Research Highlights

Research Highlights
Exploring How Soybeans Respond to Nitrogen and Sulfur Fertilization

Soybeans growing in the experimental fields at the Kansas River Valley Research Station. Photo: Ignacio Ciampitti

By Carol Brown

The nitrogen cycle is a complicated system in Earth’s biology, occurring both above and below ground. Biological nitrogen fixation, or BNF, is one of the most important processes in plant production. Soybeans contribute to nitrogen fixation through the microbes that live in and around their roots and carry out BNF, making the crop an important player in the nitrogen cycle.

Ignacio Ciampitti, professor of crop physiology and digital agriculture at Kansas State University, is conducting research on how soybean nutrition, including nitrogen (N) and sulfur (S), affects yield and seed quality. The project, supported by the Kansas Soybean Commission, is also being conducted independently in 11 other states with Ciampitti amassing data from each state to reach regional conclusions.

“The main idea behind this project is to quantify the overall contribution of BNF to soybean plant nitrogen demand and how sulfur interacts with BNF and nitrogen,” says Ciampitti. “We want to see how the responses from these nutrients affect soybean yield.” 

Ciampitti and his team found after the first year of the project, in 2022, that soybeans responded to sulfur rather than nitrogen. Each plot received one of five different treatments applied at planting: N, S, a combination of N and S, a full treatment of N and S, which was applied at both planting and at growth stage R3, as well as a control plot with no nutrient applications.

“In the places that had yield responses, those were mainly linked to sulfur,” he explains. “Sulfur seems to be helping the BNF process and improving the nitrogen status in the plant.”

Ciampitti says that many people may think plant growth and development is limited by N, but that isn’t necessarily true. The benefits of applied sulfur shown in Figure 1 provide a clear example of a co-limitation, when other factors limit yields.

“We are seeing that plants are actually limited by sulfur, and when we apply small amounts, it is helping the plant to be more proficient with nitrogen fixation,” he says. “And that helps the plant to grow and improve the plant’s nitrogen status overall.”

In 2021, soybean yield ranged from 59 to 71 bu/acre across the locations, and yield gaps correlated with S response (Figure 1A).

Figure 1. A: Seed yield (bu/acre) for all treatments (check, N, S, NS, and full application of NS). B: Yield gap shows a clear correlation with yield response to S application (responsive sites). 

The results in Kansas are aligning with the findings from the other states as well, when growing conditions are taken into account. Some states have more sandy soil, and some areas have low soil organic matter, which are both expected to have greater responses to sulfur, Ciampitti says.

When research results from the 12 states were combined, eight sites out of 26 showed positive yield responses. A general conclusion for all sites was that N fertilization didn’t have an effect on soybean yields, regardless of the application rate or timing (Figure 1B).

The Kansas team also explored whether N and S applications or planting date affect end of season dry-down, and if the treatments helped to speed up plant maturity for better dry-down conditions.

“In Kansas, we found that the treatments had little effect on dry-down. Rather, dry-down is heavily dependent on the environment — relative temperature, humidity and precipitation,” says Ciampitti. “Planting date can exacerbate dry-down as well. If soybeans are planted too late, they are coming to maturity in low temperature environments and late-season precipitation, which delays drying time.”

Following similar trends, Ciampitti says the second year of the project indicates that S fertilization has a positive effect on reducing yield variability, particularly in soils with sandy textures or low levels of organic matter mineralization. Furthermore, when water stress is low, S fertilization appears to increase N uptake.

Funding for this project is supported in full or in part by the respective state soybean associations as well as the USDA Hatch program of the National Institute of Food and Agriculture and Science For Success.

Additional Resources

Soybean Yield Response to Nitrogen and Sulfur Fertilization in the United States: Contribution of Soil N and N fixation Processes – European Journal of Agronomy article

Kansas State Extension/Science for Success publication

Researchers and states involved with this project:
Ignacio Ciampitti, Kansas State University
Luiz Felipe A. Almeida, Kansas State University
Adrian Correndo, Kansas State University
Trevor Hefley, Kansas State University
Jeremy Ross, University of Arkansas
Mark Licht, Iowa State University
Shaun Casteel, Purdue University
Maninder Singh, Michigan State University
Seth Naeve, University of Minnesota
Rachel Vann, North Carolina State University
Jose Bias, North Dakota State University
Hans Kandel, North Dakota State University
Laura Lindsey, Ohio State University
Shawn Conley, University of Wisconsin
Jonathan Kleinjan, South Dakota State University
Peter Kovacs, South Dakota State University
Dan Berning, Corteva Agriscience
Mark Reiter, Virginia Tech
David Holshouser, Virginia Tech 

Published: Feb 4, 2024

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.