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The research and debate on applying supplemental nitrogen (N) on soybeans continues. The ILSoyAdvisor has posted its share of articles on the topic and I have written my portion to answer that question. There is no clear answer and just applying nitrogen can be costly with an added risk of adding more nitrogen to the groundwater—two negatives. 

After studying the topic for five years and listening to many viewpoints it is hard for me to recommend supplemental nitrogen because of three unknowns: yield potential, nitrogen fixation level and the soil’s nitrogen mineralization output. Soybeans require 4 – 5 lbs. N per bushel and can fix enough N for a 50- to 60-bushel yield, so if yield potential is 75 – 80 bushels or higher, there won’t be a response. Soybeans fix nitrogen, but how much they can fix depends on soil and plant condition. They may fix enough for 40 bushels or, in some situations, 80 bushels. Lastly, how much N can the soil supply through mineralization, 20 lbs. or 100 lbs.? If we can’t answer those questions by the R1 to R3 stage, it will always be hard to make a recommendation that’s consistently responsive and profitable.

Fred Below completed three years of research looking at the N response of soybeans in Illinois. He presented “What We’ve Learned from Nitrogen-On-Beans Research” last winter. Dr. Below shared these generally positive conclusions:

1.    How much of the soybean’s nitrogen comes from the nodules?

• 50%
• The rest comes from the soil

2.    Soybeans are a huge nitrogen-requiring crop

• Soybeans required 4 – 5 pounds of nitrogen per bushel

3.    Facts about soybeans and nitrogen

• There is no such thing as a soybean nitrogen credit
• Soybeans remove about a pound of nitrogen from the soil for each bushel produced

4.    Can supplemental fertilizer nitrogen increase soybean yield?

• Yes
• There was no negative impact on nodule development and applying 100 lbs. of nitrogen preplant or V3 tended to increase nodule activity

5.    What was the best time to apply nitrogen fertilizer to soybeans?

• Although yield increases were observed for all application times, preplant applications regularly increased yield over 3 years

6.    Do fertilizer nitrogen applications inhibit nodule formation and activity?

• No, fertilizer nitrogen applications only had minor effects on nodule number and color

Shawn Conley, soybean specialist at the University of Wisconsin, led a team of university researchers evaluating “Soybean Response to Nitrogen Application Across the U. S.” This work was supported by the Wisconsin Soybean Marketing Board, United Soybean Board and North Central Soybean Research
Program. The authors concluded:

• Nitrogen application decisions had a small effect on soybean yield
• Major management decisions (e.g., irrigation, seeding rate) interact with nitrogen response
• Limited nitrogen responses suggest that positive economic returns from nitrogen are unlikely

The scientists aggregated yield data from a range of experiments from 1996 to 2016 at 105 locations in 16 states across the Corn Belt. They looked at N application or not, method, timing and rate in small plot research, primarily on university research farms. Application of N had a measurable but small effect on soybean yield. However, these increases would be unlikely to produce a profit. The group concluded that N management could only be optimized within a system that looks at irrigation and seeding as part of the system. Their findings are in tune with most universities’ reports that yield responses are usually smaller and often do not occur.

Soybeans are big N users and their yield can respond to supplemental N. However, any yield benefit comes at a cost that usually outweighs any yield gain. Someday we may all be producing 100-bushel soybeans at a time when soybeans will either be fixing more N than they can today, or we will be strategically applying N at times when we get the most yield and profit benefit. I will probably be retired by then.

Soybean agronomist Daniel Davidson, Ph.D. posts blogs on agronomy-related topics. Feel free to contact him at djdavidson@agwrite.com or ring him at 402-649-5919.