ISU researcher: Iowa has lower-quality topsoil than 50 years ago

AMES, Iowa -- In the past half century, topsoil in Iowa has lost much of what made it special, says a researcher at Iowa State University.

Topsoil depth, referred to by researchers as A Horizon soil, has remained the same over 50 years, but the quality of the A Horizon soil has decreased. That's according to Jessica Veenstra, a researcher in ISU's Agronomy Department, who looked at soil studies from 50 years ago and compared them to her current readings.

Veenstra says some of the topsoil has moved from hilltops and higher elevations to lower areas due to runoff. Her findings show that hilltops have lost soils while valleys have gained soil. The averages between the losses and the gains point to A Horizon soil remaining about the same.

"All the plowing, mixing and tilling and all the activity on the landscape is making the topsoil thicker is some places and thinner in others, and overall the soil quality has decreased," she said.

The soil has lost quality because of how tightly it is packed.

Veenstra examined how the soil particles are arranged. When Iowa was primarily prairie, the soils had a more granular structure that was like a sponge. Air and water could move easily through the soil because density was very low, according to Veenstra.

Due to years of tillage, the soil is now much denser and water and roots don't move through it as easily, her research shows.

"Imagine you've got a cup of marbles," she said. "The marbles have many air pockets and space between them. That is how the soil was. Roots, water and air could move easily through it. Now imagine a cup full of dice, with the dice stacked tightly together. That is how the soil is now with very high density. The topsoil might be the same depth, but the quality is not as good."

Veenstra looked at 89 different locations from all around the state that were described in soil studies done by National Resource Conservation Service in the 1950s and 1960s. She located the same sites and took the same readings using the same protocols as the earlier studies.

"We are doing the exact same thing the NRCS did initially," she said. "The same procedures, the same conventions -- everything the same way to see how these soils have changed."

Many locations were difficult to find as the Iowa landscape has changed dramatically in the past half century.

"The original locations were measured off fence lines that are now mostly gone," said Veenstra. "Bigger and bigger fields, bigger and bigger equipment are used now and old fences are gone. I had to use aerial photos from 1950 and measure off the photos to find exact locations," she said.

Veenstra was able to pinpoint most samples with six feet of the original location and used global positioning system equipment to mark exact locations for future reference, she said.