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.