Nitrogen fertilizer and soybean yield: what we learned from multi-year trials in Illinois

2025 crop progress

Despite intermittent delays in southern Illinois and replanting in some areas, the 2025 planting season has largely tracked the 5-year average of 2020-2024. The winter wheat development lagged through May, but picked up in June, with harvest progress similar to recent years. That meant normal doublecrop soybean planting progress.

Corn crop ratings improved through June, rising from 63% good + excellent (G+E) on June 2 to 74% by June 22, before a slight dip to 68% on July 6. Soybean ratings were around 60% G+E early in the month and falling to 54% by July 6. That’s 10 percentage points below 2024 levels, but 20% higher than in 2023, when dry weather affected much of the state. This year, the U.S. Drought Monitor shows that much of the northern half of Illinois (57% of the state) was at least abnormally dry on July 8, including D1 (moderate drought) over much of northern Illinois, and pockets of a few pockets of D2 (severe drought), mostly in Kane and Will counties.

With 38% of soybeans blooming as of July 6, much of the crop showing good growth now, and fields moving into reproductive stages, the perennial question about whether soybean yield might be boosted by an application of nitrogen fertilizer has arisen. In this article, we’ll review findings from a multi-year project conducted across a range of soils and environments in Illinois to evaluate soybean yield response to broadcast urea applied at different growth stages.

Soybean and nitrogen: a recap

With high yield potential and with seed high in protein (38–40%), the soybean crop requires a lot of N—roughly 4.5 lb of N per bushel, with about 3.5 lb of that removed with the grain. A 60-bushel soybean crop (similar to the current Illinois average) would need approximately 270 lb of N per acre, while an 80-bushel crop, which is not uncommon today, needs to accumulate about 360 lb of N.

Historically, soybeans have met their N requirements through a combination of N supplied from the soil (through mineralization – release of N from soil organic matter) and biological N fixation (BNF), a process carried out by symbiotic bacteria in root nodules. On average, BNF supplies 50-60% of the total N, with the remainder coming from the soil. However, as average yields have increased in Illinois, particularly in high-yielding fields (exceeding 80 bushels/acre), there has been an ongoing discussion about whether fertilizer N is needed to supplement natural sources in high-yielding soybeans.

Soybean N uptake is relatively low (less than 20% of total uptake) during pre-flowering vegetative growth stages, but it increases rapidly during reproductive stages, peaking usually around stage R5.5 (full podding, usually early August). Studies have reported maximum N uptake rates of 3–4 lb N/acre/day during R4 and R5. While some studies have reported modest yield increases from application of fertilizer N during these stages, especially in irrigated or double-crop systems in Nebraska and the Mid-South, many other studies have shown little to no benefit. Responses to fertilizer N have varied widely depending on soil type, weather, timing, and application rate. Nothing, including what yield potential of the crop might be, has helped to predict when the crop will respond to N, but overall, the practice has not been profitable.

Field trial: locations and methodology

To explore this question further, we conducted nine field trials in Illinois between 2014 and 2017. Sites included university research farms near Urbana, Monmouth, and Brownstown, and on a farmer’s fields near Chillicothe in Peoria County, which were irrigated through an overhead pivot system as needed. These sites spanned a range of soils, from coarse-textured soils with <2% organic matter to very productive fine-textured soils with >3.5% organic matter.

Six N fertilizer treatments were evaluated at each site:

• Untreated control
• Single applications of inhibitor-treated urea surface-applied at planting, R1, R3, or R5 growth stages
• A treatment with all four application timings combined (i.e., planting + R1 + R3 + R5)

Soybeans were planted in May with regionally adapted varieties and managed using standard agronomic practices. All plots followed corn, with appropriate weed and disease control, including fungicide seed treatment at sites where needed. Rainfall was generally average to above average, and soybean yields ranged from good (~60 bushels/acre) to outstanding (>90 bushels/acre) among sites and years.

Site-specific results

Brownstown

The soil at Brownstown had ~1.7% organic matter and a natural claypan (Cisne silt loam), making the crop susceptible to both soil saturation and drought. Despite a relatively late planting date (May 22) and unusually high June rainfall (10.7 inches), yields were good: the untreated control averaged 61.1 bu/acre (Table 1). Applying 100 lb N/acre at any single time or at all four timings did not produce statistically higher yields compared to the control, although applying N at planting (or at all four times) yielded some 4.5 bu/acre less than the untreated control.

