By Nicolas Cafaro La Menza
Soybean has a large nitrogen (N) requirement. Indeed, soybean requires about four times more N per bushel produced than corn. On average, soybean needs to absorb 4.8 lbs of N per bushel produced. Hence, a soybean crop that produces 50 bu/ac (similar to current Nebraska average) will need to absorb 240 lbs N per acre. In contrast, a well-managed, irrigated crop that produces 80 bu/ac will need about 384 lbs N per acre.
Except for a small dose of N fertilizer applied as “starter” in some fields, most soybean crops rely almost exclusively on N supplied by soil organic matter mineralization and N fixation. The latter is a symbiotic association between a bacteria and the plant. The bacteria fixes N from the air and makes it available for the plant in exchange for carbohydrates that come from plant photosynthesis.
As the yield levels increase, so does the N requirement, leading to uncertainty relative to the degree to which the N supplied from soil organic mineralization and fixation is sufficient to meet crop N requirements. Therefore, it seems critical to know the level at which soybean yield becomes limited by N supply, if it ever does. However, it is challenging to evaluate N limitation in soybean for two major reasons. First, soybean absorbs 60% of the N after R3 (beginning of pod setting), so it is difficult to ensure an ample N supply just when it is really needed by the crop. Second, application of N fertilizer in soybean (and other legume crops) typically results in a decrease in N fixation. In other words, applying N fertilizer reduces N fixation so that the amount of N absorbed by fertilized versus non-fertilized crops may end up the same. These two reasons help explain why yield response to N fertilizer has been found to be small and inconsistent in past soybean research.
Our Experiments
As a first attempt to understand the degree of N limitation across yield levels in soybean, we designed an experiment that includes:
- a “full-N” treatment that received ample N supply during the entire soybean crop growing season, and
- a “zero-N” treatment that did not receive any N fertilizer.
Our experiments were conducted in irrigated soybean in Nebraska (four producer fields near Mead, Saronville, Atkinson, and Smithfield) and Balcarce (Argentina) from 2015 to 2017. The experiments covered a wide range of environments, with yield ranging from 40 to 90 bu/ac. Side-by-side comparison of yield in the full-N versus zero-N treatment at each site would provide a good indication about the degree of limitation across this wide yield range.
Our full-N treatment received large N fertilizers amounts (ranging from 300 to 780 lb N/ac), which were determined based on 1) site-specific yield potential as determined by climate and genetics and 2) the soybean N requirement per bushel produced (4.8 lbs N per bu). Because of the “trade-off’ between N fertilizer application and N fixation, we ignored N fixation for our calculation of N fertilizer requirements. Again, these N fertilizer amounts were applied to test our hypothesis about N limitation in soybean and not to make recommendations regarding N fertilizer application in farmer fields!
To guarantee a high N supply during the entire growing season, the total N fertilizer amount calculated for the full-N treatment was split into five applications (V2, V4, R1, R3 and R5 stages). The amount of N fertilizer in each application was proportionally adjusted according to the expected crop N requirement at each stage: 10% at V2, 10% at V4, 20% R1, 30% at R3, and 30% at R5. In other words, we “spoon-fed” our soybean crop during the entire growing seasons to ensure that N supply was perfectly synchronized with the crop N demand.
What Did We Find?
Figure 1 shows the comparison between measured yields in the full-N treatment (vertical axis) versus the zero-N treatment (horizontal axis). The colored symbols denote the yields of soybean at each site and year. The solid x=y line denotes equality in yield if soybean yield is not impacted by N fertilization. However, the chart’s dashed line shows that soybean yield was indeed impacted by N fertilizer addition. On average, seed yield was 11% higher in the full-N treatment compared with the crops that did not receive any N fertilizer (zero-N) (Figure 1). However, the yield responses depended upon the yield level of the environment. For example, there was no seed yield difference between full-N and zero-N treatments for yield levels around 40 bu/ac. In contrast, there was a 13-bushel yield increase due to N fertilizer application at yield levels near 90 bu/ac. To summarize, our results indicate that soybean yield is indeed limited by N supply, especially in high-yield environments.
Figure 1. Seed yield in the full-N versus zero-N treatments across experiments in Argentina and Nebraska. Green arrows indicate the measured yield differences due to N fertilization at high (40 bu/ac) and low (90 bu/ac) yield levels.
Figure 2. Seed protein (left) and oil (right) concentration (at 13% of seed moisture) in the full-N versus zero-N treatments across experiments in Argentina and Nebraska.
N protein concentration typically decreases with higher yields. We found the opposite response in our experiments: despite higher yields, seed protein concentration was higher in the full-N versus zero-N treatments (36.0% versus 34.7%). In contrast, oil concentration decreased slightly in the full-N treatment (Figure 2).
Take-Home Messages
Although our study used N rates that are far from being economically profitable and environmentally sound, it clearly shows that:
- Nitrogen supply from soil organic matter mineralization and fixation are NOT sufficient to fully satisfy soybean N requirement, especially in high yield environments.
- Yield response to large N fertilizer amounts were modest and depended on the yield level of the production environment (5 bu/ac at 50 bu/ac yield level, but 13 bu/ac at 90 bu/ac yield level).
- Seed protein concentration increased with N fertilizer addition, a surprising finding worthy of more research.
- As soybean yield continues to increase, the N limitation will become more and more important. Hence, future research should be directed to 1) find agronomic practices that can “break” the trade-off between N fertilizer addition and N fixation and 2) increase N fixation.
- If you are considering an N application in soybean, keep expectations at a reasonable (low) level and give priority to fields with consistent high yields in previous years.