Iowa Study Shows Digestate Boosts Soil Carbon and Farm Sustainability

By Andi Anderson

Making renewable fuel from manure and crop residue could transform both energy and farming. A new long-term Iowa State University study shows digestate, the byproduct of anaerobic digestion, improves soil organic carbon, enhances crop yields, and reduces reliance on synthetic fertilizers.

Digestate is the slurry left after bacteria break down organic matter without oxygen, producing renewable natural gas. On Sievers Family Farms near Stockton, Iowa, digestate has nearly replaced synthetic nitrogen on cornfields.

Over 12 years, researchers analyzed 421 soil samples from 14 fields, finding an average soil carbon increase of 714 pounds per acre annually. Fields with low starting carbon levels saw gains as high as 47%.

“It validates what we have long believed—that we’re seeing enhancements in the health of our soil which has translated into improved yields,” said farmer Bryan Sievers, who co-authored the study. “Now we can start to assign a monetary value to these best management practices.”

Soil organic carbon is vital for water retention, erosion control, and nutrient cycling, but is typically depleted by row-crop farming. Lisa Schulte Moore, director of Iowa State’s Bioeconomy Institute, called the results “remarkable,” noting that most practices aim to retain, not rebuild, carbon.

The study also highlighted challenges. Digestate can cause phosphorus buildup, though researchers are developing ways to filter phosphorus for other uses.

Still, the overall benefits—fertilizer replacement, improved soil health, and added farm income—position digestate as a promising tool.

When the Sievers began their project, Iowa had only two farm digesters; today, about 20 are in operation, not including large industrial plants. The findings are part of Iowa State’s C-CHANGE Grass2Gas project, which explores how digesters could create new markets for winter crops and perennial grasses, encouraging conservation practices that reduce erosion and pollution.

“We can use something that we think of as a waste to produce energy, improve soil health and fight climate change,” said ISU researcher Sebastian Villarino.

With Iowa farmers facing rising input costs and soil challenges, long-term results from a local farm may convince more producers that digestate offers a practical and profitable path to sustainable farming.

Photo Credit: gettyimages-sasiistock