Nitrogen-Fixing Trees

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.

What are nitrogen fixing trees?

Nitrogen fixing trees can help improve soil fertility by fixing nitrogen from the atmosphere. This makes the soil more able to support plants and crops. Nitrogen-fixers are a group of plants and organisms that play a key role in the nitrogen cycle on this planet. They are an important part of any healthy garden ecosystem and are a great way to improve the nitrogen content in your garden’s soil without resorting to chemical fertilisers.

Incorporating nitrogen fixing trees in your garden

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, so if you’re looking for the best nitrogen fixing trees, read on!

Examples of native nitrogen fixing trees


Alder Trees – all Alder trees (including Speckled Alder, Grey Alder, White Alder and other native Alders) have a symbiotic relationship with Frankia Alni, a bacteria known for its nitrogen-fixing properties. It lives in the roots of the Alder trees and helps the tree to absorb nitrogen from the air via the soil. Therefore, the alder is able to improve the soil where it grows and is a useful pioneer species. They can really vary in size from the Green Alder averaging around 5m (16ft) to the Red Alder growing as tall as 30m (100ft).

Buckbrush/California lilac/soap bush/ceanothus – these are all nitrogen-fixing trees/shrubs that are part of the buckthorn family. Similar to the Alder trees, they fix nitrogen in the soil via their relationship with Frankia Alni. The shrubs are between half and 3 metres tall (approx. 1-9ft) but the trees in this genus can grow up to 6 or 7 metres (up to 23ft).

Nettletree/Capulin – part of the Elm family (although recent research suggests it’s actually part of the hemp family of trees), the Nettletree (Trema micrantha) grows up to 10m (33ft) in height and is native to South/Central America and the southern parts of North America. As with other nitrogen fixing trees, they exist in symbiosis with Frankia Alni, which enables them to absorb nitrogen into the soil.


Laburnum Alpinum (Scottish Laburnum) or hybrid, Laburnum X Watereri
In more northern reaches, 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. Note, however, that while it can be useful to provide nitrogen to an edible garden, all parts of the tree are highly poisonous and should never be eaten.

Caragana arborescens

Siberian Pea Tree (Caragana arborescens) - 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.