-Farm News photo by Darcy Dougherty Maulsby
The most recent glacier that pushed into Iowa (roughly 15,000 years ago) created the fertile Des Moines Lobe and left about 600 billion tons of loess in Iowa. The farm shown here is located in the Loess Hills of western Iowa.

Iowa is blessed with some of the world’s most fertile soil.

Ever wondered why? It goes much deeper than the prairies that once covered the state.

“Iowa’s soils start with glaciers that moved across this region thousands of years ago,” noted Dr. C. Lee Burras, a professor of agronomy at Iowa State University. “Glaciers are huge masses of flowing ice that have shaped our world.”

Thousands of years ago, Iowa was covered by a series of glaciers. Glaciers are akin to a river with a turbulent current, Burras added. “The bottom of all that moving ice breaks rocks free. Rocks break apart. When the glaciers got far enough south of the North Pole, they started to melt, and all that sediment dropped out.”

The most recent glacier that pushed into Iowa (roughly 15,000 years ago) created the Des Moines Lobe, which covers all or parts of 22 counties in north-central, northwest, west-central Iowa and central Iowa. The level to gently rolling land in the Des Moines Lobe is intensively cropped.

-Farm News photo by Darcy Dougherty Maulsby
During his career, Dr. C. Lee Burras, a professor of agronomy at Iowa State University, has presented a number of programs about Iowa's amazing soils. He is shown here visiting with audience members at a Practical Farmers of Iowa meeting in Ames about five years ago.

In the world of glaciers, the one that formed the Des Moines Lobe was a tiny bit of ice.

“From a human perspective, however, it was huge,” said Burras, who discussed the history of Iowa’s soils during an Iowa Learning Farms webinar earlier this year. “The Des Moines Lobe covers about 12,000 to 14,000 square miles. In certain places, it’s well over 100 feet thick. That tiny bit of ice left us more than 800 billion tons of ground-up sediment that help make our soils so productive in parts of Iowa.”

Nurturing life, building soil

As glaciers moved into the region that would become Iowa, they kept bringing new material that could nurture life and build soil. The glaciers stopped in Iowa thousands of years ago, because this is the “midway” point between the North Pole and the equator, Burras said.

“Iowa is where glaciers fall apart.”

What occurred “off glacier” is also key to understanding Iowa’s soils, Burras added. Every time a glacier pushed into Iowa, it would ablate (meaning a lot of melting occurred), so there was a huge discharge every summer.

“Think of a nearby stream you can hop across,” Burras said. “Back when a glacier was feeding it, that stream was the size of the Mississippi River.”

That water carried boulders, rocks and other minerals. This flow froze in the winter. Sand and gravel would fill the valleys on the landscape. As intense winds blew across Iowa, this whipped that material in the valleys upward and deposited loess on the upland. In western Iowa, soils like Galva, Primghar and Sac were formed from loess blowing northeast.

“The most recent glacier, which created the Des Moines Lobe, left about 600 billion tons of loess in Iowa,” Burras said. “If you go over to Sioux City, there’s well over 100 feet of loess in that area.”

Why isn’t New England

part of the Corn Belt?

The glacial boundary (the region representing the farthest advance of a glacier that has retreated) corresponds with America’s Corn Belt. Glaciers also covered New England, Burras said, so why isn’t that area part of the Corn Belt?

Those glaciers pushed over a different type of bedrock in New England. “They didn’t pick up the minerals that are essential for crop production,” Burras explained. “Our glaciers overrode a lot of limestone and feldspars (groups of minerals that contain calcium, magnesium or potassium). Those rocks the glaciers ‘stole’ from Canada had exactly the right materials to turn into great soils in Iowa.”

From a soil classification perspective, Iowa has six soil orders, 10 major soil regions and 507 soil series (including locally named soils like Fayette or Tama). Yet it all comes down to two major groupings of soils — mollisols, which correspond to areas that were prairie ecosystems for thousands of years, and alfisols, which occur where there were major forest areas for thousands of years, usually near river valleys.

“Both mollisols and alfisols are incredibly productive soils naturally,” Burras said. “They are inherently fertile throughout the soil profile. It all goes back to the geology of the parent materials of those soils.”

Parent material can include glacial till in the Des Moines Lobe (where Clarion, Nicollet and Webster soils are common), deep loess (in the Loess Hills) and alluvium (in the Missouri River Valley).

“The glaciers gave us our fundamental landscapes and the fundamental rooting material that plants grow in,” Burras said. “As that rooting material weathers, it releases nutrients naturally. Any time you expose fresh sediment, living organisms capitalize on the landscape.”

Those living organisms range from soil microbes to plants. As these organisms grow and die, this adds organic matter and influences the plants that can grow in the soil.

Those plants have changed through the centuries. Once the glaciers receded and loess stopped being deposited, four major climate-ecosystem “waves” progressively moved across the region that would become Iowa, including tundra/conifer forest to deciduous forest to prairie to deciduous forests, Burras said.

“The world never stays the same,” he said. “The landscape and soils evolve due to the weather, the vegetation, and the way the weather and vegetation interact with the geology.”

“We see what we know”

It’s easy to forget this history, however, and overlook these changes, since “we see what we know,” Burras said. He cites the example of an aerial photo taken northwest of Jewell in Hamilton County.

He points out that there’s a big blob in the middle of the picture that looks quite different from the surrounding landscape. That large blob was Lake Cairo, which was drained years ago. The former 1,300-acre former lake is now farmland that grows corn and soybeans, according to Prairie Rivers of Iowa. Lake Cairo is one example of countless drainage projects that shifted the ecology of Iowa, setting into motion changes that would affect the state’s communities, economy and the environment for decades to come.

“We often think something doesn’t matter if we can’t see it,” Burras said. “When I take people out to where Lake Cairo used to be, for example, they only see crop fields.”

Burras acknowledges that land in Iowa is farmed intensely, and the goal is to farm it even more intensively in the future.

“I always have to ask myself, ‘How important is natural pedology [soil science focused on the formation, nature, ecology, and classification of soil], when most people in Iowa see soils as economic features that are to be intensely managed?'”

Most people who have questions about soil in Iowa ask Burras about the Corn Suitability Rating index (CSR2), which provides valuable insights into soil’s potential productivity.

“People want to know how good or bad a soil is for growing corn or other crops,” Burras said. “They also want to know how can they manage that soil to keep yields going up.”

To increase yields, farmers change the soil, said Burras, citing tons of manure, nutrients and other amendments that farmers have added to the soil through the decades. “We’ve been intensely modifying our soils in Iowa for 190 years, and we’re going to keep modifying the soil. Iowa has incredible soils that continue to change.”