Theme: Gardening
for a Sustainable Future
Peas, Beans, and...Bacteria?
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.
Author: Eve Pranis
© 2008
National Gardening Association