Nitrogen is the most common element in the earth’s atmosphere and is also an essential element for plant growth that is found in the soil. Non-organic gardeners often add nitrogen rich chemical fertilisers to the soil to aid leafy growth. Organic gardeners, on the other hand, go for more natural methods to maintain soil health and improve plant health and yield. Nitrogen fixing plants are a group of plants (and organisms they work with) that play a key role in the nitrogen cycle on this planet. They are an important part of any healthy garden ecosystem.
Why Plant Nitrogen Fixing Plants in Your Garden?
Nitrogen is, along with phosphorus and potassium, one of three key nutrients that plants need in order to grow. Nitrogen, like carbon, has its own natural cycle. Understanding and making use of that cycle can be key to success in organic polytunnel gardening.
The most important elements of the nitrogen cycles are the bacteria and fungi which give us a helping hand beneath the soil surface. While we may refer to several plants as ‘nitrogen fixers’ – – it is not the plants themselves that transfer the atmospheric nitrogen to the soil but nitrogen-fixing bacteria living in the root nodules of those plants. Placing nitrogen-fixers into the soil will allow those nitrogen-fixing bacteria to thrive and to do their jobs.
Nitrogen fixing plants are used in order to build soil fertility. Sometimes, these plants will be part of our year round growing and eating efforts. Some are permanent additions to a garden ecosystem. At other times, a cover crop may be planted which will be chopped and dropped to feed the soil and protect the soil ecosystem in your polytunnel. Planting a cover crop that fixes nitrogen can be particularly beneficial where you will later by growing leafy, nitrogen-hungry crops.
How Nitrogen Fixing Works
Bacteria living in the root nodules of nitrogen fixing plants help transfer atmospheric nitrogen to the soil, where it can be taken up by other plants,or used by the plants themselves. These form a part of the natural nitrogen cycle on our planet.
Other nitrogen fixing bacteria present in soil also fix nitrogen from the air. Meanwhile, bacteria are also amongst those organisms which help to break down rotting plant and animal waste material in a process known as ammonification. Nitrifying bacteria take the Ammonium created by nitrogen fixing and decomposing bacteria and turn it into Nitrites and then to Nitrates – the form in which nitrogen can be taken up by plants. Denitrifying bacteria complete the cycle by returning nitrates to the air in the form of atmospheric nitrogen.
Plants that are known as nitrogen fixers have developed mutually beneficial relationships with bacteria which live in their root nodules. The plant gives the bacteria carbohydrates obtained from photosynthesis and in return, the bacteria provide fixed nitrogen to the host plant.
Which Bacteria are Involved in Nitrogen Fixing?
There are two main groups of microbes that plants partner with in order to use atmospheric nitrogen to fuel their growth. These are Frankia, a type of Actinobacteria, and Rhizobium or Bradyrhizobium bacteria. The former bacteria form relationships with plants such as those in the Elaeagnaceae, Rosaceae and Rhamnaceae families.The latter work predominantly with legumes, and are by far the most important nitrogen fixing bacteria.
What Nitrogen Fixing Plants Should Be Considered for the UK?
There are a wide range of nitrogen fixing plants that could be beneficial additions to UK gardens. Let’s take a look at some of the plants UK gardeners could consider:
Nitrogen Fixing Trees
If you have a medium to large garden then you will have enough space to incorporate some trees into your garden design. Trees can help you to make the most of the space in your garden through vertical growth, can provide shade and other yields and can generally improve the look and feel of your garden. Above and beyond all that, some trees also act as nitrogen-fixers – taking nitrogen from the air and, through a number of complex mechanisms, making it available in the soil for the use of other nearby plants. These nitrogen-fixing trees could be a key component in a food forest or edible garden in the UK:
Alder (Alnus glutinosa)
Alder has a symbiotic relationship with a nitrogen-fixing bacterium called Frankia alni, which is found in its root nodules. Therefore, the alder is able to improve the soil where it grows and is a useful pioneer species. Other alders, such as the Italian Alder (Alnus cordata) and other Alder species, though non-native, may also be useful nitrogen fixers in gardens in some parts of the UK.
Laburnum Alpinum (Scottish Laburnum) or hybrid, Laburnum X Watereri
In more northern reaches of the UK, this is the largest nitrogen fixing tree that you are likely to see. Well known Scottish forest gardens, for example, use these colourful yellow trees to provide a constant stream of cuttings which will distribute nitrogen around the whole system.