Chillicothe (irrigated, coarse-textured soils)

The most consistent and substantial yield responses to N were observed at the Chillicothe site, which had coarse-textured soils with lower organic matter (~2%) (Warsaw and Cresent loam), and pivot irrigation. Applying N at planting increased yields by 35% (22.4 bu/acre) in 2015 and 38% (19.7 bu/acre) in 2016 compared to the untreated control (Table 2). Applying N four times during the growing season also increased yield over the untreated control, but nearly all of this came from the planting-time application alone. None of the single, in-season (R-stage) N applications increased yield in any year at this site. In 2017, on a coarser-textured soil (Dickinson sandy loam) and under drier conditions (despite increased irrigation), single N applications did not significantly increase yield, although, four applications produced 87.2 bu/acre, compared to an average of 78.7 bu/acre across all other treatments.

The large yield increase from planting-time N applications was surprising, given that the literature generally suggests that N at planting can reduce nodulation and limit BNF. However, plots that received N at planting were visibly greener and more extensive than those without N, and the differences persisted throughout much of the season. The fact that a single application of N at planting produced as much yield as four applications of N indicates that early N applications may have stimulated early growth and root development, enabling plants to support N fixation and nutrient uptake more effectively.

Urbana and Monmouth

Yield responses to applied N on the deep prairie soils at Monmouth and Urbana were relatively small and inconsistent (Table 2). Out of five site-years, only one showed a significant response to a single N application, and that was an approximately 6% (5.2 bu/acre) yield decrease from the planting-time application at Urbana in 2015. This decrease coincided with a high control yield (91 bu/acre), with the higher N rate (100 lb N/acre) used, and with high rainfall in May (6.1 inches) and June (9 inches). At stage R6 (late August) we measured more than twice as much soil N following the planting-time N application than in the untreated control. Higher soil N can limit nodule formation, and we think this may have lowered BNF and lowered N availability later in the season.

Only the four-time application produced yield increases at these sites, and those were modest and inconsistent. At Urbana, applying N four times increased yield by 9% (6.4 bushels per acre) in 2014, and by 7–8% in other years, although these increases were not always statistically significant. These yield gains were not nearly enough to cover the costs of fertilizer and application.

Nitrogen response versus yield level

There is a common perception that high-yielding soybeans, due to their greater N demand, are more likely to respond to supplemental N fertilizer. We did not find this: across the nine site-years, there was a negative relationship between yield response to N and overall yield levels—that is, yield responses to N decreased as the untreated control yield increased (Figure 1). This was particularly evident at the Chillicothe site, where irrigated, lighter-textured soils tended to respond more to fertilizer N than at sites where soils provided more water and N, and where panting-time N was more likely to decrease than to increase yield.

These results align with other research, including work by Mourtzinis et al. (2018), which showed minimal soybean yield response to fertilizer N across more than 207 sites (including those sites discussed here) in the U.S. Corn Belt. In these environments, weather conditions that favor high yields also tend to favor N mineralization and a more consistent water supply, both of which help sustain BNF and the ability of plants to produce and fill the large number of seeds needed for high yields. The large yield responses to N that we found in this study were observed on irrigated soil with a coarse texture and lower organic matter content than at the other locations; such soils would have provided less mineralized N, and any organic N that was mineralized would be more subject to leaching loss.

Final thoughts and practical implications

Overall, this study reinforces the conclusion that most soybean fields in Illinois are unlikely to benefit agronomically, and even less likely to benefit economically, from N fertilizer applications. While multiple applications sometimes produced modest yield increases (6–9%), the gains were generally too small and inconsistent to justify the cost, particularly under current market conditions. With USDA projecting a season-average of $10.25 per bushel for the 2025 marketing year, down from $14.20 in 2022, making the return on investment even more critical.

It is clear that BNF and soil N mineralization supply enough N for very high soybean yields in most productive soils in Illinois. Those growing soybeans on lighter-textured, irrigated soils might consider trying a planting-time application of urea (100 lb of product, or 46 lb N) to a small strip or area of a field to see if this increases early growth and greener leaves. If this happens consistently over fields and years, it might be considered as a future practice.

In economic environments like this, the best approach is to focus on the basics and cost-effective management practices (e.g., maintaining adequate soil test pH, P, and K levels, selecting adapted and high-yielding varieties, timely planting, optimal seeding rate, weed control, etc.) Applying in-season N to “make sure” that soybeans have enough N to produce high yields is a poor bet, both economically and environmentally, and should be taken off the list of possible inputs for the vast majority of Illinois fields.

Adapted from: Vonk, K., Nafziger, E., & Preza Fontes, G. (2024). Soybean response to nitrogen fertilizer in different soils. Crops, Forage & Turfgrass Management, 10, e20304. https://doi.org/10.1002/cft2.20304

References

Mourtzinis, S., G. Kaur, J.M. Orlowski, C.A. Shapiro, C.D. Lee, et al. 2018. Soybean response to nitrogen application across the United States: A synthesis-analysis. Field Crops Research. 215: 74–82. https://doi.org/10.1016/j.fcr.2017.09.035