April 22, 2011

Q:  Is an inoculant necessary for flooded fields that are being planted to soybean?

Some lowland fields, particularly in southeast Nebraska, experienced flooding this week and more may over the next few weeks as we see additional snowmelt from the Rockies. Jim Specht, UNL soybean geneticist, shared the following response to this question and suggests that growers who find themselves in this situation consider whether an inoculant application might be worthwhile insurance.

In theory, a soil completely covered with flood water and completely saturated (very little air present) for a period of time would be expected to be lethal to most aerobic microorganisms, such as rhizobia. In practice, however, it is often difficult to know if the oxygen in a saturated soil has really been depleted to zero for a long enough time to kill all aerobic microorganisms.

More importantly, some aerobic organisms can survive by apparently shifting to a free-living state that involves anaerobic metabolism (for example, denitrification of soil N). Such metabolism can provide them with sufficient energy to live at a low level, compared to an oxygenated environment. In fact, this adaptability would likely be a selective advantage of rhizobia adapted to cropping systems where a soybean crop follows a flooded rice crop.

Certainly, the concentration of free-living rhizobia in a flooded soil will decrease, perhaps from millions to thousands (or less) per gram of soil. The argument then is probably not whether there are any rhizobia left after a flooding event, but whether there are sufficient numbers so that a new soybean crop could achieve enough N-fixation to avoid yield loss.

Given this is likely an unknown for a specific field, many agronomists suggest producers with flooded fields use an inoculant as a form of crop "insurance."

Learning Opportunity

Consider taking this opportunity to do your own field research on this question, testing both options — applying and not applying inoculant to flooded fields before planting soybean this year.  Whichever option you choose for the majority of the field, consider using a small strip of the field to test the other option.  This will help you learn more about how your field responds in this situation, which may be helpful for future management decisions.

More Information

There is not a lot of research on this topic; however, two UNL NebGuides provide further information on soybean inoculation

• Soybean Inoculation: Understanding the Soil and Plant Mechanisms Involved
• Soybean Inoculation:  Applying the Facts to Your Fields

Jim Specht
UNL Soybean Geneticist