Bradyrhizobium japonicum is the nitrogen-fixing bacteria specific to soybeans. Other species colonize roots of different legumes. These bacteria infect roots, taking on a mutually beneficial relationship with the host plant to help convert atmospheric nitrogen into ammonium compounds that are needed by the host. Drought conditions of 2012 helped remind us that the Bradyrhizobium bacteria strains have environmental limits for survival and colonization of legume hosts.

Good news! In general, the cold soil conditions of this past winter are unlikely to greatly reduce survival of nitrogen-fixing bacteria. While these bacteria prefer temperatures between 40 and 80 degrees, Fahrenheit, normal winter cold temperatures generally have little effect unless soils are also extremely dry. Conversely, high soil temperatures, like those experienced in 2012, have been shown to adversely affect Bradyrhizobium survival.

Environmental conditions that negatively impact Bradyrhizobium bacteria populations include the following:

• Low pH (Acid Soils)
• High Temperatures
• Extremely Dry or Extremely Wet Soil Conditions
• Low Organic Matter
• Certain Crop Protection Products (Limited Data)

It has become clear that some strains of Bradyrhizobium japonicum are more persistent in the soil than other strains. In order to be certain that the best strains of nitrogen-fixing bacteria are available to colonize soybean roots, it is recommended that soybean seed be inoculated frequently, particularly if stressful environmental conditions, listed above, exist in any field.​​