The Q&A Archives: Inoculate garden beans and peas?

Question: I like treating my bean and pea seed with legume inoculant. Is it pointless to inoculate seed treated with fungicides? Loretta Kelley Washington, DC

Answer: In most cases it's pointless to inoculate your garden beans and peas with rhizobium at all, but not because of the fungicides on the seeds, says Tim Hartz, vegetable crop specialist at the University of California, Davis. Most garden soils are already heavily amended with manures and organic matter. If legumes can get enough nitrogen from mineral sources, they won't bother forming rhizobium nodules, even if the bacterial are present in the soil, Hartz explains. Nodule formation doesn't necessarily mean a dramatic increase in nitrogen fixation. For the inoculant to be effective, the correct strain of rhizobium has to be matched with the correct variety of legume, says Hartz. The general inoculants sold to home gardeners may stimulate nodule formation on legumes, but the amount of nitrogen fixed will vary, depending on the strain of inoculant and variety of legume, he says. It's a shot in the dark to get the right match for high nitrogen fixation and often the result is an insignificant increase in the plant's growth, adds Hartz. There is a place for legume inoculants, however. If you have low fertility soil with low organic matter content or soil that hasn't been gardened for years, treating your legumes with even a general inoculant may help, says Hartz. Rhizobium is a bacterium, not a fungus, so the fungicides on seeds shouldn't harm it. The best way to apply inoculants is to dig a furrow for your seed, sprinkle a band of inoculant along the bottom of the furrow and then plant the seed in the furrow, recommends Hartz. The roots will come into contact with the inoculant as they germinate. To check if the roots are inoculated, pull up a few pea or bean plants and look for nodules forming on their roots. If the nodules are pink inside when you cut them open, the inoculant is working.

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