Inoculating Soybean With Bradyrhizobium Japonicum For Nitrogen Fixation

DAVID MOSELEY, RASEL PARVEJ, AND JIM WANG 

BATON ROUGE, LOUISIANA

Questions have recently arisen regarding inoculating soybean for nitrogen fixation. Nitrogen fixation is a process where soybean plants can utilize nitrogen from the air by a symbiotic relationship with Bradyrhizobium japonicum bacteria and is essential for economical soybean production. The Science For Success group (A national team of soybean agronomist) published an article on the Soybean Research & Information Network website stating that nitrogen fixation can account for 40 to 70% of the nitrogen requirement for soybean. 

Inoculating with Bradyrhizobium japonicum

The bacteria that is responsible for nitrogen fixation in soybean is Bradyrhizobium japonicum. If a field has never been planted to soybean, it is likely the seed will need to be inoculated. The bacteria can survive many years until the next soybean crop, but the common recommendation is to inoculate seed if soybean has not been planted for 3 to 5 years. However, research has indicated that an increase in yield is possible if seed is inoculated in a regular soybean rotation. Adverse conditions can reduce the activity of the bacteria, including drought or flooded conditions. An important reminder is that the soybean plants require specifically Bradyrhizobium japonicum for nitrogen fixation. Other legume plants benefit from nitrogen fixation but are associated with other rhizobium bacteria. For more information on different plants and specific inoculants, see this Penn State Extension publication. When applying Bradyrhizobium japonicum, it is important to remember the inoculant is a live organism. Attention to proper storage and timing of application can help minimize a decrease in viability of the bacteria.

Molybdenum Applications in Acidic Soils

Molybdenum (Mo) is an important nutrient for nitrogen fixation. In low pH soils (acidic soils) with low Mo availability, adding Mo may be necessary. It is important to remember that a Mo application can harm the inoculant. If an application of Mo is necessary, it is important to not combine it with the inoculant unless the application is immediately before planting. ∆

DAVID MOSELEY, RASEL PARVEJ, AND JIM WANG: LSU AgCenter