Plants that put nitrogen back into the soil are called nitrogen-fixing plants, and by growing these in your garden you can reduce the amount of fertilizer you apply. Plants need nitrogen to grow, but they can't use the nitrogen in the air. They rely on soil microorganisms in specialized root nodules to change the nitrogen into a form roots can absorb. Nitrogen-fixing plants include certain trees, shrubs, perennials, vegetables and crops.
The nitrogen the microorganisms produce is the same as is found in many commercial fertilizers.
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Flowering and Shade Trees
Trees that fix nitrogen can also offer flowers or shade in gardens.
Common laburnum (Laburnum anagyroides) features hanging clusters of dense, bright yellow late-spring flowers and dull green or gray-green leaves in groups of three leaflets. Growing 15 to 25 feet tall and wide, this deciduous tree is hardy in U.S. Department of Agriculture plant hardiness zones 5 through 7.
All parts of laburnum are poisonous.
Carob (Ceratonia siliqua) is a drought-tolerant shade tree that grows 30 to 40 feet tall and wide or wider. Hardy in USDA zones 9 through 11, carob is an evergreen. Greenish-yellow blooms appear late fall through early winter, maturing to red and followed by long, green twisting pods. The flowers sometimes exude a musky fragrance.
Carob seed pod pulp is edible and tastes like chocolate.
Nitrogen-fixing shrubs grow well with little need for fertilizer.
Pineapple broom (Cytisus × kewensis), which is hardy in USDA zones 6 through 8, grows 3/4 to 1 foot tall and 1 to 5 feet wide. This deciduous shrub bears white to pale yellow, fragrant late-spring flowers on arching stems.
Rooibos (Aspalathus linearis) flowers in summer, producing yellow blooms. Growing up to 6 feet 7 inches tall, rooibos is hardy in USDA zones 8 through 11.
The nitrogen fixed in certain perennials' roots helps the plant return year after year.
Lupine (Lupinus × hybrida) produces erect spikes of blooms late spring through midsummer in shades of blue, purple, white, red, pink, yellow or bicolors, depending on the variety. Most cultivars grow 3 to 4 feet tall and 1 to 1 1/2 feet wide, but dwarf types grow 1 1/2 to 2 feet tall. Lupine is hardy in USDA zones 4 through 8.
Chinese indigo (Indigofera decora) is a shrubby perennial that offers plentiful light pink blooms 3/4 inch long early through midsummer and intermittently to early fall. Dark green leaflets contrast with the flowers. Chinese indigo grows 1 to 2 1/2 feet tall and 3 to 4 feet wide, and is hardy in USDA zones 5 through 8, sometimes dying back in harsh winters in zone 5 but returning in spring.
Beans and Peas
Beans and peas are legumes, a nitrogen-fixing plant family.
Common bean (Phaseolus vulgaris) is an annual vining or bush vegetable that grows 2 to 15 feet tall and 2 to 3 feet wide, depending on the variety. White, yellow, pink or red flowers are followed by yellow, green or purple bean pods.
Garden pea (Pisum sativum) is a family of annual vegetables that produces common garden peas, and snow, sugar and snap peas, which have edible pods. Pea shoots are also edible. Garden pea grows 1 to 1 1/2 feet tall and 2/3 to 1 foot wide.
Food and Fodder Crops
Nitrogen-fixing food and fodder crops reduce farmers' reliance on artificial fertilizers, which can become pollutants.
Soybean (Glycine max) is an annual crop plant. Blooming mid- through late summer, soybean produces clusters of three to five hairy pods that contain two to four seeds. Soybean grows 2 to 6 feet tall and 2 to 3 feet wide.
Alfalfa (Medicago sativa) is a perennial crop that's hardy in USDA zones 4 through 8 and grows 3 feet 3 inches tall. Purple flowers appear early through midsummer and seeds ripen in late summer. Alfalfa attracts wildlife to the garden.
Soy bean fixes 20 to 275 pounds of nitrogen, and alfalfa fixes 70 to 200 pounds of nitrogen, per acre per year.
Alfalfa leaves are mildly poisonous to humans when eaten in large amounts.