Soybeans have a large nitrogen requirement. By comparison, soybeans require about four times more N per bushel produced than corn.

On average, soybeans need to absorb 4.8 pounds of N per bushel produced. So, a soybean crop that produces the Nebraska average of about 50 bushels per acre will need to absorb 240 pounds N per acre. A well-managed, irrigated soybean crop that produces 80 bushels per acre will need about 384 pounds 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. It seems critical to know the level at which soybean yield becomes limited by N supply, if it ever does.

However, it's 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's hard 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. So, applying N fertilizer reduces N fixation so that the amount of N absorbed by fertilized versus non-fertilized crops may end up the same. This might be why yield response to N fertilizer has been found to be small and inconsistent in past soybean research.

N treatments
As a first attempt to understand the degree of N limitation across yield levels in soybean, University of Nebraska-Lincoln researchers designed an experiment that includes:

• a “full-N” treatment that received ample N supply during the entire soybean crop growing season

• a “zero-N” treatment that did not receive any N fertilizer.

These 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 full-N treatment received a range of 300 to 780 pounds of N per acre. Rates were determined based on site-specific yield potential as determined by climate and genetics, and the soybean N requirement per bushel produced. Because of the “trade-off” between N fertilizer application and N fixation, the experiment ignored N fixation for calculation of N fertilizer requirements.

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. In other words, researchers “spoon-fed” the crop to ensure that N supply was synchronized with crop N demand.

On average, yield was 11% higher in the full-N treatment compared to crops that received no N fertilizer (zero-N) (see table). However, the yield responses depended upon the yield level of the environment. For example, there was no yield difference between full-N and zero-N treatments for yield levels around 40 bushels per acre. In contrast, there was a 13-bushel yield increase due to N fertilizer application at yield levels near 90 bushels per acre. Results indicate soybean yield is indeed limited by N supply, especially in high-yield environments.

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 vs. zero-N treatments (36.0% vs. 34.7%). In contrast, oil concentration decreased slightly in the full-N treatment.

Take-home messages
Although the study used N rates that are far from being economically profitable and environmentally sound, it 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.

• 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. So, future research should be directed to find agronomic practices that can “break” the trade-off between N fertilizer addition and N fixation, and increase N fixation.

• If considering an N application in soybeans, keep expectations at a reasonable low level and give priority to fields with consistent high yields in previous years.

This report comes from UNL CropWatch.