Nitrogen Fixation
How it Works and a Look at Some Super Nitrogen Fixing Trees, Shrubs and Herbs
An essential component of any regenerative landscape will be the Nitrogen-fixing perennial plants within the community of fruits, nuts and herbs and other plants.
During this post, we will look at why Nitrogen is important for plants, how Nitrogen can be biologically sourced and we’ll profile some of our favourite Nitrogen-fixing trees, shrubs and herbs.
Nitrogen is an essential nutrient for plant growth and development and although around 78% of the earth’s atmosphere is nitrogen, plants cannot utilise this. Plants instead depend upon combined or fixed forms of nitrogen, such as ammonia and nitrate. Currently, the majority of this nitrogen is provided to cropping systems in the form of industrially produced nitrogen fertilisers. The use of these fertilisers has led to worldwide ecological problems, such as the formation of coastal dead zones, and requires a high energy input to produce. Biological nitrogen fixation, on the other hand, offers a natural means of providing nitrogen for plants.
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| Legume (aka Pulse Crop) in association with Rhizobium bacteria. |
Biological Nitrogen fixation is an important component of regenerative agriculture,organic gardening/farming, forest gardening, and other polyculture practices. Through a partnership with micro-organisms in their roots, some plants can turn atmospheric nitrogen into nitrogen fertilisers useful to themselves but also becoming available to their neighbours over time through root dieback, leaf fall, and chop and drop pruning. These are known as the nitrogen-fixing plants.
This is a mutually beneficial relationship with the plant providing carbohydrates obtained from photosynthesis to the microorganism and in exchange for these carbon sources, the microbes provide fixed nitrogen to the host plant. While it does not replace the need to bring in other nutrients depleted by harvests such as phosphorus and calcium, nitrogen fixation provides a valuable biological source of an essential fertiliser.
There are two main groups of microbes that plants associate with in order to utilise the atmospheric nitrogen to fuel growth. They are Frankia and Rhizobium.
Frankia
Many plants partner with micro-organisms called Frankia, a group of Actinobacteria. These plants are known as the actinorhizal nitrogen fixers.
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| Frankia can be seen above as the nodules forming around the roots of one of our Elaeagnus umbellata saplings in our nursery. |
- Betulaceae, the birch family.
- Myricaceae, the bayberry family.
- Casuarinaceae, the Austrian “pines”.
- Elaeagnaceae, the oleasters.
- Rosaceae, the rose family.
- Rhamnaceae, the buckthorn family.
These plants tend to thrive in nitrogen-poor environments and are often the pioneer species in plant communities playing an important role in plant succession.
Rhizobium
By far the most important nitrogen-fixing symbiotic associations are the relationships between legumes (plants in the family Fabaceae) and Rhizobium and Bradyrhizobium bacteria. These plants are commonly used in agricultural systems such as alfalfa, beans, clover, cowpeas, lupines, peanut, soybean, and vetches.
The Rhizobium or Bradyrhizobium bacteria colonize the host plant’s root system and cause the roots to form nodules to house the bacteria. The bacteria then begin to fix the nitrogen required by the plant. Access to the fixed nitrogen allows the plant to produce leaves fortified with nitrogen that can be recycled throughout the plant. This allows the plant to increase photosynthetic capacity, which in turn yields nitrogen-rich seed.
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| Rhizobium colonies clearly seen as nodules on the plant rootsof Spartium junceum |
So now you know what nitrogen fixation is, lets take a look at some of my favorite Nitrogen Fixing Trees – Shrubs and Herbs that we use throughout our polyculture gardens.
Nitrogen Fixing Trees
Italian Alder – Alnus cordata
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| Alnus cordata – Italina Alder – Nitrogen Fixing Tree |
Black Locust – Robinia pseudoacacia
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| Robinia pseudoacacia – Black Locust |
Overview: Robinia pseudoacacia – Black Locust is a rapidly growing, deciduous tree that is native to North America. This member of the pea and bean family (Leguminosae) has now naturalised in many parts of Europe. It is in flower in June, and the seeds ripen from November to March. The species is hermaphrodite. It’s a good tree for establishing on degraded land but can become invasive due to its prolific seed production, and it also spreads by suckering from the roots. Prefers well drained soil and sunny positions.
