Research HighlightsScrutinizing Soybean Nitrogen Credits
Highlights:
- Nitrogen credit recommendations for crops following soybeans are not figured consistently.
- Multi-year research trials at 29 sites in 15 states aim to answer questions about how and when soybean N credits become available to the next crop.
- Trials aim to determine how latitude, residue load, soil cation exchange capacity and tillage affect soybean N credits across the country.
By Laura Temple
Nitrogen.
Crops need it to thrive. But it’s the most limiting nutrient in crop production.
Farmers know that feeding N to crops leads to yield. They know that soybeans fix their own N. And they know that in fields planted to soybeans last season, they can apply less N for the next crop.
But how much less?
Depending on the source, the N credit recommendation from soybeans varies widely. For example, in Minnesota the N credit to a subsequent corn crop is 30 pounds per acre, but to a subsequent wheat crop, it’s only 20 pounds per acre. Neighboring Wisconsin recommends a minimum credit of 40 pounds per acre to corn. In Maryland, recommendations range between 15 and 40 pounds per acres, but in Delaware, Virginia and West Virginia it’s a 0.5 pound of N credit per bushel of soybean yield — though if yield is unknown, it’s 20 pounds per acre. Nearby Pennsylvania recommends nearly double that amount. Iowa uses a nitrogen rate calculator that incorporates soybean N credits based on the ratio of the price of N to the price of corn, but a private lab in Nebraska recommends 40 pounds per acre N credit for soybeans yielding more than 20 bushels, or a credit of over 55 pounds per acres for soybean yields above 55 bushels. At the high end, Oklahoma reports up to a 60-pound-per-acre N credit following soybeans.
“Nitrogen credit recommendations following soybeans are disjointed, even among contiguous states,” summarizes Mike Mulvaney, Hartwig Endowed Chair for soybean agronomy at Mississippi State University. “Even the definition of N credits isn’t consistent.”
Based on his extensive review, what’s found about N credits in the literature doesn’t necessarily align with extension recommendations.
“Shouldn’t recommendations be supported by data?” he asks. “Why are N credits from soybean indexed to previous corn? What happens in cotton-soybean-corn rotations? Do N credits still apply?”
He gives compelling reasons to clarify N credits from soybeans. Those credits can save farmers money on fertilizer in subsequent crops. But they also create an economic risk from not applying fertilizer correctly.
“Truly understanding N credits could also lower the carbon intensity score for soybeans, which has implications for them as a globally traded commodity,” he adds.
Mulvaney is tackling soybean N credits questions on a unified scale. He is leading national-scale research to understand and quantify N credits from soybeans, thanks to combined support from the Atlantic Soybean Council, Mid-South Soybean Board, Mississippi Soybean Promotion Board, North Central Soybean Research Program, Southern Soybean Research Program and United Soybean Board. Using the Science for Success network, researchers in 15 states are executing a common rotation trial at 29 sites.
He has many questions about how N becomes more available following a legume crop like soybeans based on how the N cycle makes the nutrient available to plants. He expects this research to begin answering those questions.
Nitrogen Availability to Plants
Living organisms, including plants, use N to produce protein. For crops, that protein becomes the building blocks for growth, as part of enzymes that enable photosynthesis and all other plant functions.
After N cycles from the atmosphere through living organisms, two steps in the N cycle directly impact the availability of N to crops:
- Immobilization locks up N so that it is not available to other organisms.
- Mineralization releases tied-up nutrients into plant-available forms.
The N in organic matter, including plant residue, is immobilized. When plants die, the N reside begins to decay and mineralize N, making it available to other plants.
“Residue is chewed on by microbes,” Mulvaney explains. “When those microbes die, the N is released as ammonium or nitrate, the two major N forms that can be taken up by plants.”
He continues, “Plants and soil microbes compete for the available N in the soil. The N consumed by microbes isn’t available to plants until they release it as waste or die and decompose themselves.”
The carbon-to-nitrogen, or C:N, ratio can indicate how organic material breaks down. In general, higher C:N ratios decompose more slowly than lower ratios, immobilizing more soil N in the process. Corn stalks, with a C:N ratio around 60:1, break down more slowly than soybean residue, commonly reported in the range of 15:1 to 40:1 — though that value depends on when the measurement is taken.
