Soybean is nodulated by three distinct types of rhizobia bacteria; Rhizobium fredii, Bradyrhizobium japonicum, and Bradyrhizobium elkanii. These soil dwelling bacteria have the unique ability to penetrate and fix nitrogen in association or symbiosis with the soybean roots.
The soybean plant gets nitrogen from the bacteria, while providing the bacteria with carbohydrates. When soybean seed germinates, the bacteria invade the root hairs of the seedling and begin to multiply. Nodules, which house the bacteria, form on the roots.
Under field conditions, the first nodules form within a week after seedling emergence and become visible as they increase in size. Active fixation begins in the V1 to V3 stage, after which the number of nodules and the amount of nitrogen fixed continue to increase. The soybean demand for nitrogen is highest from the R5 to R8 stages.
The amount of nitrogen fixed by the plant will increase with the number of nodules. The number of nodules on the soybean root will increase until the time seed is forming in the upper pods of the soybean plant.
Healthy nodules have a pink or red color inside and are actively fixing nitrogen. A green, brown or white color indicates that the fixation is not taking place. One can check the nodules by gently digging out plants with a small trowel and rinsing the plant in water. Pulling the plant from the soil may cause the nodules to slough off. The nodules can be cut open with a knife to inspect the inside color. It is suggested to check a few places in the field. While digging up plants one should also observe if there are root rots visible.
Soybeans will first take up available nitrates before they will actively start fixing their own nitrogen. Nodule numbers may be low in fields where soybeans are grown for the first time and no seed inoculation (with the right bacteria) took place. Other factors resulting to low nodule numbers include fields that were saturated early in the season or areas where root rots are present.
Good nodulation usually will not occur in fields under extreme dry conditions. At the V2 growth stage the soybean plants should have roots six inches down in the soil and at the V3 to V4 stage of growth, there should be 8 to 10 large healthy nodules per plant.
Many soybean acres are planted to glyphosate resistant varieties, in which a resistant gene has been inserted in the plant to make it tolerant to glyphosate herbicides. After application of the herbicide on the canopy, it translocates downward through the soybean plant with most of the herbicide remaining in root tissue. Although these soybean plants are resistant to glyphosate, the bacteria which live on the root and nodules are not resistant.
University of Nebraska researchers applied glyphosate on either V1 or V4 soybeans and they reported that there was no effect on nodulation. They also found no effect on nodule counts later in the season following a V9 glyphosate application. Yield was not reduced due to any of the different glyphosate rates or application timings. To read more on this report, go to: http://cropwatch.unl.edu/archives/2004/crop04-17.htm#glyphosate.
The Unversity of Minnesota’s Rhizobium Research Laboratory has more information on nodulation in its FAQs, online at www.rhizobium.umn.edu.
Sources: Hans Kandel, University of Minnesota Extension Service; Iowa State University Extension Service