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Soybeans require nitrogen and lots of it because it’s a protein crop and proteins contain 16% nitrogen. For decades we have relied on the soybean plant’s ability as a legume to fix its own nitrogen. And it can fix a lot, with the rest supplied by the soil. But as soybean yields ratchet upwards into the 70- and 80-bushel range in whole fields, nitrogen will become increasingly limiting.

The challenge to making a nitrogen recommendation is to know when yields are going to be high, and how much nitrogen the soil and plant can provide. There are no easy answers yet, but we are beginning to understand more about how soybeans interact when supplemental nitrogen is applied.

The Illinois Soybean Association has been funding some research into understanding the concept of “Nitrogen on Soybeans.” Dr. Fred Below at the University of Illinois and Tim Smith with CropSmith in Farmer City are carrying out the field work over this 3-year project during 2015, 2016 and 2017. Below is what they have learned to date.

• Soybeans produce only about 50% of the nitrogen they need from their nodules, with the rest coming from the soil.
• Soybean is a big nitrogen-requiring crop, having to accumulate between 4 and 5 lbs. of N per acre for every bushel of grain produced.
• Nitrogen applied preplant at 30 to 60 lbs./A can benefit establishment without negatively impacting nitrogen fixation.
• Supplemental nitrogen does not always shut down nitrogen fixation as previously thought, and rates as high as 100 lbs./acre can be applied without a detrimental effect on nodules or nitrogen fixation.
• Somewhat surprisingly preplant applications of N were the most consistent in increasing yield, probably because they provided young plants with needed N before the nodules became established and effective at nitrogen fixation.
• Nitrate containing and controlled release sources (ESN) were the most effective at all postapplication stages, while urea and ammonium sulfate were most inconsistent.
• Soils with a high nitrogen mineralization rate (ISNT measure) will support a higher soybean yield.
• Although soil can be a major source of nitrogen, it may not always provide enough beyond what the plant can fix.
• Soils can mineralize more nitrogen when soil conditions (organic matter, temperature and moisture) are ideal, as in 2016.
• At a given site substantial yield increases can be obtained by applying the right source at the right time, but year-to-year weather variations can impact response.
  
  While a small preplant application is probably of consistent benefit, recommending nitrogen on soybeans after the crop is up is still a guessing game and consistent results remain elusive. While it is difficult to estimate actual nitrogen fixation today, it’s possible to estimate soil contributions by measuring nitrate availability and mineralization using ISNT (Illinois Soil Nitrogen Test) or SLAN (Soil Labile Amino Nitrogen).

Stay tuned as the “Nitrogen on Soybeans” story continues to unfold.

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.