This week, I visited a producer concerned of an insufficient amount of active nitrogen-fixing nodules. Soybean plants are able fix most of their nitrogen requirement through a symbiotic relationship with Bradyrhizobium japonicum bacteria. The bacteria form nodules on soybean root hairs and convert nitrogen gas (N2) into ammonium (NH4+), a form usable by the plant.
However, there are several reasons that can cause soybean plants to be deficient in nitrogen, including an insufficient number of active nodules.
If the plants have symptoms consisting of light green to yellow leaves, they may be deficient in nitrogen. However, a deficiency in sulfur can cause very similar symptoms. A tissue sample can be helpful to determine if the plants are deficient in one or more nutrients.
Another good visual is looking at the nodules attached to the roots, but care should be taken to remove the plants with a large amount of soil still intact around the roots, and then the soil should be gently washed from the roots.
If the plant is pulled from the soil or if the soil is shaken off the roots, many of the lateral roots and the attached nodules will fall off. If the soil is washed from the roots, the lateral roots and nodules should remain.
If the plants are deficient in nitrogen from a lack of nodulation, an application of nitrogen can be economical. Ideally, the application should be applied between the R1 and R2 growth stage. However, applications at R3 have also been reported to show an increase in yield.
If the plants have a nitrogen deficiency for another reason, such as a lack of oxygen in the soil, then applying supplemental nitrogen may not have a positive effect on yield.