Nitrogen-fixing plants span a wide range of ecosystems and uses, from reforestation and agroforestry to ornamental gardening and agriculture. These plants have the unique ability to convert atmospheric nitrogen, which is not directly usable by most plants, into a form that can be assimilated into plant tissues. This process is called nitrogen fixation and is essential for plant growth and overall ecosystem health.
The process of nitrogen fixation involves a symbiotic relationship between certain plants and bacteria. These bacteria, often belonging to the genus Rhizobium, reside within nodules on the roots of the plants. The bacteria possess the necessary enzymes to convert atmospheric nitrogen (N2) into ammonia (NH3), which is then used by the plant for growth and development.
The list of nitrogen-fixing plants for agriculture is quite versatile and includes, among others: Beans: fava (aka faba, broad), alfalfa, green (aka French), runner, field, sweet, peanuts (aka groundnuts), soybeans, cream, black-eyed, or purple-hulled beans, lupins, lentils, cowpeas, chickpeas.
Groups of Nitrogen-Fixing Plants
Legumes: The most well-known group of nitrogen-fixing plants is the legume family (Fabaceae), which includes plants such as beans, peas, lentils, soybeans, alfalfa, and clover. These plants form a symbiotic relationship with Rhizobium bacteria.
Actinorhizal Plants: Another group of nitrogen-fixing plants includes alder, casuarina, and some other woody plants. These plants form symbiotic relationships with bacteria belonging to the genus Frankia.
Benefits of Nitrogen-Fixing Plants
- Soil Enrichment: By fixing nitrogen, these plants enrich the soil, making it more fertile for other plants to grow.
- Sustainable Agriculture: Nitrogen-fixing plants are valuable in sustainable agriculture as they reduce the need for synthetic fertilizers, which can be harmful to the environment.
- Ecological Importance: Nitrogen-fixing plants contribute to the overall health of ecosystems by increasing soil fertility and supporting diverse plant communities.
Nitrogen-Fixing Trees
Black Locust (Robinia pseudoacacia)
- Fast-growing deciduous tree with fragrant white flowers. Tolerates poor soils and is excellent for erosion control.
Alder (Alnus spp.)
- Includes species like red alder and black alder. Ideal for riparian zones, it improves soil fertility in wet or compacted areas.
Acacia (Acacia spp.)
- Thorny or thornless trees/shrubs common in arid regions. Known for improving dry, sandy soils.
Honey Locust (Gleditsia triacanthos)
- A hardy tree often used in agroforestry. It provides dappled shade and is drought-tolerant.
Mimosa (Albizia julibrissin)
- A tropical-looking tree with fern-like leaves and pink, fluffy flowers. Thrives in warm climates.
Indian Beech (Pongamia pinnata)
- Common in tropical areas, it’s used for reforestation and soil improvement.
Leucaena (Leucaena leucocephala)
- A fast-growing tree with small white flowers, widely used in agroforestry and fodder production.
Mesquite (Prosopis spp.)
- Drought-tolerant trees/shrubs found in arid climates. Improve soil in desert regions.
Carob (Ceratonia siliqua)
- Mediterranean tree producing edible pods, ideal for dry and rocky soils.
Gliricidia (Gliricidia sepium)
- Used as a living fence, it fixes nitrogen and provides green manure.
Cassia (Senna spp.)
- Tropical or subtropical trees with showy yellow flowers, used for ornamental and soil restoration purposes.
Calliandra (Calliandra spp.)
- Ornamental shrubs or small trees with feathery red or pink flowers, widely used in agroforestry systems.
Coral Tree (Erythrina spp.)
- Trees with striking red flowers, commonly used as shade trees in tropical areas.
Parkinsonia (Parkinsonia aculeata)
- Known as Palo Verde, a drought-tolerant tree with green bark and small yellow flowers.
Flame Tree (Delonix regia)
- Tropical tree with fiery red-orange flowers and fern-like leaves.
Albizia (Albizia spp.)
- Includes species like silk tree (Albizia julibrissin), known for feathery foliage and nitrogen-fixing abilities.
Moringa (Moringa oleifera)
- A highly nutritious tree with edible leaves and pods, thrives in dry and nutrient-poor soils.
Tamarind (Tamarindus indica)
- A tropical tree producing edible pods, known for its nitrogen-fixing root nodules.
