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Country road with red barn, rolling hills, and farmland.

How did we get 12 inches of topsoil?

NRCS Notes: Here’s a look at how Illinois soil became the deep, fertile, crop-growing wonder it is today.

When I discussed forest resources in the Prairie State in the last column, I mentioned that early settlers didn’t understand the complex ecology that formed our tallgrass prairie. Geologists tell us this part of the country has been affected by continental glaciation four times. The last of these vast ice sheets covered nearly one-third of the state of Illinois about 25,000 years ago.

The climate was cooler back then. Because much of the fresh water was tied up in glacial ice, our climate was also drier. As it warmed, land smoothed by glaciers was exposed as ice sheets melted. Weather patterns produced winds that picked up fine particles of soil (loess) out of the Mississippi and Illinois river valleys, spreading them out on top of the newly exposed glacial till plain.

Fibrous root systems of grasses took a quick foothold in those cool, dry conditions. The grasses grew well in the short summers and then died back each year, but their roots stayed alive underground during harsh winters. Summers gradually grew longer as our climate became warmer and wetter. Normally, trees develop under these conditions, but that didn’t happen in this part of the country. The reason? Fire.

Whether started by lightning or by native hunters on the flat, windswept plain, fires would burn until they encountered a natural break. These fires occurred so regularly that they kept all but the hardiest of trees from becoming established. Grasses also adapted to longer growing seasons and increased available moisture, until they reached the climax ecological state of what became known as the “tallgrass prairie.”

Grass development
Because of how grasses first developed in short summers, they put forth most of their growth in the warmest part of the year. That’s why we call them warm-season grasses — as opposed to cool-season grasses that green up early in the spring, go through a summer growth slump, and then green up again in the fall. Think of Kentucky bluegrass in your yard. Some species, such as big bluestem and indiangrass, could easily reach 7 to 8 feet high on moist, flat plains. Species such as cordgrass grew well in swampy areas. Others, such as little bluestem and side-oats gramma, adapted to the drier hill country.

Plants like these comprised the endless seas of grass described by our region’s first explorers. In their experience, good soil grew good trees. Since they saw no trees growing here, they thought something was wrong with the soil. But even if they wanted to farm the ground, the mass of grass roots formed such thick sod, they couldn’t till it with equipment of the day. It wasn’t until 1837 when John Deere invented a new self-cleaning steel plow that they could successfully cut through the sod.

Growing and dying
Only then did settlers discover that thousands of years of grasses growing and dying in the same spot gave us some of the richest, most productive soils in the world. Naturally, this type of land became highly sought after for farming. While prairie vegetation once covered about 40% of the land in Illinois, native prairie is now only a fraction of 1% of our total land cover. It’s ironic. Some of the best remnants of prairie cling to bits of land that lie along railroads, roads and cemeteries left by the settlers who were themselves responsible for the demise of our tallgrass prairie.

I’m not suggesting we convert large acreages back to native prairie. But native grasses can be effective for pollinator habitat, wildlife corridors, even grazing. If you are interested to see how native prairie plants might fit into your conservation or grazing plans, I urge you to contact your local Natural Resources Conservation Service office. Let’s not lose the resource that gave us the nickname the Prairie State.

Dozier is the Illinois state conservationist. Direct comments or questions to

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