Updated February 11, 2025:
Project Summary
We leverage the Science for Success team, a coalition of nationwide Soybean Specialists, to quantify soybean N credits across the US. There has never been a nationally coordinated effort to quantify soybean N credits. If accurately quantifying N credits from soybean residue can reduce the N application rate to subsequent crops, this will provide evidence of the economic and environmental value of soybeans as a sustainable rotational crop.
Major Accomplishments (Jan 1, 2024 to Dec 31, 2024)
• We completed the first rotation of the experiment, establishing crop history treatments
• We presented a review on soybean N credits at the American Society of Agronomy Conference (ASA).
• We presented preliminary results at stakeholder meetings
1.1 Project justification and rationale
There has never been a nationally coordinated project to definitively address the amount of N made available to subsequent crops. Currently, N credit recommendations from soybean consist of a hodgepodge of disparate values, even among contiguous states (Figure 1). A coordinated, multi-regional effort to quantify N credits from soybean is needed to quantify the amount of fertilizer N savings and provide scientifically verified data for soybean sustainability initiatives.
Economically, it is important to accurately quantify N credits from soybean given high N fertilizer prices because producers may be able to reduce the amount of N applied to subsequent nonlegume crops. For example, if the N rate to a subsequent nonlegume could be reduced by 20 lbs N/ac while maintaining yield, at current prices ($475/ton urea), a farmer would save $10.33/ac. However, we expect that this amount should change based on environment and the amount of residue remaining in the field. Current N credit recommendations must be revised, especially considering new soybean varieties, management practices, record yields, and weather conditions. This project is expected to provide a robust framework to quantify N contributions from soybean residue using modern cultivars and management practices representative of soybean farmers across the USA.
1.2 Goals & Objectives
The goal of this project is to quantify soybean N credits across the US. The objectives of this research are to:
1. Quantify total C and N of soybean residue after harvest, across a wide range of weather, soil, management practices, and yield levels.
2. Determine total soybean N credits to the following nonlegume crop.
4. Identify weather, soil, and crop management variables that could be used to predict soybean N credits across the US soybean regions.
5. Disseminate results to soybean farmers and other stakeholders through the United Soybean Board’s Science for Success initiative.
1.3 Project Deliverables
1. Quantification of soybean residue C and N after harvest from 14 states across the USA.
2. Create an N response curve of nonlegume crop response to cropping history to quantify soybean N credits.
3. Produce at least 1 Fact Sheet, 1 webinar, and 5 videos by project completion.
4. Train at least one PhD student and 10 undergraduate students by project completion.
1.4 Benefits to soybean farmers
This project has the potential to save soybean producers money by applying soybean N credits to subsequent crops. For example, if the N rate to a subsequent nonlegume could be reduced by 20 lbs N acre-1 while maintaining yield, at current prices, a farmer would save $13.60 acre-1. Current N credit recommendations need to be revised, especially in consideration of new soybean varieties, management practices, record yields, and weather conditions. The N credits project is expected to provide a robust framework to quantify N contributions from soybean residue using modern cultivars and management practices representative of soybean farmers across the USA.
2.0 Methods
2.1 Study sites
A fourteen site-year study was conducted across some of the major soybean-producing states in the US (MS, OH, SC, WI, NC, IA, MO, MI, IN, AL, NE, AR, TN, and LA) as shown in Figure 1. An additional two sites (MD, PA) were selected as collaborators on the project. Sites were selected based on participation in the United Soybean Board’s Science for Success program, availability of funding, and capacity to execute this relatively complicated experimental design.
2.2 Experimental design
The experimental design was a split-plot randomized complete block design with four replications. The main plots consisted of history treatments (corn, soybean, and fallow). Fallow plots were maintained weed-free throughout the first year of the rotation. Subplots consist of N rate (0, 80, 160, 210, 260, 310 lbs N ac-1) to a subsequent corn crop. A maximum rate of 310 lbs N ac-1 was selected to obtain adequate resolution near the agronomic optimum N rate (AONR). The experimental units are 4 rows wide by at least 35 feet long, respectively.
