Corn Belt Soil


Thanks to Verlyn Klinkenborg for this essay:

How the Loss of Soil Is Sacrificing America’s Natural Heritage

A new study points to a stunning loss of topsoil in the Corn Belt — the result of farming practices that have depleted this once-fertile ground. Beyond diminished agricultural productivity and more carbon in the atmosphere, it is a catastrophic loss of an irreplaceable resource.

A corn field being planted in Hull, Sioux County, Iowa. MELINA MARA/ THE WASHINGTON POST VIA GETTY IMAGES

Geologically speaking, I grew up in a small farm town on the Des Moines lobe, a huge tongue-shaped remnant of glacial activity that reaches south across central Iowa. All around us were mollisols with a deep A-horizon — a type of rich black topsoil visible in farm fields for miles in every direction. In school we were taught only one thing about that soil: to be proud of it. It was a given, a blessing, a moral fact. In a sense, it seemed to have no history. Yet when I was very young, I surely must have met old people — relatives from northwest Iowa — whose elders had helped break the prairie in the late 19th century, using heavy sod-plows and the great teams of animals needed to pull those plows through tenacious tallgrass. The way I was taught, it felt, somehow, as though the prairie’s providential job had been to keep the soil ready for a time when we would need it. By the time I was in school it was hard to find living prairie anywhere in Iowa. It had nearly all been turned.

You hear many different numbers regarding that black Iowa soil. It’s often repeated that the topsoil — the nutrient-rich A horizon — was some 14 to 16 inches deep when the prairie was first broken, a fantastic depth of fertility rivaled only by some regions in the Ukraine. By the mid-1970s — roughly a century after the prairie was broken — it was reported that, in places, half of that topsoil had already been lost to erosion from wind and runoff. There was a lot of talk about soil conservation, of course — about contour plowing and set-aside programs that paid farmers to keep marginal land out of cultivation. Yet year by year, the soil loss went on. There were also large-scale erosion events, like the floods of 2008 and 2013, in which parts of Iowa lost in a week what experts maintained was a sustainable yearly loss: 5 tons of soil per acre. It was possible to get a local sense of how much topsoil was being lost — in particular fields and drainages. But it’s been hard to get a region-wide, landscape-scale sense of the extent of Midwestern soil erosion — until now.

In late February, three geoscientists from the University of Massachusetts — Evan Thaler, Isaac Larsen, and Qian Yu — published a paper called, “The extent of soil loss across the U.S. Corn Belt.” Using high-definition satellite imagery, a recent soil carbon index, and soil spectral data, they were able to show that across the Corn Belt — which includes all of Iowa and parts of Minnesota, Wisconsin, North Dakota, South Dakota, Missouri, Illinois, and Indiana — A-horizon soil was essentially no longer present on convex slopes. What they found on those slopes was B-horizon soil — subsoil in other words, with minimal fertility, which is only exposed after A-horizon soil has been removed. What does that look like? The paper includes a satellite photo of a bare field near Clear Lake, Iowa. The low areas in the field are medium to dark brown — an indicator of A-horizon soil. But the high spots are tan and beige — the color of B-horizon soil. By calculating the exposure of B-horizon soils across the region, the scientists were able to estimate the overall loss of A-horizon soil.

The number they arrived at is shocking. “We predict,” they wrote, “[that] the A-horizon has been completely removed from 35±11% of the cultivated area of the Corn Belt.” Plus or minus 11 percent is a large range of uncertainty. But its meaning is plain. At best, 24 percent of the topsoil in the Corn Belt has been completely removed by farming. At worst, 46 percent has been lost.

It’s worth being clear here. The authors aren’t talking about reduced soil fertility or loss of mineral nutrients. They’re talking about the complete removal of the medium in which crops are grown — the utter bankruptcy of the organic richness that lay for centuries under the tallgrass prairie. The authors argue, in a sense, that we’ve been farming in the dark, though they’re never quite that blunt. Previous estimates of erosion, they write, “may have greatly underestimated the extent of A-horizon loss, and therefore the thickness or mass of soil that has been eroded from hillslopes in the Corn Belt.”…

Read the whole essay here.

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