Most industry experts say it is important to inoculate soybeans every year. “It’s commonly believed that if you keep soybeans in your rotation or plant soy on soy that you will have sufficient rhizobia for effective nodulation,” Locke says. “But over time, rhizobia native to the soil actually can become ineffective at nodulation.”

Weather plays a role in rhizobia populations. “In 2009, we had extended soil moisture while we waited to get corn harvested — not a great environment for rhizobia survival,” Maloney says. “Last fall, we had extended periods of dry weather and possibly a negative effect on rhizobia. The best plan is to treat soybeans every year with a quality, aggressive commercial inoculant.”

Will your soybeans produce root nodules if you do not add an inoculant? “If you have grown soybeans only once in a field over the past 10 years, the soybeans will still form root nodules,” Maloney says. “But the bacteria will be lazy and poorly convert nitrogen to the plant.” The longer a strain of bacteria has survived in the soil, the better it gets at surviving without fixing nitrogen. “A commercially applied seed inoculant is bred to form nodules and fix nitrogen,” Maloney says. “That’s what you want.”