Soybean field
LIMITING FACTOR: While yield environment plays a big role, UNL research indicates soybean yield is indeed limited by nitrogen supply, especially in high-yield environments.

Is soybean yield limited by nitrogen supply?

While it's hard to measure, it will be important in the future to know at what point soybean yields are limited by nitrogen; UNL research shows it varies by yield and environment.

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.

Nicolas Cafaro La Menza

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.

TAGS: Fertilizer
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