How do beans help corn and squash grow in a milpa field?

In the traditional milpa agricultural system, beans provide essential support to corn and squash primarily through their role in soil enrichment.

Beans act as a natural fertilizer for the other crops by hosting nitrogen-fixing bacteria within their root systems. These bacteria convert atmospheric nitrogen into a form that plants can utilize for development, which helps replenish nitrogen in the soil. This process is particularly beneficial because it reduces the need for synthetic fertilizers, making the milpa system a sustainable method of cultivation.12

Additionally, the corn plants serve as a natural trellis for the beans to climb, while the beans provide the necessary nutrients to support the growth of both the corn and the squash. This symbiotic relationship is a hallmark of the "Three Sisters" planting tradition, where each plant contributes to the health and productivity of the others.1