Agronomists are often asked if supplemental nitrogen and sulfur applications can enhance soybean yield. To answer these questions, a team of researchers conducted a multi-state project powered by the Science for Success initiative. They mined through two decades of datapoints, then tested findings with replicated research in 2019 and 2020.  

To further investigate, they calculated the profitability of the applications. Ultimately, the team concluded that supplemental nitrogen and sulfur applications may slightly increase soybean yield, but the overall benefit is not cost-effective.   

Jeremy Ross, Extension soybean agronomist at the University of Arkansas System Division of Agriculture, shared details about the studies funded by the Arkansas Soybean Checkoff with additional support provided by the United Soybean Board.  

He presented the results on Jan. 17 at the Arkansas Crop Management Conference in North Little Rock, Ark.  

Sifting through historical data 

The first study was a metanalysis of historical data from 1996 to 2016 to determine the impact of nitrogen fertility on soybean yield. Research was collected from 16 soybean producing states throughout the U.S., creating a database of 5,991 region-specific datapoints from 105 locations. 

Ross described it as a treasure trove of data. Researchers scoured the wide range of nitrogen application timings, rates, methods, and combinations. After sifting through the data, they found that nitrogen impact on soybean yield could be measured, but the overall response was limited.  

In their findings, single nitrogen applications only yielded 0.9 bushels per acre greater than the control, and split applications were only 1.6 bushels per acre greater than the control.  

Out of all the timings, the greatest return on investment (ROI) was the combination of an at-planting fertilizer application with an additional application during the reproductive growth stage. Still yet, the ROI was not enough to justify the cost of the nitrogen. 

Rate was also found insignificant. Ross said 300 pounds of nitrogen per acre was considered the theoretical rate to maximize yield. Even at that rate, researchers determined there was no real benefit.  

The metanalysis concluded that an increase in soybean yield could not be associated with nitrogen applications alone. For optimum soybean yield, nitrogen management decisions must be based on the entire cropping system. This includes factors like soil fertility and pH, irrigation, row spacing, and seeding rate.  

To verify the relevancy of the results, they followed up with two years of current replicated tests. 

Current nitrogen and sulfur tests 

Tests were conducted in 2019 and 2020. The Science for Success team focused on sulfur and nitrogen applications at 52 sites across ten soybean producing states. Delta locations included sites in Arkansas and Mississippi. Other sites spanned into the upper Mississippi River Valley and the lake states, westward into the northern plains, and eastward into the Ohio River Valley and along the Appalachian seaboard.  

“These tests covered a significant acreage of soybean production to make sure we had a good balance. We also balanced treatments at 5 to 6 reps each with sulfur only, nitrogen only, and a combination of sulfur and nitrogen at all sites,” Ross said. 

Low, medium, and high treatment rates were applied. Gypsum was applied at 63, 125, and 188 pounds per acre. Urea was applied at 19, 39, and 56 pounds per acre, and ammonium sulfate was applied at 42, 83, and 125 pounds per acre.  

Average yields from treated replications were compared to an untreated control. For the entire study, the control averaged a yield of 61.5 bushels per acre. Of the treated replications, average yield ranged from 63.2 bushels per acre to 61.7 bushels per acre.  

While some of the higher application rates produced a yield increase, those increases were insignificant when they ran the statistics. The team then calculated partial profits across all sites with grain price scenarios at $8.70 per bushel and $15.00 per bushel. 

Trent Irby, Extension soybean agronomist at Mississippi State University, also participated in the study. He discussed the results in an interview with Farm Press.  

“The take home message is that we can get an increase in yield with supplemental nitrogen applications, but that increase is not always enough to pay for the cost of the application,” he said.   

In addition, sulfur applications alone increased yield at only four of the 52 sites. Ross noted that while sulfur applications did not provide any cost benefit in this study, these recommendations could potentially change in the next 10 to 15 years.  

“Soybeans get enough sulfur through deposition from the air and soil cycling,” he said. “Theoretically, as we clean up our air quality, this could reduce the amount of free sulfur available in the atmosphere. If that happens, deficiencies will need to be addressed with supplemental applications.” 

Recommended management practices 

While nitrogen and sulfur applications are not economical from a yield enhancing standpoint, they are necessary in situations to correct failures or deficiencies. 

“If there is a yield limiting problem, then the application can often pay for itself,” Irby said.  

It is important to know the symptoms of deficiencies to correct any issues. Deficiencies usually present with stunted growth and plant yellowing during the reproductive growth stages. 

Sulfur deficiency impacts the younger plant tissue first. Nitrogen deficiency, on the other hand, begins with yellowing on older leaves on soybean plants. Left uncorrected, nitrogen deficiency may lead to leaves turning brown or necrotic. 

Ross reiterated that best management practices like proper inoculation, soil testing, and lime application help to ensure plenty of nitrogen for soybean plants during the crucial reproductive stages. 

Inoculation establishes bacteria in the soil that develops a symbiotic relationship with the soybean root system and allows for nitrogen fixation. Nitrogen fixation alone can account for 50 to 75% of the soybean nitrogen needs, and accumulated residue from previous crops and soil nitrogen can meet the remaining needs.  

Both Ross and Irby reminded that soybean inoculation provides cheap insurance for nitrogen fixation, with cost ranging from $1.50 to $3.00 per acre. 

Irby added, “As a general rule of thumb, inoculation is recommended if a field has not been in soybeans in the last three years. If there is any doubt whatsoever, spend the money to inoculate that seed because it is cheaper than paying for a nitrogen fertilizer application later in the season.”