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

Some lowland fields 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, 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 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 soybeans 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.