Soybeans have among the highest nitrogen (N) demands of agronomic crops due to a high concentration of protein in the seed. Symbiotic fixation supplies about half of the plant’s N needs, and the remainder comes from the soil and/or fertilizer.
Despite their high demand for N, soybeans have historically received little or no N fertilizer. However, some studies have indicated that fixed N alone may not be sufficient to supply the N required to produce maximum yields. In fact, adequate N2-fixing capacity of soybeans declines rapidly after the R5 stage, which coincides with the peak soybean N demand for protein synthesis in seeds.
To better understand the potential for increased soybean productivity by using additional N, DuPont Pioneer conducted research trials to determine soybean yield response to late-season N applications.
In a 2011-2012 study conducted at 5 Illinois locations, 80 lbs N/acre was applied as a polymer-coated urea at the R2 growth stage. Results showed a significant positive yield response to the N fertilizer at all 5 research locations (p < 0.10). Yield increases ranged from 1.3 to 3.7 bu/acre (Figure 1).
Figure 1. Soybean yield increases with R2 applications of 80 lbs N/acre at 5 Illinois locations in 2011-2012. All locations showed a significant (p < 0.10) yield increase.
In addition, a larger study conducted at 55 Illinois and Indiana locations over 2 years also showed a positive yield response to 100 lbs N/acre applied at the R2 growth stage. Across all locations, the average yield increase from supplemental N application was 1.1 bu/acre (Figure 2). The yield response to the added N was 0.4 bu/acre for short-season (< 3.0 MG) varieties and 2.1 bu/acre for full-season (> 3.0 MG) varieties.
Figure 2. Influence of 100 lbs N/acre applied at R2 vs. no applied N on soybean yields at 55 research locations in Illinois and Indiana over a 2-year period.
An iron deficiency can also hinder soybean productivity. Soybean iron deficiency chlorosis (IDC) is a nutrient deficiency disorder with symptoms that include chlorosis (yellowing) of the soybean foliage and stunting of the plant. This condition is yield-limiting in many soybean fields in the northern and western Corn Belt as well as parts of the southern U.S. Because soybean varieties vary widely in tolerance to IDC, variety selection is the first and most important step in managing this problem.
DuPont Pioneer conducted a study to determine if an in-furrow application of an iron chelate treatment at planting would help to mitigate IDC symptoms at 11 locations with a history of IDC in Nebraska and Kansas. Results showed that several soybean varieties with a range of IDC tolerance scores had a positive yield response to the iron chelate treatment (Figure 3). Visual differences, such as greener and more robust plants, were noted as well.
Figure 3. Yield response to iron chelate treatment averaged across 11 locations with a history of IDC in Nebraska and Kansas in 2012.
In another study, DuPont Pioneer and Auburn University investigated the use of a wheat cover crop to manage IDC on high pH soils in the Black Belt Region of Alabama.
Results from this 2012 study showed that using a cover crop increased yield of several soybean varieties that had a range of IDC tolerance scores. In fact, a cover crop increased the yield of the most IDC sensitive variety by 45 bu/acre. Other varieties showed yield increases ranging from 20 to 31 bu/acre in response to a cover crop (Figure 4).
Figure 4. Soybean yield response comparing the impact of a cover crop vs. a fallow field in 2012. Pioneer® brand varieties used in the study were 95Y40 (RR), 95M82 (RR), 96Y70 (RR, STS), and 96M60 (RR).