Research Highlights

Research Highlights
Measuring How Agronomic Systems Change Soil Health and Crop Yield

Overview of the cover cropping systems study plots at the Pontotoc Ridge-Flatwoods Branch Experiment Station of Mississippi State University. Photo: Mississippi State University

By Laura Temple

The elements of an agronomic system interact in countless ways, both simple and complex, to support crop production. How do system changes impact crop yields, soil properties and profitability?

Mark Shankle, research professor of agronomy for Mississippi State University, tackled these big questions in dryland soybeans with a long-term research project supported by the Mississippi Soybean Promotion Board.

“Awareness of soil as a living ecosystem is increasing,” Shankle says. “With that, farmers are showing interest in sowing cover crops in the fall, no-tilling cash crops in the spring and applying organic fertilizer sources as an alternative to commercial inorganic fertilizer. Inconsistent soybean yields are a problem for many dryland soybean producers, due to the lack of rainfall at critical times during the growing season, so we wanted to learn if a system focused on improving soil health could support more reliable yields.”

He designed a complex study to look at how multiple practices work together in a system. His team compared five different cover crop options, three fertilizer programs and two soybean planting dates. For five years, they have been gathering plot data, including soil characteristics and yield. 

“We collected soil samples from each plot every year to determine nutrient requirements based on soil test recommendations, like growers would,” Shankle explains. “And we kept the same cover crop and fertilizer combination in the same plot each year to gauge the cumulative effect on soil properties.”

To see how planting date influences both cover crop biomass production and soybean yield, the team terminated cover crops and planted no-till soybeans during April in half the field as an early planting system. They repeated the same course of action in May for the second half of the field to study a late planting system. For both planting dates, they terminated cover crops with a burndown herbicide treatment about two weeks before planting. However, they applied fertilizer treatments to both the early and late planting systems at the same time in the spring. This promoted cover crop biomass production in the late planting system. 

Vetch cover crop in the cover cropping systems study at the Pontotoc Ridge-Flatwoods Branch Experiment Station of Mississippi State University. Photo: Mississippi State University

Management Factors to Support Soil Health

Shankle’s research focused on what farmers want to know about management practices. 

The cover crop treatments included cereal rye, vetch, wheat and the NRCS-recommended mix of cereal rye and mustard. The control was native cover that grew over the winter. However, as volunteer cover crops from other plots encroached, the team managed the control plots to reflect bare soil for the last two years of the study.

“We also looked at wheat alone as a cover crop, because farmers have said they are more willing to adopt it due to presumed lower cost and proven management practices,” Shankle says. 

Poultry litter is readily available as a fertilizer option for many Mississippi farmers, so the fertilizer treatments compared poultry litter to commercial fertilizer, with a standard control of no fertilizer. Laboratory recommendations based on annual soil testing dictated fertilization rates. 

“We focused on practices that add soil organic matter,” he explains. “Both cover crops and poultry litter provide organic matter. The early and late plantings of soybeans allowed us to observe crop and soil responses to different amounts of organic matter relative to treatments in each system.”

The cover crops in the late planting system provided more biomass, due to a longer growing period prior to termination and because fertilizer treatments were applied while they were still growing. The cover crops in the early planting system had less time to grow, with fertilizer applied after termination. 

Soil Microbiome Changes

This research took a unique look at how these practices impacted soil health. In addition to annual soil tests, the team’s soil microbiologist, Shankar Ganapathi Shanmugam, assistant research professor at Mississippi State University, studied the change in soil organisms over time.

“The richness and diversity of the microbes, especially bacteria and fungi, living in the soil indicates its ability to support growth,” Shanmugam says.

Soil sample from the cover cropping systems study that contains a diverse array of microorganisms, as well as visible earthworms. Photo: Mississippi State University

He extracted microbe DNA from soil samples at the beginning of the project and then again in 2022 for comparison. The volume of DNA data required supercomputers to sequence it and identify the types of microbes in the soil. 

“In the plots with cover crop treatments and with poultry litter, we saw big increases in the diversity of the microbiome,” he reports. “That makes sense, because both practices add carbon to the soil, which serves as a food source for those microbes.”

Shanmugam notes that the increase of nitrogen-fixing bacteria, or Rhizobium species, shown by the DNA tests indicates a corresponding increase in the nitrogen transformations in these soils as a result of management changes. He also measured an increase in richness, or the total number of taxa of microbes living there. 

“We observed improvements in soil characteristics like aggregate stability, water infiltration and carbon availability in the plots with cover crops, poultry litter or both,” he adds. “All those qualities relate to diversity changes in the soil microbiome.”

Poultry litter especially correlated to the increase in soil aggregate stability. Yet, soil nutrients from the fertilizer treatments did not build up in the soil, showing that the soybeans and cover crops used the available nutrients every year. 

“While we didn’t see correlations between soil microbes and specific cover crops, we did see that cover crop biomass tied to greater microbe diversity,” Shanmugam says. “Grasses create more biomass and increase soil microbial biomass carbon, and that supported diverse microorganisms in those plots.”

Impact on Yield and Profitability

The team aligned their findings with farmer priorities by having an economist analyze program costs to compare system profitability. 

“We observed that cover crop systems impacted soil health, but they did not have an effect on soybean yield,” Shankle reports. “Cover cropping systems did not have a positive or negative effect on soybean yield.”

Soybeans in the early planting system (foreground) have emerged while the late-system soybeans are being planted through a terminated cover crop, visible in the background. Photo: Mississippi State University

Yields did vary by planting date, impacted by the amount and timing of rainfall events during reproductive growth stages R1 through R7. 

“Typically, soybeans planted before May 1 are associated with higher yields,” he says. “However, our cover cropping systems study indicates that yield can be higher with early or late-planted soybeans for any given year, depending on weather patterns. If rainfall events, and thus soil moisture, coincided with critical stages of growth and development, it would favor either planting date.”

Because yields remained consistent, the economic analysis found that adding cover crops decreased profitability. However, that analysis did not account for participation in programs designed to defray the costs of cover crops.

Applying fertilizer did increase profitability. Poultry litter proved more profitable than commercial fertilizer. Shankle says fertilizer treatments, especially poultry litter, impacted crop growth and yield, in contrast to using cover crops. Soybean leaf tissue from fertilized plots contained more phosphorus and potassium than unfertilized plots.

“One unexpected result of the cover crops was the influence of their biomass on deer browsing,” he adds. “We found that when cover crop biomass stubble remained above the emerging soybeans, deer were deterred from reaching down to the soybean plants. Therefore, deer browsing of soybean vegetation was more prevalent in plots with no cover crop treatments.”

With consistent yields and proven soil health changes, he believes this research will help farmers take an in-depth look at poultry litter and cover crops in dryland soybean production systems. The economic model can help them identify cover crop programs that would make up for the change in profitability from adopting a system focused on improving the sustainability of soil health for the future.

Published: Oct 9, 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.