Trials document cover crop benefits in soil

LEXINGTON, Ill. — A team of Illinois State University researchers are evaluating the impact of cover crops on carbon sequestration and organic matter.

Rob Rhykerd, ISU Department of Agriculture soil science professor, detailed the early results of the trials at University Farm’s recent cover crop field day.

“One of the things with cover crops is the talk about being able to increase the organic carbon content in the soil,” Rhykerd said.

“If we can increase the organic matter content, that has a lot of benefits to our soils. Increased organic matter increases water-holding capacity, you have more organic matter, more nitrogen in the soil, that’s going to mineralize and be available to the crops. It helps the field to drain better, it helps improve soil structure and we could go on and on.”

The short-term impact of cover crops on the soil profile and organic carbon content study featured 2-acre plots of pennycress, cereal rye and a pea/clover/radish/oats mix, along with a check plot.

The trials began in the fall of 2020. Corn for silage was harvested early, allowing for the cover crops to be planted in September.

Biomass

The cover crops in the plots were terminated in the spring and the biomass was measured.

The cereal rye had 20.5 tons of above-ground biomass per hectare, or 2.47 acres, on a dry weight basis. The pea/clover/radish/oats mix produced 10.9 tons of biomass per hectare and the pennycress 5.7 tons per hectare.

Soil samples were also collected and analyzed.

“When we traditionally take a soil sample, we pull the core, 6, 7, 8 inches whatever, we homogenize it, send it off to the lab and we mix all of that up. So, we took a slightly different approach and with cover crops we want to see is there a short-term impact,” Rhykerd said.

Soil samples were collected in the fall of 2020 and June 5-12, 2021. Soil samples were collected to a depth of 20 centimeters, or 7.87 inches, and petitioned into 2-centimeter sections for the first 10 centimeters down through the profile.

“We really wanted to see in the short-term if we are building up organic matter in the surface that we’re missing when we take a homogenized sample. We then took another sample from 10 to 15 and 15 to 20 centimeters,” Rhykerd noted.

“When compared the samples from fall to spring, what was amazing was on the surface from the cereal rye, we nearly doubled the amount of organic matter. It went from about 5% in the fall and was up to 11% in the spring. We have the data from the fall of 2021 and it is still higher in the top 2 centimeters at the surface than it was the year before.

“The organic matter in the pea/clover/radish/oats mix also jumped. It was closer to 8% and went down a little bit in the fall. The pennycress was right there, as well.

“We saw a little bit of an increased from 2 to 4 centimeters, but by the time we got down to about 4 centimeters in the soil we’ve lost that.”

Carbon Sequestered

The total amount of carbon sequestered through the 20 centimeters of the soil profile from fall to spring was 2.8 tons per hectare for pennycress, 7.8 tons per hectare for pea/clover/radish/oats mix and 13.3 tons per hectare for cereal rye.

When soil samples are typically collected, the residue is cleared from the top of the soil and a sample is collected with a probe into the clear surface.

“We didn’t do that. We punched right through the residue because I wanted to know how much carbon was there in the system,” Rhykerd said.

“We’re going to keep looking at this long-term and see how quickly the organic matter moves down the soil profile. We’ve now added golden pennycress to the wild pennycress and we’ll compare those. We also added an annual rye cover crop last fall to the trials.

“One thing that’s really neat is where cereal rye was planted in 2020; you can still see that cereal rye on the surface. It’s still there and it’s still decomposing.”