Sea Buckthorn – Hippophae rhamnoides
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| Hippophae rhamnoides – Sea Buckthorn |
Nitrogen Fixing Shrubs
Elaeagnus umbellata – Autumn Olive
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| Elaeagnus umbellata – Autumn Olive |
Overview: Elaeagnus umbellata – Autumn Olive – A large deciduous shrub from E.Asia, growing 4.5 m high and 4.5 m wide, tolerates part shade, very drought tolerant. Branches are often thorny, leaves are bright green and silvery beneath. Yellowish white, fragrant flowers are produced in May-June, followed by rounded silvery brown (ripening red) fruits in Sep-Oct. Sometimes cultivated for its edible fruit. There are many named cultivars. Flowers are rich nectar and very aromatic. Plants can fruit in 5 yrs from seed. This specie is considered invasive in the U.S.
Uses: Hedging plant, ornamental and tolerates maritime exposure succeeding in the most exposed positions. The plant is used as a nurse tree, when planted with fruit trees it is reported to increase the overall yield of the orchard by 10%. It can also be grown as a biomass crop on a 3 year rotation.
Nitrogen Fixing Potential: The species is classified by USDA as being a MEDIUM nitrogen fixer with estimated yields of 85-160lbs/acre or 39-72kg/4050m² or 0.014g /m2.
Propagation: Seed – best sown as soon as it is ripe in a cold frame. It should germinate in late winter or early spring, though it may take 18 months. Stored seed can be very slow to germinate, often taking more than 18 months. A warm stratification for 4 weeks followed by 12 weeks cold stratification can help. The seed usually (eventually) germinates quite well. Prick out the seedlings into individual pot as soon as they are large enough to handle and plant out when they are at least 15 cm tall.
Caragana arborescens – Siberian Pea Tree
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| Caragana arborescens – Siberian Pea Tree |
Overview: Caragana arborescens – Siberian Pea Tree – A deciduous shrub originating from Central Asia belonging to the Fabaceae (legume) family growing to 5-6m high and 4m wide with an upright habit. It grows vigorously. Flowers are borne from buds on the previous year’s wood and are typical of flowers from this family. Flowering occurs in May. Pollination is via bees, usually wild bumble bees. Pods develop from flowers – looking like small pea pods, they are 4-5 cm long. The pods ripen to amber or brown from June -July onwards and seeds fall by August. The plant is extremely hardy tolerating winter temperatures of -40. Prefers a continental climate with hot dry summers and cold winters.
Uses: Windbreaks and shelter belts, wildlife-erosion control plantings, Extensive root system that stabalizes the soil. Plants make good wildlife fodder and can be used to as poultry food. A fiber is obtained from the bark and used for rope making.
Nitrogen Fixing Potential: The species is classified by USDA as being a MEDIUM Nitrogen fixer with estimated yields of 85-160lbs/acre or 39-72kg/4050m² or 0.014g /m2
Biodiversity – The shrubs will begin to flower in the 4th or 5th year after planting and are attractive to a wide range of pollen and nectar feeding invertebrates from Apil – May.
In time as the hedge thickens up with regular pruning, suitable nesting habitat will form inside the lower part of the hedge. Birds such as Wren – Troglodytes troglodytes, Chiffchaff – Phylloscopus collybita and Robin – Erithacus rubecula are commonly found in dense low hedging. These birds can help to keep common vegetable pest populations low.
Cytisus scoparius – Broom
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| Cytisus scoparius – Broom |
Overview: Cytisus scoparius – Broom A hardy Nitrogen fixing shrub native to Europe growing to 2.4 m by 1 m at a fast rate. Its bright yellow flowers appear in spring, from May to June and attract a range of invertebrates. A versatile plant well suited to many soil types that can grow in nutritionally poor soil. Prefers a sunny position but tolerates some shade and will succeed in exposed conditions including maritime exposure. A deep root system means they are very drought tolerant once established and grow well on dry banks. Very tolerant of cutting, it regenerates quickly from the base.
Uses: Ornamental, Maritime exposure tolerated – stabalizes sand dunes. Urban plant – tolerates atmospheric pollution. An essential oil from the flowers is used in perfumery. Great plants for fiber, basketry and good brooms (hence the commom name!). You can either place stems on the ground and go over them with a rotary lawn mower to break the biomass into smaller pieces or leave as rough mulch.
Nitrogen Fixing Potential: The species is classified by USDA as being a HIGH Nitrogen fixer with estimated yields of +160lbs/acre or +72kg/4050m² or 0.018g /m2.
Elaeagnus angustfolia – Oleaster, Russian Olive
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| Elaeagnus angustifolia – Russian Olive |
Overview: Elaeagnus angustifolia – Russian Olive – A deciduous large shrub or small tree from Europe and W.Asia, growing approx 7m high and 7m wide. Tolerates part shade, salt and air pollution. It has silvery branches often thorny, with silvery scales when young, silvery willow-like leaves, silvery flowers in June and yellowish-silvery fruits ripening in October. Plants prefer continental climate. This species is often cultivated in Europe and Asia for its edible fruits (there are many named varieties some of which are thorn less). The plants begin to flower and fruit from three years old. It is very tolerant of pruning even right back into old wood. The flowers are sweetly scented. Fruits hang on the plant for much of the winter providing a valuable source of winter food for birds. The fruit is readily eaten and disseminated by many species of birds. This species is considered invasive in the United States.
