In a recent study from the University of Illinois Urbana-Champaign, researchers tested whether modern high-yielding soybeans benefit from nitrogen fertilizer, with results suggesting additions are largely unnecessary. 

Soybeans form a natural partnership with bacteria that colonize the plant’s roots. In exchange for sugars, the bacteria convert atmospheric nitrogen into a form the plant can use, supplying much of the nitrogen needed for growth and grain development. It has historically been accepted that this supply, along with nitrogen from the soil, meets the plant’s nitrogen needs, but ever-increasing yield potential raises the question about whether this is still the case, say researchers in the Department of Crop Sciences and Illinois Extension; both units are part of the College of Agricultural, Consumer and Environmental Sciences (ACES) at Illinois.

High-yielding soybeans raise questions about nitrogen fertilizer needs

Soybean is a high-protein crop, and protein does not come cheap. Nitrogen is a key component of amino acids, which are the building blocks of proteins. As soybean yields increase, the plant needs to produce more protein, which in turn requires more nitrogen. Therefore, some researchers and farmers believe adding nitrogen fertilizer helps the plant meet this increased demand. 

“Ongoing genetic improvement steadily raised soybean yield potential and the need for nitrogen, raising questions about supplementing the supply with fertilizer,” said Emerson Nafziger, professor emeritus in crop sciences and Extension specialist. “People wondered whether soybeans wouldn’t be able to supply all of the energy for high yields as well as the energy needed to fix such a large amount of nitrogen.”

This notion is spurred on by highly publicized ‘record’ yields that are often attributed to intensive practices including nitrogen fertilizer, Nafziger says. These anecdotal claims are not backed up by available research and are partly what prompted Nafziger and his colleagues Giovani Preza Fontes and Joshua Vonk to tackle the issue. 

Field trials suggest minimal yield benefit from nitrogen applications

To test whether soybeans respond to extra nitrogen, the U. of I. team ran nine field trials across four locations in Illinois between 2014 and 2017. They looked at soybean yield when nitrogen fertilizer was applied at each of four growth stages: planting, flowering, pod setting, seed filling, and at all four of these stages. 

The researchers found that applying nitrogen to the crop at all four stages increased yield in most cases, but not enough to cover the cost of the fertilizer. Nafziger stresses that the yield improvements they saw from repeated applications are not a recommendation for farmers to follow suit. 

“While this increase from repeated applications was interesting, any treatment that costs two to three times the value of the extra production is a sure way to lose money,” he said. “People who are trying to set yield records might do it anyway, but it should never be done in normal production fields.” 

In all years and locations, soybean yields were good to outstanding, regardless of nitrogen application. There was no consistent yield benefit from single nitrogen applications during flowering, pod setting, or seed filling. 

“We found little response to applying nitrogen at individual reproductive stages ranging from flowering to seed filling, despite the fact that these cover the period when crop demand is high as pods are formed and seed filling begins. This indicated that nitrogen availability during critical stages of yield formation was not a significant limitation to yield,” said Preza Fontes, assistant professor in crop sciences. 

Exception found in specific soils and planting conditions

There was, however, one exception. A single nitrogen application at planting significantly increased soybean yield in two out of three years in a loam soil in a farmer’s field near Chillicothe, Illinois. This was unexpected as other studies have found that when nitrogen is applied during planting, the plant invests less in its symbiotic relationship with nitrogen-fixing bacteria, sometimes leading to deficiencies later in the season. 

Nafziger partly attributes this yield increase to the site’s lighter soil texture and lower organic matter, which may have been less favorable for early growth compared to the other sites. In one of the two years, the single nitrogen treatment also seemed to lessen the symptoms of sudden death syndrome, a soil-borne fungal disease. 

“We can’t count on this for disease control, but in this case, it seemed to be part of the reason it increased yield,” Nafziger said. “A little boost early in the season might have been enough to get plant and root growth ahead and keep them ahead the rest of the season.” 

A single application at planting could help soils with limited early-season nutrient availability, but in most cases, farmers can trust nitrogen-fixing bacteria to do their job. Free-living soil bacteria can also contribute to early growth by releasing nitrogen from soil organic matter, which may explain why early-applied nitrogen had little effect in soils with more organic matter. 

“Nitrogen fertilizer should only be considered in areas where the crop often gets off to a slow start due to soil factors. It’s a practice that we don’t think people should rush into,” Nafziger said. “In fact, those with lighter soils might hand-apply nitrogen fertilizer to a small plot and watch to see if the crop grows better there. If it doesn’t, then the fertilizer did no good. 

“We need nitrogen for high corn yields, but one of the things that makes soybean such a great part of the crop rotation on most Illinois farms is that it produces high yields without the expense and environmental consequence of applying nitrogen fertilizer.” 

The study, “Soybean response to nitrogen fertilizer in different soils,” is published in Crop, Forage, and Turfgrass Management [DOI: 10.1002/cft2.20304]. Authors include Joshua Vonk, Emerson Nafziger, and Giovani Preza Fontes. The research was supported by the Illinois Nutrient Research & Education Council [2014-3-360422-398].