Nitrogen Credit Questions
Mulvaney says N credits are generally defined as an N fertilizer replacement value, or the N that can reduced when fertilizing corn following soybeans compared to corn following corn.
“The question is, where does that difference in N response come from?” he asks. “Does it come from soybean residue? Or is the difference due to N immobilization from the microbes breaking down corn residue? Or perhaps it is confounded with residual fertilizer N from previous corn crops.”
He raises several other questions based on the science behind residue decomposition, N mineralization and the idea that N credit recommendations should be consistent.
- Some amount of N remains in soybean residue after harvest. Shouldn’t the amount of residue affect the soybean N credit?
- Soybeans translocate most of the N they take up in the grain, which is harvested. Less N is stored in the rest of the plant that becomes residue, so how much N does post-harvest residue really provide?
- Soil microbial activity slows and then stops when the ground freezes. Should we expect differences in N mineralization based on latitude?
- Should N credits be lower in the south than in the north because residue breaks down more quickly in warmer temperatures?
- If soybean residue decomposes quickly, shouldn’t more N credits be applied to a winter wheat crop than a spring corn crop?
- Soils with high cation exchange capacity, or CEC, hold nutrients better, so do these soils hold more plant available N from decomposing residue?
- How does tillage impact N mineralization and immobilization as crop residue breaks down?
True Nitrogen Availability Comparisons
To answer these questions, Mulvaney designed a unique trial protocol. The multi-year trial establishes crop history during the first year: soybean, corn and a summer fallow as a control. During the second year, corn is planted over all previous treatments with various N rates.
“Most N credit trials compare soybean-corn rotations to corn-on-corn,” he says. “Comparing soybeans to summer fallow will help tell us where the N response is coming from.”
Each year, research teams in 15 states collect data on pre-plant soil N, including total N and inorganic, or plant-available, N. In addition, they are monitoring stand counts, crop biomass, soil C:N ratios, residue fiber quality, yield, and C and N content in post-harvest grain samples.
To better understand soybean residue decomposition and N mineralization following soybeans, trial sites in Indiana, Michigan, Mississippi, Missouri and Nebraska have an additional litter bag study. Bags stuffed with soybean residues are periodically retrieved to monitor decomposition dynamics across a north-south transect. Some litter bags lay on top of the soil to mimic no-till conditions and others are buried in the soil to reflect tillage.
“The litterbag portion of this study is being done at a variety of latitudes to help us understand how temperature, weather and tillage affect N mineralization following soybeans,” Mulvaney says.
He plans to gather and analyze at least 28 site-years of data to start answering the questions he has been asking. He believes these data will create a foundation for more consistent science-based fertilizer recommendations for crops following soybeans. This information could also ensure soybeans get the appropriate credit for their role in reducing the carbon footprint within common U.S. crop rotations.
“I am really excited about this research and what we will learn,” Mulvaney says. “It has been a dream of mine to do N credit research on this scale.”
Additional Resources
Breeding Research Lays Foundation to Investigate Links Between Fixing Nitrogen and Protein Concentration – SRIN article
Study Finds Nitrogen Mineralization From Crop Residues May Not Be Enough – SRIN article
Can Soybean Hulls Become Another Nitrogen Source for Crops? – SRIN article
Adding Wet Distillers Grain to Soybeans for Nitrogen Fixation Benefits – SRIN article
Soybean Nitrogen Nutrition – Science for Success YouTube video
Soybean Yield Response to Nitrogen (N) and Sulfur (S) Fertilization: Contribution of Soil N and Biological N Fixation Processes – Science for Success fact sheet
Nitrogen (N) Deficiency of Soybean – Crop Protection Network publication
Meet the researcher: Mike Mulvaney SRIN profile | University profile | Science for Success profile
The Soybean Research & Information Network (SRIN) is funded by the Soy Checkoff and the North Central Soybean Research Program. For more information about soybean research, visit the National Soybean Checkoff Research Database.
Published: Mar 23, 2026
