Research Highlights

Research Highlights
Planting Practices Influence Residue Breakdown in Soybean Fields

Soybean row spacing appears to influence the rate of residue breakdown on the soil surface. The volume of residue in the 15-inch rows on the left is noticeably less than in the 30-inch rows on the right. Photo: University of Delaware Extension

By Laura Temple

The “trash,” or residue, from previous crops left in the field surface holds soil in place and stores organic matter and nutrients. Residue can suppress weeds and conserve soil moisture, but it can also harbor pests, like insects and pathogens, and influence the release of nitrogen and other nutrients to crops. Farmers have learned to manage residue at planting to ensure strong stands of the following crop. 

Because of the many factors that residue influences, Jarrod Miller, assistant professor and extension specialist in agronomy at the University of Delaware, is taking a fresh look at how it breaks down in the following soybean crop.

“A couple years ago, I observed a dramatic visual difference in the corn residue left in different plots in a soybean population study,” Miller says. “Row spacing appeared to impact how that residue decomposed during the growing season. That raised a lot of questions. For example, does an earlier canopy preserve soil moisture, allowing for increased residue decomposition? Or that earlier canopy may increase evapotranspiration, reducing overall soil moisture.”

The Delaware Soybean Board is supporting Miller’s research to start answering those questions. The study, which began in 2022, compares decomposition of corn crop residue and a rye cover crop terminated two weeks before planting across plots of soybeans at different planting densities and row spacings. The team planted soybeans at 80,000; 100,000; 120,000; 140,000 and 160,000 seeds per acre, testing each density in both 15- and 30-inch rows. The plots were irrigated and monitored with drone imagery to track biomass. 

To estimate breakdown of both types of residue, each plot held multiple decomposition bags. The bags, containing either corn fodder or rye biomass, allowed the team to compare the weight of dried samples at the end of the season to base measurements taken at planting.

Miller’s goal is to link soybean planting practices to residue management and yield. That information would help farmers decide how best to maximize the benefits of decomposing residue in their agronomic systems. 

“While we had some challenges managing the plots throughout the season, our initial data indicates that weird things are happening,” Miller explains. “However, beyond showing us unexpected results, we don’t have enough information to answer many questions yet.”

Complex Interactions

Miller notes that complicated relationships exist between residue, crop canopy, carbon to nitrogen ratio in the soil and more. While soybean yield didn’t vary by plant population, plots with a better crop canopy yielded better, regardless of density. 

“Rye biomass decomposed more in plots with higher yield,” he reports. “While corn residue decomposition showed no relationship to yield, it broke down more when rye residue broke down less.”

He adds that plots with a lower carbon to nitrogen ratio also corresponded to higher yields. And like he observed before, row spacing made a difference. Residue decomposed at a higher rate in narrow rows. The drone imagery data also indicated a slight correlation between higher biomass and lower soybean germination rates.

“After the first year, we have even more questions,” Miller says. “What is the interaction between corn and rye residue as it breaks down? What is happening in the microclimate under the soybean canopy, and how does that influence decomposition?”

In an effort to answer more of these questions, future plots will contain decomposition bags with both corn and rye residue mixed together. He also hopes to figure out how to allow larger insects that may aid decomposition to enter the bags naturally without losing biomass through holes in the bags. Installing soil moisture sensors in the plots will provide more accurate moisture and temperature data to track the microclimates at the soil surface.

“Based on our initial data, clearly something is going on,” Miller says. “We plan to continue this research to figure it out. Whatever the interactions, they clearly link to many different factors. And, with the growing potential for carbon sequestration, learning more about what’s happening under the canopy will give growers more ways to improve crop management.”

Published: May 22, 2023

The materials on SRIN were funded with checkoff dollars from United Soybean Board and the North Central Soybean Research Program. To find checkoff funded research related to this research highlight or to see other checkoff research projects, please visit the National Soybean Checkoff Research Database.