Legumes & Rhizobia

Legumes like beans can extract nitrogen from the atmosphere


By: Sandy Feather ©2010
Penn State Extension

Q. I grew my first vegetable garden last summer, and things grew pretty well. My question is about the inoculant that the catalogs recommend using with beans. What does it do, and how important is it to use? I prefer to garden organically if possible.

A. Beans are classified as legumes. Legumes are plants that have the unique ability to use nitrogen directly from the atmosphere to fuel their growth. This is possible because they have a symbiotic (mutually beneficial) relationship with rhizobia bacteria. These soil-dwelling bacteria infect the roots of leguminous plants to form nodules. The bacteria lack chlorophyll and are therefore unable to manufacture their own food via photosynthesis.

However, the bacteria are able to break atmospheric nitrogen down into a form immediately usable by the plants. In return, the plants provide the carbohydrates and sugars the bacteria need to live.


Leguminosae is a huge family that includes many familiar plants:

  • All types of beans, peas, clovers and lentils

  • Flowers such as baptisia, hyacinth bean, and sweet pea

  • Trees such as black locust, honeylocust, Kentucky coffee tree and redbud

  • If you stop and look at the flowers and fruits produced by these different plants, you can see the similarities. The fruits are usually flattened (sometimes rounded) pods that contain individual seeds.

Legume Inoculant

The inoculant recommended for use with beans and peas ensures that the appropriate strain of rhizobia bacteria is present in your garden soil. Although there is a specific strain of rhizobia that is appropriate for each crop, many of the inoculants you purchase and apply are combination products that will work for both beans and peas.


Although your beans would still grow and produce without the inoculant, your yields will be higher and your plants more vigorous if you use it. Inoculant is nontoxic and compatible with organic production. Remember that bacteria are living organisms and that the inoculant has a shelf life of roughly a single growing season.


Vegetable gardens

Plant preferences

Proper watering