A Look at Legumes: Rhizobia and Root Nodules

Legumes have long been known to be good for your garden by fixing nitrogen and improving soil fertility. These legumes come in the form of common peas and beans, as well as cover crops that act as green manure in the off season. But how exactly do they “fix” nitrogen in the soil, and what does that mean anyway?

The next time you pull up a pea or a bean plant, take a closer look at its roots. Those small white sacs are known as nodules, and they form on the roots of the legume family (Fabaceae).

A strain of bacteria called rhizobia lives in the soil year-round, but when specific plants are present — in this case, legumes — the rhizobia establish a symbiotic relationship with them. Legumes secrete compounds called flavonoids from their roots, which in turn trigger the secretion of nod factors in the rhizobia. Coming full circle, the nod factors spark a reaction in the legumes, causing the roots to swell and form the nodules you see here. It is within these nodules that rhizobia live in harmony with their host plant.

In a naturally occurring process called nitrogen fixation, the rhizobia pull (or “fix”) nitrogen gas from the atmosphere and convert it into ammonium compounds, which provide essential nutrients for the plant.

When the host plant dies, the fixed nitrogen is released into the soil and made available for other plants to use. Even though there is abundant nitrogen in the air, it is present in a form that cannot be used directly by plants; therefore nitrogen fixation is an important process in the life cycle of legumes, which enrich rather than deplete the earth.

Because of this symbiosis, rhizobia always require host plants and cannot fix nitrogen independently. They can live in the soil for some time without food, but to sustain soil health, it’s important to “feed” the bacteria by rotating symbiotic crops such as peas, beans, clover, and alfalfa through different beds every season.

If you grow legumes in your garden year after year (even if you simply sow cover crops), you’ll help the rhizobia population thrive and improve soil fertility without ever using fertilizer!