Evaluating soybean nodulation

Producers should evaluate a few soybean plants in each field to determine if nodules are present in sufficient numbers and actively fixing nitrogen.

Soybeans have a high demand for nitrogen. Approximately, five pounds of nitrogen is required to produce a bushel of soybeans. Fortunately for soybean producers, most of this nitrogen is provided through biological fixation of atmospheric nitrogen by bacteria living in colonies (nodules) on the soybean roots. Biological fixation accounts for 50 to 75 percent of the soybean crops’ total nitrogen requirement. The remainder is obtained from the soil.

Because biological fixation is the major source of nitrogen for soybeans, producers should evaluate a few soybean plants in each field to determine if nodules are present in sufficient numbers and actively fixing nitrogen. This is easy to do and the information gained can be used to correct an in-season nitrogen deficiency or develop strategies for improving nodulation the next time soybeans are grown in the field.

Begin checking roots for nodules about five to six weeks after planting. The nodules should be large and active by this time and supplemental nitrogen fertilizer can still be applied if needed. Always use a shovel to carefully remove as much of the root system as possible from the soil. Dig up at least 10 plants from representative areas in each field and immerse the roots in water to remove the soil.

There should be 8 to 20 large (2 to 4 mm) and active nodules per plant just prior to flowering. If you are sampling early in the season and there are less than five nodules per plant, wait one week and resample the field. Note the location of the nodules. Nodules formed on the tap root are probably a direct result of this season’s inoculation. Nodules formed on the lateral roots result from bacteria existing in the soil.

Nodules that are actively fixing atmospheric nitrogen will be pink to red when cut open. If the nodules are very small and white, they may be immature and have not begun fixing nitrogen. Green, brown or mushy nodules are not fixing nitrogen. If you find mushy nodules and the field has not been flooded, look for small white maggots, or larvae of the soybean nodule fly, in the nodules. The soybean nodule fly is a minor pest, but was found in Ingham County last summer. Record the sampling locations, plant growth stage, nodule numbers and nodule colors so that this information can be analyzed after harvest.

Poor nodulation and reduced nitrogen fixation are most likely to occur in the following situations.

  • New soybean fields, due to low bacteria populations in the soil.
  • Fields containing high levels of residual soil nitrogen from a previous forage legume or manure application.
  • Coarse-textured soils due to inadequate moisture levels to sustain bacteria.
  • Flooded or saturated soil conditions lasting seven days or more due to oxygen deprivation.
  • Soil pH below 5.7 or above 7.3.
  • Compacted soils due to reduced oxygen availability.

Fields showing nitrogen deficiency symptoms (light green and stunted plants) due to low nodule numbers or inactive nodules may respond to supplemental nitrogen. Applying 60 pounds of actual nitrogen per acre prior to full bloom should provide an economic return. Liquid fertilizers applied at this rate will damage the foliage, so dry materials are recommended.

Because soybeans require large amounts of nitrogen and biological fixation is the major source of nitrogen, producers need to learn how to maximize the amount of biological fixation occurring in their fields to achieve higher yields and be more profitable.

This article was produced by the SMaRT project (Soybean Management and Research Technology). The SMaRT project was developed to help Michigan producers increase soybean yields and farm profitability. Funding for the SMaRT project is provided by MSU Extension and the Michigan Soybean Checkoff program.

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