2.3 Time zero soil samples
Time-zero soil samples were taken from multiple points within each field to account for variability. The samples were collected at a depth of 0-6 inches and the collected soil samples were bulked to obtain a homogeneous sample. Collected samples were immediately stored in cool, dry conditions to preserve their integrity. Each sample was labeled with detailed information, including the field location, date of collection, and soil depth. A subsample was sent to the laboratory for physico-chemical analysis.
2.4 Data collection
Stand counts were measured after emergence. Residue biomass was collected from the main plot history treatments by harvesting plants within two 1-meter rows (non-harvest rows). Samples were taken from representative areas per plot and dried to a constant weight at 60°C. Soybean and corn were combine harvested from the middle two rows of the plots, and yield were determined on 13% and 15.5% moisture content respectively.
2.5 Data analyses
Soil samples were analyzed for texture, pH, cation exchange capacity, soil organic matter, nitrate-N, and ammonium-N using standard laboratory procedures. Soybean and corn residue biomass were ground to pass a 1 mm sieve and then ball milled using the Mixer Mill. Samples were analyzed for C, N, and nitrate-N concentration based on standard laboratory procedures. Residual N concentrations were determined by multiplying the percent N of biomass by biomass dry weight. The Harvest index was determined by dividing the grain yield by biomass yield.
2.6 Statistical analyses
Data were analyzed using descriptive statistics. Yield data were presented using a bar plot. Principal component analyses were performed to identify the relationship among traits using the FactoMineR package (Sebastien et al., 2008) and Pearson’s correlation was used to show the relationship between the traits of soybean. All analyses were conducted using R version 4.4.2 (R Core Team, 2024).
3.0 Results
3.1 Time zero soil properties at the study sites
Soil properties across the research locations in 2024 are shown in Table 1. A wide variation in soil textural classes, pH, CEC, nitrate-N, and ammonium-N among research sites will provide a range of environments to test our hypotheses. We expect that clay soils, with higher CECs and water-holding capacity, may retain more N from soybean residue than light-textured soils, which do not hold nutrients and are prone to nutrient leaching. The variability in soil organic matter (0.7 to 3.2%) was representative of farms in soybean growing areas (Table 1).
3.2 Residue biomass, and residue nutrient content
Soybean residue biomass ranged from 2,813 to 11,092 lbs acre-1 with a mean value of 6,492 lbs acre-1 (Table 2). We expect more biomass to lead to increased N credits across soybean-growing regions. The average soybean residue biomass carbon content across sites was 41.7 (%). When combined with residue biomass data, this resulted in carbon (C) application rates from 38.3 to 45.1 lbs C acre-1 (Table 2). Residue N content from soybean ranged from 0.6 to 2.0% with a mean value of 1.1%. Interestingly, soybean residue C:N ratios ranged from 22.8 to 76.5 with a mean value of 45.0 across the study sites, indicating that N is likely going to be immobilized before it is subsequently mineralized. Soybean residual N ranged from 30.6 to 155.4 lbs N acre-1. This result provides an upper limit on how much N can be transferred from soybean to subsequent nonlegume crops through residue decomposition. Soybean harvest index ranges were quite variable, from 27.8 to 53.3%. There was a positive correlation between residue biomass and residual N (P=0.01) as shown in Figure 6. There was a negative correlation between C:N and residue N, and harvest index and residue N (P=0.001) while other traits follow a similar trend as shown in Figure 6.
Principal component analyses (PCA) were conducted on all response variables. The first two principal components with an eigenvalue greater than one accounted for most of the variability in the dataset (67.69%), shown in Figure 4. Residue biomass, residue N, residual N, and yield correlated positively to PC1, while harvest index, C:N, and residue C correlated negatively to PC1. A similar pattern in the relationship was found among traits with PC2 (Table 4 and Figure 4).
