When soybeans are planted where they haven't been grown before, growers need to inoculate the seed to establish nodulation.
Even then, some are concerned that seed inoculation won't be enough - they add nitrogen to give soybeans a jump-start. But Jay Goos, a North Dakota State University (NDSU) soil scientist, has another suggestion.
He's found that applying soybean inoculant to wheat seed planted the year before the first soybean crop can "pre-establish" the rhizobia in the soil.
"When inoculating soybean seed for planting in soil devoid of rhizobia, you can get good nodulation in the vicinity of the original seed location, but much of the topsoil is still devoid of rhizobia," Goos says. Having pre-established rhizobia in the soil before planting first-year beans increases the amount of nodulation on soybean roots throughout the topsoil, he explains.
In his studies, Goos adapted Brazilian research in which rice or wheat seed was sown with soybean inoculant to help pre-establish soybean rhizobia in the soil.
"The wheat was attractive to us because both wheat and soybeans are predominantly solid-seeded in North Dakota. If you can obtain some modest growth and proliferation of the bacteria after inoculation of wheat, subsequent tillage will help distribute the bacteria throughout the topsoil," he says.
Goos conducted two studies. In the greenhouse, he grew spring wheat with and without treating it with commercial soybean inoculant at the same rate recommended for soybean seed.
After wheat harvest, an estimated 9,000 soybean rhizobia per gram of soil were found where wheat seed had been inoculated. No soybean rhizobia were found in uninoculated soils.
Field studies were conducted in 1999. Goos and Pat Carr, NDSU Dickinson Research Extension Center agronomist, planted wheat, with and without inoculation, in a soil devoid of soybean rhizobia. After harvest of the wheat, soil samples were taken and used to grow soybean plants in the greenhouse.
Soybeans grown in uninoculated wheat ground produced one nodule per soybean plant by the second trifoliate stage, possibly due to contamination. Soil taken from around the crowns of previously inoculated wheat plants produced almost 150 nodules per plant, Goos says.
Soybean seed should be inoculated when planted in new soil, even when rhizobia have been pre-established with the previous wheat crop, he adds. "That would be just to make sure that there were plenty of rhizobia in the soil from the very first year.
"And inoculation in itself is not an expensive proposition. It's a buck or two bucks an acre. If a farmer put on 40 lbs of nitrogen, that would cost about $8/acre."
Inoculating wheat didn't hurt or help that crop, Goos adds. "I did find one paper from India that said that soybean rhizobia would help the wheat, but I never observed that."