These plants help fertilize the soil by extracting atmospheric nitrogen and making this essential natural fertilizer available to surrounding plants through their roots. These plants can be grown right under fruit trees and next to veggies that are "heavy nitrogen feeders", and are used as winter cover crops to prepare the soil for spring planting.
All legumes (beans, peas, and thousands of legume species from small plants to large trees) fix nitrogen.
Other good nitrogen fixers to plant in temperate climates are:
Alfalfa (good human and livestock feed)
Goumi Berry, Autumn Olive, and all plants in the Elaeagnus Family (edible fruits).
Clovers (many medicinal uses)
Lupines (CA native)
Ceonothus (CA Native)
CA Redbud (CA Native
Carob (great over story tree in food forest, drought tolerant, sweet edible pods)