Most of the nitrogen (N) requirement for soybean is supplied through the N-fixation process, which is a result of a beneficial relationship between the plant and specific soil bacteria. If necessary, soybean seed should be inoculated with the appropriate bacteria to facilitate nodulation. Healthy and active nodules are pink to red on the inside and 2-4 mm in diameter. Light green, stunted soybean plants at the beginning of reproductive growth stages may indicate poor nodulation or other deficiency or environmental stress.
What is Nitrogen Fixation?
About 5 lbs of nitrogen (N) is needed to produce a bushel of soybeans.1 Anywhere from 50-75% of the N comes from N-fixation, the process by which atmospheric N is converted to a form usable by the soybean plant.1 For this to occur, soil bacteria called Bradyrhizobia japonicum must be available in the soil or be applied to soybean seed. The bacteria act by invading the soybean plant root hairs and multiplying to form nodules on the roots. These nodules house the N-fixing bacteria. The bacteria have a mutually beneficial relationship with the plant; the plant provides them with carbohydrates while the bacteria provide N-fixation for the plant.
Nodule formation can be seen within a week of emergence, but active fixation usually begins within 10-14 days later, around V2-V3 growth stages, at which point they can supply most of the plant’s N requirements.2,3 An individual nodule remains active for 6-7 weeks before beginning to break down. New nodules continue to form throughout much of the plant’s life, until ceasing during pod fill stages. Fixation production increases to its peak around the R5.5 growth stage, after which nodules decline and fixation decreases.2,3
Evaluating Nodule Presence and Activity
Soybean roots should be examined for nodulation 5-6 weeks after planting.1 Choose at least 10 plants from representative areas in each field to sample. Using a shovel to remove as much of the root system from the ground as possible, dig up the plants and immerse the roots in water to remove the soil. There should be 8-20 large (2-4 mm diameter) active nodules per plant prior to flowering.1 If sampling is done earlier and there are less than 5 nodules per plant, resample in a week. When split open, nodules are pink to red when actively fixing N (Figure 1). Small, white nodules have yet to fix N (immature) and green, brown, or mushy nodules are not fixing N. Inactive nodules may result from stresses like flooding or the soybean nodule fly maggot. Nodules on the tap root are usually a result of this season’s inoculation, while those appearing on lateral roots generally result from bacteria present in the soil or from soil applied inoculants.
Conditions Resulting in Poor Nodulation include:
- New soybean fields are likely to have low populations of the appropriate soil bacteria.
- Fields with high levels of residual soil N from a previous forage legume, manure, or fertilizer application.
- Soil compaction and cool soil conditions resulting from no-till practices can reduce nodulation.
- Extremely dry or wet conditions following soybean planting.
- Coarse-textured soils may have inadequate moisture levels to sustain bacteria.
- Flooded or saturated soil conditions can cause oxygen deprivation. Nodules may form and die due to flooding (3 or more days saturation can cause nodule rotting and death).4
- Plants that have been weakened by hail damage, root diseases, or IDC (iron deficiency chlorosis).
- Fields that have soil at a pH level below 5.7 or above 7.3.1
What is Inoculation and When Might it be Needed?
If soils do not carry adequate numbers of Bradyrhizobium bacteria for N fixation, they can be added as either a liquid or granular peat inoculant, or a peat-based powder. Inoculants can be seed applied or applied in-furrow. The viability of inoculants will depend on storage time and storage conditions. An expiration date should be listed for commercial inoculants, but they can be killed by direct sunlight or excessive heat prior to the shelf life’s end.
Inoculants can have a positive impact in many growing situations including the following:
- If a soybean crop has not been grown in the field for 3-5 years and if soil pH has not been maintained above 6.0.2,3
- Fields never planted to soybean should be double inoculated with both a seed applied inoculant and a soil applied inoculant.
- Fields with sandy soil (low organic matter soils) need to be inoculated every year.
- Fields that were flooded the previous season may need to be inoculated due to the anaerobic conditions reducing the level of Bradyrhizobium bacteria in the soil.2,3
Sometimes the conditions in first-year soybean fields or fields with a high residual N can result in inoculation failure. With first-year soybean planting, inoculant can be applied to the seed and in-furrow to the soil. Another method to encourage nodulation in first-time soybean fields is to consider planting a soybean crop for two consecutive years, inoculating both years.4
Conditions most likely to cause inoculation failure:
- Planting a soybean crop into a field for the first time.
- Soil pH levels below 5.5 and above 8.0.4
- Residual soil nitrogen levels greater than 40 pounds per acre.4
- Improper inoculant application to the seed or poor quality inoculant resulting from improper storage.
Nitrogen Application Considerations
In general, applications of N fertilizer to soybean should be avoided. A greater supply of N available in the soil from fertilizer results in reduced N fixation by the nodules. As the N soil supply increases, nodule number and activity level decline proportionately. Soybean planted in fields with excessive residual nitrate should be closely monitored. If nodulation has been severely inhibited and N deficiency symptoms appear, additional N during pod fill may be helpful.5 Research has shown conflicting results regarding late-season N applications, with the greatest success occurring when N was applied via irrigation system in high-yield situations.5
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1Staton, M. 2011. Evaluating soybean nodulation. Michigan State University Extension. http://msue.anr.msu.edu
2Pedersen P. 2009. When to inoculate soybean seed in Iowa. Integrated Crop Management. Iowa State University. www.ipm.iastate.edu
3Pedersen, P. 2003. Soybean seed inoculation. Integrated Crop Management. Iowa State University. www.ipm.iastate.edu
4Staton, M. 2014. Identifying and responding to soybean inoculation failures. Michigan State University. http://msue.anr.msu.edu
5Larson, K., Rice, C., and Roozeboom, K. 2012. Successful soybean nodulation without rhizobia. 344 Agronomy e-Updates, Kansas State University Extension. Web sources verified 5/9/2018.