N on Beans: A Different Perspective

Published on 6 May 2015, 21:00 PM • by: Dan Davidson

Are you interesting in applying nitrogen on soybeans? Have you tried it yet?

The past year or two agronomists, farmers and university faculty have been writing and talking about putting nitrogen on soybeans. Two things I have noticed are that farmers are interested in trying it to increase soybean yield and university specialists state it doesn’t pay.

The scientific reality to date is that applying nitrogen on soybeans is inconsistent. Sometimes it works and sometimes it doesn’t and no one is exactly sure when it will work so it is difficult to make a decision and equally difficult to make a recommendation.

So most recommendations coming out of universities encourage growers to refrain from applying nitrogen on soybeans because they probably won’t respond. They also state that soybeans fix their own nitrogen so applying nitrogen will just reduce nitrogen fixation. But sometimes I feel there is another aspect to the story on why soils don’t respond to nitrogen consistently.

I suggest that the reason soybeans don’t consistently respond to nitrogen is due to the following factors:

  • Yield potential is too low
  • Nitrogen fixation is very high
  • Soil is capable of delivering greater amounts of nitrate than we think

If your soybean crop is only going to produce 60 to 70 bushels, nitrogen fixation and soil nitrogen reserves (available nitrate and mineralizable nitrogen) will meet the crop’s nitrogen requirements.

A plant’s nitrogen fixation potential is variable in the field depending on nodulation, how effective the rhizobia are in capturing and processing nitrogen and the soil conditions in which they exist. Currently there is no method to gauge nitrogen fixation in the field and the presence of nodules that are red on the inside is not a measure of nitrogen fixation potential. Maybe the soybean plant is only fixing 20 percent of its needs or maybe 60 percent, or maybe 100 percent.

Soybeans will take up nitrogen if the soil has soluble nitrate available and maybe 50 or more lbs. that can feed the plant. But the greater untapped reserve is the soil organic matter and the soil’s ability to mineralize substantial amounts of nitrogen that the plant can tap into. Between soluble nitrate and mineralization the soil can supply 100 to 200 lbs. of nitrogen. And if the organic matter levels and microbial activity are high, mineralization can be quite substantial.

Inconsistency in nitrogen response is probably related to applying nitrogen when yield potential is too low, when nitrogen fixation is high, or the soil has the ability to provide substantial amounts of nitrogen. If you don’t track and account for nitrogen fixation potential and soil nitrogen reserves and mineralization it will always be difficult to make a nitrogen recommendation and responses will always be inconsistent.

Have you tried applying nitrogen on soybeans and was there a consistent yield response?

Agronomist Dr. Daniel Davidson posts blogs on agronomy-related topics. Feel free to contact him at djdavidson@agwrite.com.

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