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Research Highlights
Long-Term Research Exploring Synergies in Best Practices for Soil Health

Soil samples developed a baseline for long-term research on the potential synergies of manure applications, cover crops and reduced tillage. Photo: Iowa State University

By Laura Temple

Reducing tillage, applying manure, and growing cover crops all influence soil characteristics. However, little is known about how these best management practices work together to impact soil health, especially in soils already high in organic matter and carbon, as demonstrated by the soil carbon index in Iowa1.

“To find answers to both long-term and short-term questions about the impact of these practices in what soil tests show are Iowa’s carbon-rich soils, we have established a new research site,” says Michelle Soupir, agricultural and biosystems engineering professor at Iowa State University. “We will be evaluating nine strip-till systems that combine various best practices for nutrient application timing and cover crops.”

She is leading multi-year research at the new site that will monitor many soil health indicators, including bulk density, water holding capacity, total soil carbon and nitrogen levels, particulate organic matter and the carbon and nitrogen fractions in that matter, aggregate stability and much more. At the same time, the research will annually monitor crop yield in a soybean-corn rotation and water quality leaving the site.

Nine strip-till systems are combining various best practices for nutrient management and cover crops to understand the long-term impact on soil health. Photo: Iowa State University

“The goal is to provide farmers with data from these systems that demonstrate the positive impact of combinations of these best practices on yield, water quality and soil health,” Soupir says. “We are incorporating a range of strategies to provide a suite of options so farmers can identify what works best for their individual operations.”

Soy checkoff dollars from the Iowa Soybean Association are funding soil and water sampling and analysis. Funding from the Iowa Egg Council was instrumental in getting the site designed and drainage installed for collection of water quality samples. The research team is seeking additional funding to study microbial activity and nutrient cycling, as well.

How Will Cover Crops Interact with Nutrients?

Many of the questions Soupir hopes to answer center on the role of cover crops in keeping nutrients available to crops and protecting water quality. Either poultry manure, urea ammonium nitrate (UAN) or a combination will be applied in early winter, spring, or with split application timing. One approach will apply manure based just on soil tests, keeping the phosphorus index below the maximum recommendation.

“Iowa farmers prefer to apply manure, especially swine manure, in the fall,” she says. “We are focusing on timings as late as possible in the fall or early winter so cold temperatures stabilize nitrogen through the winter, and also spring. Our previous work with poultry manure had found that spring application is better for water quality.”

Five systems in the study will include a cover crop of winter cereal rye following every crop. The other systems will provide comparisons of nutrient applications without cover crops.

Poultry manure is one of the nutrient sources being applied either in early winter, spring or with split application timing, to learn how it interacts with cover crops and reduced tillage to protect water quality, while supporting crop yields.  Photo: Iowa State University

“We hope cover crops can help stabilize nutrients and prevent leaching in the spring,” Soupir explains. “We expect that the cover crop will take up nitrogen and reduce losses to tile drainage. But we also want to know when the nitrogen stored in the cover crop will be available to the crop. We’ve added an agronomist who specializes in nutrient cycling models to help us understand when nitrogen will be plant-available.”

She also notes that poultry manure provides phosphorous, which doesn’t move through soil and leach the way nitrogen does. It stays near the soil surface. However, it can be lost to surface runoff.

“We will also be monitoring to see if cover crops reduce losses of soil-bound phosphorus by limiting surface runoff,” she adds.

Over time, she expects soil organic matter to increase both through manure applications and cover crop biomass. With this long-term study just underway in 2021, baseline soil samples and soybean yields were collected. Unfortunately, water quality samples could not be collected due to drought conditions.

Exploring Answers to Additional Questions

Soupir anticipates that this long-term study will allow the team to research answers to many other questions. For example, many cover crop studies indicate they can play a role in weed control.

The research team plans to do a weed seedbank analysis of the site in the spring of 2022. Soil samples collected from each plot within the research site will be taken, and whatever is there will be grown out in the greenhouse. They hope to repeat this analysis in three years or so, to see how the weed seedbank changes over time in these systems.

“Changes in soil health take years to see, but it is exciting to start a new site,” Soupir says. “We hope to maintain this research for 10 years or more, and we expect to be able to make adjustments to answer new questions about crop yields, water quality and soil health as they arise. We are very thankful that the Iowa Soybean Association is partnering in this project.”

1 Development of a Soil Carbon Index for Iowa Mineral Soils, Department of Agronomy, Iowa State University, 2012.

This project was funded by the soybean checkoff. To find research related to this research highlight or to see other checkoff research projects, please visit the National Soybean Checkoff Research Database.