Uses: Hedging plant (NB does not form a dense screen), biomass crop,tolerates maritime exposure. Edible fruit -raw or cooked as a seasoning in soups, and the expected fruit yields are 7-9kg per plant. The taste is dry sweet and mealy. The seed oil, flowers and leaves are used medicinally. An essential oil obtained from the flowers is used in perfumery. Leaves are used as goat and sheep fodder. The wood is hard, fine-grained and used for posts, beams, carving, domestic items and makes good fuel.
Nitrogen Fixing Potential: This specie is classified by USDA as being a HIGH nitrogen fixer with estimated yields of 160+ lbs/acre or 72>kg/4050m²
Biodiversity: When in flower the plants are attractive to a range of pollinators and I’ve often observed our plants teeming with flying insects during the flowering period. When trimmed the plants will ramify well and can form a dense hedge-like appearance. The interior of the plants in this condition is perfect for nesting birds and for small mammals and lizards to retreat into when under threat.
Propagation: Establishment and reproduction of Elaeagnus angustifolia is primarily by seed, although some spread by vegetative propagation also occurs. Cold stratification of the seed is required for 30-60 days. Sowing when fruits ripe will probably provide the best germination results.
Nitrogen Fixing Herbs
Trifolium repens -White Clover
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| Trifolium repens – White Clover |
Overview: Trifolium repens – White Clover – White clover is a dwarf, prostrate, mat-forming perennial that can spread via stems which freely root along the ground at the nodes. Easily grown in average, medium, well-drained soils in full sun to part shade. Prefers moist soils in light shade, but tolerates full sun and moderately dry soils.Uses: White clover has been described as the most important forage legume of the temperate zones. Besides making an excellent forage crop for livestock, clovers are a valuable survival food: they are high in proteins and although not easy for humans to digest raw, this is easily fixed by boiling the harvested plants for 5–10 minutes. Dried flower heads and seedpods can also be ground up into a nutritious flour and mixed with other foods, or can be steeped into an herbal tea. The plants ability to spreads aggressively by creeping stems makes is a good ground cover plant. The plant is also used as a companion plant when undersown with cereals or tomatoes.Nitrogen Fixing Potential: The species is classified by USDA as being a HIGH Nitrogen fixer with estimated yields of +160lbs/acre or +72kg/4050m² or 0.018g /m2.
Other sources state up to 545 kg of N per hectare per year is possible.
Biodiversity: The plants provide a source of nectar and pollen for a number of native bees as well as the honey bee.
Propagation: Best propagated by seed. Pre-soak the seed for 12 hours in warm water and then sow in spring in situ. Division is also possible in the spring and autumn.
Planting Material: For covering an area quickly seed is the best option.
Onobrychis viciifolia – Sainfoin
Uses: Green manure, soil stabalizer (due to deep tap root),companion plant. Grown for pasture, hay or silage since it is very palatable to livestock.
Nitrogen Fixing Potential: The species is classified by USDA as being a MEDIUM Nitrogen fixer with estimated yields of 85-160lbs/acre or 39-72kg/4050m² or 0.014g /m2
Biodiversity: Sainfoin produces Nectar and Pollen for Honeybees and Bumblebees. This nectar is thought to be one of the highest yielding honey plants. The flowers of Sainfoin attract huge numbers of insects and some reports clain Sainfoin may attract up to ten times more bees than white clover. . Deer enjoy sainfoin, and some game birds such as turkey and pheasants shelter among the plant.
Propagation: For the best results pre-soak seed for 12 hours in warm water and sow in situ in the spring
hi
Can you please help finding a nitrogen-fixation plants here in Egypt. All the pirior plants in the article do not exist in Egypt. or maybe exist in another family!
Do you have an idea of certain nitrogen fixation plants in this climat?
Thank you
ALia
Common bean (Phaseolus vulgaris), Berseem clover (Trifolium alexanderinum) and lentil (Lens culinaris) have all proven to be Nitrogen fixers in Egypt, with the bean providing the best results. I hope this helps.
it might be an idea to use simple google search:
1. nitrogen-fixation
2. since Egypt can be desert like climate -> desert
3. researchgate
4. dowload or .pdf
hopefully you will find your answer(s)