Plant Health Initiative

Soybean Nodulation

The amount of nitrogen fixed by the plant will increase with the number of nodules. Photo credit: Hans Kandel, University of Minnesota

Soybeans are legumes, plants which have co-evolved with specific bacteria that have the ability to "fix" atmospheric nitrogen and make it available to the host plant. Legumes generally do not require N fertilizer because of this relationship with beneficial bacteria.

Nitrogen-fixing bacteria colonize host roots and form nodules (small swellings) on the root system. Bacteria that nodulate legumes are currently classified into six genera (Rhizobium, Bradyrhizobium, Mesorhizobium, Sinorhizobium, Azorhizobium, and Allorhizobium), and are generally called “nodule bacteria” or rhizobia.

Nodules formed on soybean by the soybean-specific species Bradyrhizobium japonicum can be seen shortly after soybean emergence. Nitrogen fixation begins around growth stage V2 or V3. Adequate nodulation requires about 7 - 14 nodules per plant. The number of nodules formed on the roots along with the amount of nitrogen fixed continues to increase until growth stage R5.

Nodules that are fixing nitrogen are pink or red inside. After about 50 to 60 days from the time of formation, the nodules start to degenerate. White, green or brown nodules indicate that little or no fixation is occurring.

 

Factors that influence nodulation

High nitrogen fertility inhibits the abiltiy of the bacteria to form nodules and fix N. Little nodulation may also occur in fields in which soybeans are being grown for the first time. Other factors that may depress nodulation are very wet conditions early in the season, or, conversely, extremely dry soil conditions. The presence of root-rotting fungi can also inhibit the ability of the bacteria to colonize roots and form healthy nodules.

 

Do you need to inoculate seed?

The decision on whether or not to inoculate depends on whether the field has a recent history of healthy-looking soybean. Most soils in the North Central soybean-producing area have a good population of Bradyrhizobium japonicum if soybean has been grown in recent years. However, if a field is new to soybean, or has been out of soybean for more than three to five years, it is good insurance to inoculate.

Most state Extension recommendations are to inoculate the seed if

• the field has never been planted to soybean
• soybeans have not been grown in the field in the past three to five years
• the soil pH is below 6.0
• the soil has a high sand content
• the field has been flooded for more than a week, creating anaerobic conditions

Check for nodulation

When checking roots for nodules, dig them out carefully to avoid sloughing off the nodules. Check at least 10 sites in the field. Adequate nodulation requires about 7 - 14 nodules per plant. If less than 5 nodules are present wait for about a week and take another assessment

If nodule numbers have not increased and nitrogen deficiency symptoms are beginning to appear (light green appearance of the plant or yellow lower leaves) then 50 to 60 lbs/acre of nitrogen should be applied. However, if soil nitrate levels were high at planting time, then nodululation may be inhibitied due to high N, and the crop  should be able to obtain nitrogen from the soil.

 

Adapted from

Soybean Nodulation - CropPest Ontario

Seed Inoculation - Iowa State University

Soybean Seed inoculation - Iowa State University