Soybean is a member of the Leguminosae family of plants. An important characteristic of members of this plant family is their ability to live in a symbiotic (mutually beneficial) relationship with specific bacteria — in the case of soybean, Bradyrhizobium japonicum, a rhizobial species.
While the air is 79% di-nitrogen (N2), soybean plants without B. japonicum are unable to utilize this nitrogen source. The soybean plant provides nutrients (carbohydrates and minerals) and a protective growing environment for the rhizobia. In turn, the rhizobia “fix” atmospheric nitrogen into ammonia (NH3), which can then be used by the soybean plant. B. japonicum is specific to soybean and will not fix nitrogen in any other legume.
Likewise, the rhizobial species that fix nitrogen for alfalfa or other legumes will not nodulate and fix nitrogen on soybean. For this relationship to exist and benefit both soybean and B. japonicum, effective nitrogen-fixing bacteria must be present in the soil in relatively high numbers at planting time. In field soil where soybean has never been grown, it is essential to first establish the specific rhizobia to ensure nitrogen fixation. Rhizobia are established through a process called, “inoculation.”
When present in the soil, rhizobial bacteria attach to, then colonize the soybean root on new root hairs immediately behind the growing root tip. Within 10 to 14 days after colonization, the bacteria will form a visible nodule. Not to be confused with soybean cyst nematode (SCN), which are white to yellow, the B. japonicum nodule is a wart-like structure on the soybean root that contains a B. japonicum colony. The B. japonicum nodule grows very rapidly and begins fixing nitrogen at the V2-V3 soybean growth stage.
Functional nodules have a pink or red interior. This red pigmentation (leghaemoglobin), however, does not indicate fixation efficiency. After about four weeks, nodules will reach their full size and will continue to fix nitrogen until they are 6 to 7 weeks old, at which time they begin to senesce (become dark/black in color).
The soybean plant reaches peak nitrogen fixation during the beginning seed/pod fill crop growth stage (R5/R6 soybean). A number of biotic and abiotic factors influence nodule growth and nitrogen fixation, including soil moisture, soil temperature, soil pH, diseases, and in some instances, micronutrient availability. Soil fertility/nitrogen availability, pesticide use and inoculant quality also affect the efficiency of nitrogen fixation.