Soybeans are a big user of nitrogen and need approximately 3.5 lbs. of nitrogen per bushel of grain produced. So, an 80-bushel soybean crop will uptake 280 pounds of nitrogen.
Fortunately, if we have adequate nodulation, we don’t need to be concerned with nitrogen deficiency; nature will take care of it for us. But we need to assess soybean fields for adequate nodulation.
About 5-6 weeks following planting of soybeans is a great time to scout plants and evaluate nodulation. At this time, nodules should be large enough to be active.
When digging up plants in a field, be sure to use a shovel to ensure you are carefully removing as many roots as possible. Simply pulling out the plant will likely detach roots and you will not get an accurate nodule count. Then put the plants into a bucket of water to rinse the soil from the roots.
At the V2 growth stage (image 1), nodules will be actively fixing nitrogen from the air.
Just because nodules are present does not mean they are actively working. Nodules that are active in fixing atmospheric nitrogen will be pink or red in color when cut open (image 2).
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If nodules are white in color they may be immature and haven’t yet begun to fix nitrogen. In this case, we would recommend visiting the field in another week to conduct another assessment. Nodules that are mushy and brown are not fixing nitrogen.
Be sure to note the location of the nodules on the plant. Nodules that form on the taproot are likely a result of inoculation from this season. Nodules on lateral roots are from bacteria that are existing in the soil.
A properly nodulated soybean plant should have five to seven nodules on the taproot two weeks after emergence or twelve total root nodules per inch of taproot at flowering (R1) (R. Elmore 2007). While we evaluate nodulation early in the season, the number and size should continue to increase until the R5 growth stage.
Causes of poor nodulation can include:
- Improper soil pH: soil pH should be between 6.5 and 7.0. If pH is below 6.5, the addition of Molybdenum will aid in infection and nodulation. This is a correction that needs to occur with seed treatment or very early seedlings (V1 stage).
- Planting soybeans into “virgin” ground, where soybeans haven’t been grown. Here, there are low bacteria populations in the soil, and we often see inadequate nodulation.
- Inoculant that has heated in storage. Be sure to store inoculant in a cool place, out of direct sunlight. Also, be mindful of the expiration date on inoculant.
- Flooding or very saturated soils lasting 7 days or longer will deprive the plants of oxygen.
- Compacted soils that limit available oxygen.
What can be done if your field doesn’t produce adequate nodules, or they are inactive? If fields are showing nitrogen deficiency (light green, stunted plants), we would recommend that up to 50 lbs. of a dry nitrogen fertilizer source be broadcast over the field prior to full bloom (R2 growth stage).