There was wide variability in corn residue biomass, residue carbon, residue C:N, residual N, and harvest index, but the variability in residual nitrogen was low (Table 3). The high C:N ratio indicates that when the corn residue is returned to the soil, it should lead to N immobilization. This will help to distinguish between N immobilization from N mineralization from corn vs. soybean. Residue biomass, residual N, C:N, and yield correlated positively to PC1, while residue carbon, residue N, and harvest index correlated negatively to PC1 (Figure 5). A similar pattern in the relationship was observed among traits with PC2 (Table 5 and Figure 5). Residue N, residue carbon, and harvest index correlate negatively to PC2 (Figure 5).
3.3 Yield
Soybean yield across study sites ranged from 40.8 to 86.3 bu acre-1 with a mean value of 62.7 bu acre-1 (Table 2 and Figure 2). Corn yield across the study sites ranged from 30.8 to 292.3 bu acre-1 with a mean value of 156.0 bu acre-1 (Table 3 and Figure 3). Our yield data were comparable to the national average, such that we may be confident that results from this project will be comparable to real-world conditions. Variability across sites will capture the broad range of conditions under which soybean N is transferred to subsequent crops.
Conclusion:
There were wide variations in soil nitrate, ammonium, soil organic carbon, cation exchange capacity, pH, yield, post-harvest biomass, and residual nitrogen. Yield data were comparable to the national average. We determined upper limits on soybean residue N contribution to be in the range between 30.6 to 155.4 lbs N acre-1, depending on site. The second year of the rotation will be conducted in 2025.
Showcasing the Project and Presenting Results
• In November 2024, my graduate student made an oral presentation at the ASA-CSSA-SSSA International Annual Meeting in San
Antonio, Texas, where he indicated the benefit, the project would have on USA farmers.
• Oyedele, O. Mulvaney, M. J., Olomitutu, O. E., Wallace, J., Shavers, G. M., & Hilyer, T. Nitrogen Credit after Soybean: A Review. ASA-
CSSA-SSSA International Annual Meeting, San Antonio, TX, Nov. 10 - Nov. 13, 2024.
https://scisoc.confex.com/scisoc/2024am/meetingapp.cgi/Paper/158389
• Mulvaney, M. J., Oyedele, O. (2024). Quantifying Nitrogen Credit from Soybean. Stakeholder meetings of Multi-Regional Soybean
Checkoff. St. Charles, MO, December 9th, 2024.
Future work
• We will triangulate on N credits from soybean by conducting a litterbag decomposition trial in five states
• We will gather preliminary data on greenhouse gas emissions from soybean compared to both corn and fallow ground in order to
gather evidence to determine if soybean systems actually increase GHG emissions compared to bare soil.
• The trial will be repeated for a second year across 14 states.
View uploaded report 
The soybean N credit project aims to quantify the nitrogen (N) contribution from soybean to subsequent crops. This will help US soybean growers reduce fertilizer costs and improve soil health. Our research informs the sustainability of soybean by quantifying reduced N requirements for crops following soybean in rotation.
A novel way to consider soybean N credits is to consider the amount of post-harvest biomass as a source of N. This implies that more biomass (possibly through greater yield) may supply more residual N to subsequent crops. Furthermore, given frozen winters in the North compared to the South, one might expect increased N credits in northern latitudes due to delayed biological activity. Through the USB Science for Success collaboration, our nationally coordinated project collects data from 14 soybean-producing states to answer these questions.
Our 2024 data suggest maximum N credits between 31 to 155 lbs N acre-1 from soybean residue depending on site. Interestingly, soybean residue C:N ratios ranged from 22.8 to 76.5, indicating that N immobilization is likely to occur before N mineralization. It is possible that this delay in mineralization may increase N synchronicity to the following crop.
Soybean yield ranged from 40.8 to 86.3 bu acre-1, comparable to the national average yield, ensuring that our findings are applicable to real-world farming conditions.