Siberian Pea Tree (Caragana arborescens)
While it is non-native, this small nitrogen-fixing tree has proved useful for temperature climate forest gardens. It has an extensive root system and edible seeds (which should be cooked before they are eaten) which are said to resemble lentils. The flowers are also edible and can be used in salads.
Nitrogen Fixing Shrubs
In the UK, the shrub layer of an edible forest garden contains a number of useful nitrogen fixing plants. Here are some of the useful nitrogen fixing shrubs for use in UK gardens:
There are a number of useful shrubs in this group which can provide an edible yield as well as fixing nitrogen. Amongst these are elaeagnus umbellata (autumn olive), elaeagnus angustifolia (Russian olive), Elaeagnus multiflora (goumi) and garden hybrid Elaeagnus x ebbingei. All have the potential to provide edible berries and seeds and can thrive in a wide range of conditions, though they do not always fruit in the UK. Even where they do not fruit, they are still useful in providing nitrogen to neighbouring plants and can increase the yields of fruit trees planted nearby.
Gorse (Ulex europaeus)
As a nitrogen fixer it can be of benefit to plants growing nearby and both the flowers and the wood have a number of uses. The bright yellow flowers have a pleasant scent that is something like coconut with a hint of vanilla and are used for toiletries, scented candles and dyes. The wood is good for use as kindling and the ashes are rich in potassium, are a good source of fertiliser and were traditionally used to make soap.
Broom (Cytisus scoparius)
Broom is another yellow flowered, nitrogen-fixing shrub that can be useful, especially in wildlife gardens. It can tolerate light shade, dry soils, maritime conditions and atmospheric pollution and is wonderful at attracting wildlife. Broom can be a good choice to improve soil conditions where these are currently poor. Traditionally, broom’s wood, foliage and flowers have had a number of uses, including for making brooms, baskets, dye, fibres and paper.
Sea Buckthorn ( Hippophae rhamnoides – L. )
Sea buckthorn is another shrub that can fix nitrogen. It can survive and thrive in a range of conditions but needs full sun to grow. It will do well on the sunny fringe of a woodland garden. These shrubs also produce an edible yield, providing berries that are very acidic when raw but full of vitamin C and good for use in juices when combined with other fruits.
Nitrogen Fixing Edible Crops
Most of the nitrogen fixers that are most familiar to gardeners in the UK are found in the herbaceous layer of edible gardens. Traditional edible legumes help to fix nitrogen as part of a crop rotation system for annual beds and are the most well known of all the plants that are involved in this way in the nitrogen cycle.
Edible Peas and Beans (Various)
There are plenty of edible legumes that we are used to seeing in gardens in the UK. Garden peas, broad beans, French or green beans and runner beans are all common examples. Broad beans can be particularly useful as they can be grown in many locations all across the UK and can cope with a little light shade. These crops can be sown the year prior to plants that require a lot of nitrogen and the roots left in place. Alternatively, these legumes can be used as integral parts of annual polycultures or mixed cottage garden style beds.
Nitrogen Fixing Herbaceous/ Ground Cover Plants
There are also plenty of other herbaceous nitrogen fixing plants that can find a place in UK gardens. Some good options include:
A range of different lupins can be useful ornamental nitrogen fixers and can also become a useful part of a wildlife garden’s ecosystem. These grand, annual flower stalks create beauty and drama in the garden as well as helping nearby plants.
Everlasting Sweet Pea (Lathyrus Latifolius)
Unlike traditional annual sweet peas, this nitrogen fixing flower is a perennial and is not frost tender. It can do well climbing up amongst, for example, raspberry canes and could be a great addition to mixed edible borders.
Wood Vetch (Vicia sylvatica) (and other vetches)
A number of vetches, which are in the fabaceae (legume) family are useful for providing lower layers in an edible forest garden system. Wood vetch is useful in that is is a woodland plant and so can cope well with the shade cast by trees and shrubs.
Clovers make a useful nitrogen -fixing ground cover for layered edible gardens and can also be useful for use as a green manure to renew nitrogen levels in annual vegetable beds. Both red and white clover can be beneficial when growing under apple trees, while white clover is particularly useful for attracting wildlife.
Do you have nitrogen fixing plants in your garden? Share details of how and where you have planted them, and your results, in the comments below.
Elizabeth Waddington is a writer and green living consultant living in Scotland. Permaculture and sustainability are at the heart of everything she does, from designing gardens and farms around the world, to inspiring and facilitating positive change for small companies and individuals.
She also works on her own property, where she grows fruit and vegetables, keeps chickens and is working on the eco-renovation of an old stone barn.