From Seed to Seed:
Plant Science for K-8 Educators

The legume/bacteria connection. Legumes, such as bean and pea, form relationships with the nitrogen-fixing bacteria that form nodules on the plants' roots. The legumes benefit by the increased availability of nitrogen and the bacteria benefit from access to the carbohydrates manufactured by the plant.

This is a good example of a mutualistic relationship-both organisms benefit, and, as far as we know, neither does harm to the other. It is also a good example of a relationship with a high degree of specificity. Different strains of the nitrogen-fixing Rhizobium bacteria will form relationships with certain legumes, but not others.

Rhizobia inoculants are sold in catalogs and garden centers. By adding inoculant to the soil, students can explore which legumes form relationships with the rhizobium and the differences between plants grown with and without rhizobium.