Although the air is 79 percent di-nitrogen (N2), soybean plants without Bradyrhizobium 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.

How Have Inoculants and Soybean Production Changed?

Historically, the carrier for inoculant has been nonsterile peat powder applied to the seed at planting. Recently, a number of improvements have been made in inoculant manufacturing, including the use of sterile carriers, the addition of stickers, the introduction of liquid carriers, the use of concentrated frozen products, the introduction of new organism strains, the use of preinoculants, and (more recently) the introduction of inoculants with extended biofertilizer and biopesticidal properties.

All of these improvements have resulted in more concentrated products that have a longer shelf life. Most modern products provide anywhere from 500,000 to over 1 million rhizobia cells per seed when used according to manufacturer recommendations. Despite the large numbers of bacterial cells per seed, each nodule may be the product of a different serological group of rhizobia. It is very difficult to replace the indigenous rhizobia population with an introduced strain, even if it is superior in terms of nitrogen fixation efficiency.

This article is an excerpt that originally appeared in Purdue University Extension’s Soybean Production Systems Publication and has been reposted with permission.

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About the Author: Shawn P. Conley