Legume plants enrich soil by fixing atmospheric nitrogen on their own.

Farmers knew long before modern microbiology that crops such as peas, beans, and clover could improve nitrogen-poor soil.

Because the effect followed the plant, it was natural to think that legumes themselves were pulling nitrogen directly from the air.

The difficulty is that atmospheric nitrogen is mostly nitrogen gas, N₂. Although it is abundant, the two nitrogen atoms are bound together so tightly that most plants cannot use the gas directly.

In the 1880s, Hermann Hellriegel and Hermann Wilfarth showed that legumes gained nitrogen only when their roots developed characteristic nodules under the right biological conditions. Plants grown without the necessary microbes lost the advantage.

Researchers then isolated bacteria from those nodules. The relationship is a symbiosis: both partners benefit.

The bacteria, especially a group called rhizobia, live inside the root nodules and perform nitrogen fixation. Their enzymes convert atmospheric nitrogen gas into chemical forms that can eventually be incorporated into plant molecules. The plant supplies sugars for energy and provides a protected environment in the nodule.

Calling a bean or clover a “nitrogen-fixing plant” is still useful shorthand. But in the familiar legume system, the actual chemistry of nitrogen fixation is carried out by microbial partners.