If soybean plants are chlorotic and N deficient despite being inoculated, that probably indicates the inoculant has failed.
Assessment: In the field, nodules from some soybean plants can be crushed to look at or assess their internal color. In general, pink or reddish interior color of nodules is indicative of the rhizobia actively fixing N. On the other hand, dark gray or whitish interior color of nodules indicate the ineffectiveness of the rhizobia in fixing N.
Figure 1. Close-up of soybean nitrogen-fixing nodules. Photos by Ignacio Ciampitti, K-State Research and Extension.
What factors affect inoculation response?
Several factors can result in poor nodulation or failure of inoculation:
- Poor or inadequate coverage of the seeds by the inoculum during inoculation.
- Contamination of inoculant with other foreign materials.
- Lack of competitiveness of the introduced Rhizobia strain compared to the indigenous Rhizobia.
- Lack of persistence in the soil. The introduced Rhizobia should be able to grow and remain viable in the soil and in between crops without undergoing mutation.
- Low soil phosphorus (P): Legumes need adequate phosphorus for proper growth and pod development. Low P can result in poor nodulation and reduced N fixation. Phosphorus deficiency can negatively affect seed development and pod formation leading to low yield.
- Soil pH: This an important environmental factor. Most legumes grow and nodulate well at pH 5.6 - 6.7. The best soil pH for Rhizobium lies between pH 5.8-6.8. Low pH soils require liming. In general, legume responds well to liming. Low pH (< 5) causes Al and Mn toxicity, and results in P deficiency.
- Soil nitrate and ammonium levels: High inorganic nitrogen (ammonium and nitrate) levels in the soil inhibit nodulation and N fixation. The effectiveness and competitiveness of Rhizobia are negatively affected by high inorganic nitrogen.
- Molybdenum (Mo): Soils deficient in Mo can have reduced nitrogen fixation. Mo, an essential micronutrient, is needed for the formation and function of the nitrogenase enzymes. Legumes also need other micronutrients such as iron, boron, and copper.
- Stress, either in the form of drought, excessive soil moisture, and high temperatures can reduce nodulation.
If the inoculation has failed, producers may need to apply N to their soybean crop. Depending on the projected yield potential, producers may need to apply as much as 80-100 lbs N/acre in that case.
Edwin Akley, Agronomy Graduate Student
Chuck Rice, Soil Microbiologist
Ignacio Ciampitti, Crop Production and Cropping Systems Specialist
Kraig Roozeboom, Crop Production Agronomist