NEWS for North Dakotans
Agriculture Communication, North Dakota State University
7 Morrill Hall, Fargo, ND 58105-5665
October 14, 1999
North Dakota farmers planning to add soybeans to their crop mix for the first time can benefit by inoculating wheat seed with soybean rhizobia the year before seeding soybeans, reports a North Dakota State University soil scientist.
Soybeans are a new crop for many North Dakota growers and are being introduced into areas where there is no history of soybean production.
"Whenever a legume is introduced into a new area, farmers are concerned if seed inoculation the first year will provide for adequate nodulation," says Jay Goos, professor of soil science at NDSU. Soybeans may need additional nitrogen fertilizer if soil nitrate levels are low and there is no previous history of soybeans, he says. Inoculation alone is sometimes inadequate the first time soybeans are grown.
Goos searched the literature to see how scientists in other parts of the world approached this problem. He found that in Brazil, where soybeans were also introduced into totally new areas, agronomists had successfully established adequate numbers of soybean rhizobia in the soil by inoculating rice or wheat seed the year before soybeans were to be grown.
To test the idea, Goos went to the Dickinson area, an area with no history of soybeans. "We know that soybeans aren't adapted to the dry conditions around Dickinson, but we wanted soil with no soybean rhizobia," he says. Goos and Pat Carr, agronomist at NDSU's Dickinson Research Extension Center, performed both greenhouse and field experiments to test the theory.
"The Brazilian idea worked far better than we could have guessed," Goos says.
The researchers grew spring wheat with and without treating the seed with commercial soybean inoculant at the same rate recommended for soybean seed. In the greenhouse after wheat harvest, they estimated about 9,000 soybean rhizobia per gram of soil where the seed had been inoculated, versus fewer than 100 per gram where the soil was inoculated and left fallow. No soybean rhizobia were found in uninoculated soils.
Goos and Carr collected soil samples after wheat harvest and grew soybean plants in the greenhouse. "Without inoculation of the wheat, there was one nodule per soybean plant, probably due to stray contamination," says Goos. "When we grew soybeans on soil that came from wheat that had been inoculated the previous spring we had almost 150 nodules per plant--the roots were literally covered with nodules. It was clear that inoculation of the wheat led to establishment of soybean rhizobia in the soil."
Inoculation of wheat seed the year before growing soybeans would have an additional benefit, he says. Tillage would distribute the rhizobia in a larger mass of soil than would be the case with inoculating soybean seed alone. Inoculating wheat seed the year before growing soybeans, combined with soybean seed inoculation, could provide a high degree of nodulation the first time soybeans are grown on a field, says Goos.
Goos stresses that this idea would be of little value in fields where there is a history of growing soybeans. He also stresses that inoculating wheat seed with soybean rhizobia has no observable benefit for the wheat crop.
"It seems that the soybean rhizobia multiplied to a modest degree in the wheat root zone as a free living soil bacteria--there is no specific relationship between wheat and soybean rhizobia," he says.
Will this idea work for other legumes, like alfalfa or dry beans? "We have only a small amount of data with dry beans and dry bean rhizobia, and we suspect that the same principles will apply," Goos says.
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Source: Jay Goos (701) 231-8581
Editor: Gary Moran (701) 231-7865