wind and water erosion. The soil that remains is often much lighter in color.

Thaler and his colleagues compared that color, as seen from satellites, with direct measurements of soil quality that the USDA has carried out, and found that light brown soil contained so little organic carbon, it really wasn't A-horizon soil at all. The topsoil layer was gone. What's more, Thaler found that this was consistently the case on particular parts of the landscape. "The A-horizon was almost always gone on hilltops," he says.

Thaler believes that a century of plowing is to blame. The soil gradually fell down hillsides, a little bit each year, as farmers tilled the soil. Thaler's team then expanded their study to fields of corn, soybeans, and other crops within a large area of the upper Midwest that includes much of Indiana, Illinois, Minnesota, and Iowa. They calculated that about a third of the crops were growing on erosion-prone hills. This produced their estimate that a third of all cropland in that region had lost its topsoil. That estimate is far higher than those published by the U.S. Department of Agriculture. "I think the USDA is dramatically underestimating the amount of loss," Thaler says. ...

Read the complete article at fertile-soil-gone-from-midwestern-farms.

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