Sesbania grandiflora (Agati)
- A fast-growing tree with edible flowers, commonly grown in tropical regions.
Koa Tree (Acacia koa)
- Native to Hawaii, this hardwood tree supports soil fertility and is prized for its timber.
Australian Blackwood (Acacia melanoxylon)
- A versatile timber tree that thrives in poor soils.
Redbud (Cercis spp.)
- Ornamental trees with pink or purple blossoms, often used in landscaping.
Nitrogen-Fixing Shrubs
Siberian Peashrub (Caragana arborescens)
- Cold-hardy shrub with small yellow flowers. Used in windbreaks and hedgerows.
Sea Buckthorn (Hippophae rhamnoides)
- Shrub with silvery leaves and orange berries. Improves coastal and sandy soils.
Elaeagnus (Elaeagnus spp.)
- Includes species like autumn olive and Russian olive, which thrive in poor soils.
Clover (Trifolium spp.)
- Low-growing legumes with small flowers, often planted as ground cover or forage.
Broom (Cytisus scoparius)
- A hardy shrub with bright yellow flowers, tolerates infertile soils.
Peanut Plant (Arachis pintoi)
- Ground-cover shrub often used in tropical areas to fix nitrogen and suppress weeds.
Buffalo Berry (Shepherdia argentea)
- Silver-leafed shrub with small red berries, drought-tolerant and ideal for poor soils.
Sesbania (Sesbania sesban)
- Tropical shrub used in agroforestry, particularly in wetland areas.
Gorse (Ulex europaeus)
- Thorny shrub with vibrant yellow flowers, thrives on infertile soils.
Guaje (Leucaena esculenta)
- A shrub or small tree from Mexico, valued for its edible seeds and soil improvement.
Mountain Mahogany (Cercocarpus spp.)
- Native to North America, this drought-tolerant shrub stabilizes slopes and enriches soils.
Firecracker Plant (Calliandra californica)
- A shrub with bright red flowers, excellent for desert landscapes.
Cajanus (Cajanus cajan)
- Known as Pigeon Pea, it’s a fast-growing shrub producing edible seeds and enriching soil.
Tephrosia (Tephrosia spp.)
- Shrubs with small flowers, often used in green manure and soil reclamation projects.
Tagasaste (Cytisus proliferus)
- Also called tree lucerne, it’s a fodder shrub and nitrogen-fixer ideal for grazing systems.
Groundnut (Apios americana)
- A climbing shrub with edible tubers and small purple flowers, native to North America.
Rattlebox (Crotalaria spp.)
- Shrubs with bright yellow flowers, used for green manure and soil restoration.
Manuka (Leptospermum scoparium)
- A small shrub with white or pink flowers, known for its medicinal properties and soil improvement.
Nitrogen-Fixing Flowers & Herbaceous Plants
Lupine (Lupinus spp.)
- Pea-shaped flowers in tall spikes; improves soil fertility in poor, sandy areas.
Vetch (Vicia spp.)
- A climbing legume, often used as green manure or ground cover in agricultural fields.
Bird’s Foot Trefoil (Lotus corniculatus)
- A low-growing legume with bright yellow flowers, used in pastures and erosion control.
Alfalfa (Medicago sativa)
- Deep-rooted legume used as forage and a soil improver in crop rotations.
Cowpea (Vigna unguiculata)
- Warm-season legume grown for its seeds, forage, and nitrogen-fixing properties.
Indigo (Indigofera spp.)
- A small leguminous plant historically grown for natural dye production, it also enriches soil.
Crotalaria juncea (Sunn Hemp)
- A fast-growing annual legume used as green manure and cover crop.
Faba Bean (Vicia faba)
- Produces edible beans while fixing nitrogen in the soil, widely grown in temperate regions.
Bush Clover (Lespedeza spp.)
- Herbaceous or shrubby legumes used for soil erosion control and forage.
Creeping Indigo (Indigofera spicata)
- A low-growing legume used as a cover crop in tropical and subtropical areas.
Soybean (Glycine max)
- A major agricultural crop grown for its protein-rich seeds and nitrogen-fixing ability.
Peas (Pisum sativum)
- Annual legume with edible pods or seeds, improves soil fertility in crop rotations.