Peas, Beans, and ... Bacteria?

By Eve Pranis, June 23, 2008

While researching legumes -- the family of plants that includes peas, beans, and clovers -- Page Keeley's seventh graders in Cooper's Mills, ME, learned that microbes can be magnificent, and they came to appreciate the interdependence of life on Earth.

Through their research, Page's students discovered that certain crops can be beneficial for the soil, because they are able to "fix," or turn into a usable form, nitrogen from the atmosphere. Upon further research, the students found that it wasn't actually the plants, but rather a certain type of bacteria (Rhizobium) that does the work. The bacteria forms bumps or nodules on the plant roots, then convert atmospheric nitrogen into a usable form for the plants. In turn, the bacteria, receive carbohydrates from the roots. Although many types of these bacteria occur naturally in soils, students learned that gardeners sometimes "inoculate" or mix peas, beans, and other legumes with a dried form of the bacteria. "What might happen," wondered the seventh graders, "if we tried growing peas in the classroom in sterile potting mix with and without adding rhizobia bacteria?" They hypothesized, based on their research, that they would observe some evidence of the root nodules formed by the bacteria on the "inoculated" plants. They also decided to grow some pea plants in garden soil to see if naturally occurring rhizobia bacteria would form nodules on those roots as well.

"The students noticed, even before pulling the plants out to check the roots for nodules, that the inoculate pea plants were the healthiest looking," reports Page. "When they pulled them up four to six weeks later, they counted significantly more nodules on the treated plants' roots. And there were definitely more nodules in the pot with garden soil than in the sterile potting mix. This investigation helped students appreciate firsthand the interdependence of living things and the presence of symbiotic microbial connections. It prompted further explorations of how microbes benefit humans, other animals, and plants."

Getting a Fix on Nitrogen

All living things require a constant supply of nitrogen. Although there is a lot of nitrogen in the atmosphere, it's not in a form that most living things can use. Most plants get nitrogen from the breakdown of organic materials in the soil and from fertilizers. Legumes such as peas, beans, and clover typically require less nitrogen from fertilizers because they can form symbiotic relationships with nitrogen-fixing rhizobia bacteria as described above. Consider buying a packet of dried rhizobia bacteria from a garden center or seed catalog and challenging our students to conduct some indoor or outdoor experiments to investigate symbiosis in action. Note: if legumes are growing in soil that is already fertile and rich in nitrogen, you may find little or no evidence of nitrogen-fixing bacteria.

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