Nitrogen is the primary soil nutrient that stimulates vegetative growth in plants. It is abundant in the earth's atmosphere, but it must be converted to a soluble form in soils for plants to absorb. One way this is carried out in nature is by microorganisms that live on the roots of many species in the legume, or bean, family of plants, in a process called nitrogen fixation. Incorporating these species in the garden is a natural way to return nitrogen to the soil.
Alternating crops of vegetables with a planting of nitrogen-fixing annuals is a time-tested method to maintain soil fertility. This is often done in the cool weather of fall or winter when fewer vegetables can be grown. Crimson clover (Trifolium incarnatum) is an attractive cool season annual for this purpose. It also provides a stunning flower display when blooming. Alfalfa (Medicago sativa) also prefers cool weather and is one of the heaviest producers of nitrogen. Common green beans (Phaseolus vulgaris) also add smaller quantities of nitrogen to the soil, making them self-sufficient for their own fertilizer needs.
Perennial nitrogen-fixing plants can substitute for fertilizer in lawns, perennial beds and as a groundcover around fruit trees, berry bushes and vines. White clover (Trifolium repens) is a great nitrogen fixer to mix in with lawn seed. It grows in U.S. Department of Agriculture plant hardiness zones 3 through 10 and has small white flowers throughout the growing season. False indigo (Baptisia australis) is a nitrogen-fixing legume for perennial borders. It can be grown in USDA zones 4 to 9 and has showy blue, white or yellow flowers, depending on the variety.
Many common shrubs found in hedges and foundation plantings also return nitrogen to the soil. Silverthorn (Eleagnus pungens) is a fast-growing evergreen shrub for USDA zones 6 to 9. It is tolerant of sun or shade and drought resistant, though it is an invasive species in some areas. California Lilac (Ceanothus spp.) are evergreen shrubs that are one of the few non-leguminous plants that fix nitrogen in the soil. They have fragrant blue blossoms and grow in USDA zones 7 to 10. For cold climates, try the Siberian Pea Shrub (Caragana arborescens), a hardy nitrogen-fixing shrub for USDA zones 2 to 8.
Nitrogen-fixing trees occupy an important niche in the landscape as well. Black Locust (Robinia pseudoacacia) is a leguminous tree native to the eastern United States but grows across most of the country in USDA zones 4 to 8. Like most legumes it is a fast grower, topping out at 60 feet and showering the ground with fragrant white blossoms each spring. Mimosa (Albizia julibrissin) is hardy in USDA zones 6 to 9 and has uniquely broad and flat-topped growth habit. It is a very adaptable and drought tolerant tree with puffy pink flowers in summer. Beware that it is considered invasive in some areas.