Nitrogen fixing plants, or nitrogen fixers, are plants that take nitrogen from the atmosphere and introduce it to the soil. This is a great way to add nitrogen to the soil without using artificial fertilisers.
Nitrogen is one of the three most important nutrients for growing crops, the others being potassium and phosphorous. Most crops absorb nitrogen from the soil, and use it as a main component of chlorophyll. The plants need chlorophyll to produce energy from sunlight, to produce sugars from water and carbon dioxide, and to produce proteins.
How Nitrogen Fixers Add Nitrogen To The Soil
Nitrogen fixing plants create a symbiotic relationship with soil bacteria that is mutually beneficial to the plant and the bacteria. Soil bacteria infect the plant’s root system, and make it grow nodules where the bacteria live. The bacteria take nitrogen from the atmosphere and fix it within the root nodules. The host plant then uses some of this nitrogen to grow, while unused nitrogen remains in the root nodules. In return, the plant supplies carbohydrates for the bacteria to feed on.
When the plant dies, the bacteria return to the soil where they will remain until they can infect another plant. Use the plant itself as a green manure, returning the nitrogen stored in the root nodules into the soil. The increased nitrogen content of the soil acts as a fertiliser for the next crop to be planted.
Nitrogen Fixing Plant, Shrub And Tree Varieties
There are a number of plants, shrubs and trees that fix nitrogen. Nitrogen fixing shrubs and trees tend to remain in the ground long-term, fixing nitrogen in the soil for nearby plants to use. This makes nitrogen fixing trees very useful when growing food using permaculture principles. Nitrogen fixing plants, however, are more commonly seen as annual crops which are used as a green manure to return nitrogen to the soil.
Nitrogen Fixing Plants
The most common nitrogen fixing plants used in the UK are legumes. These are often edible crops such as peas and beans. The nitrogen fixing legumes are grown prior to a crop that requires soil with a high nitrogen content. The root system of the legumes is left in the soil and the nitrogen will be absorbed by the next crop. Alternatively, include nitrogen fixing legumes as part of a permaculture food producing garden.
Some examples of nitrogen fixing plants are:
- French Beans
- Green Beans
Nitrogen Fixing Shrubs And Bushes
Shrubs can grow over many years, fixing nitrogen in the soil to be absorbed by nearby plants. There are a number of nitrogen fixing shrubs that also produce food, or attract wildlife, making them very beneficial. For example, Gorse flowers attract pollinating insects, while Sea Buckthorn produces edible berries. Some examples of nitrogen fixing shrubs are:
- Autumn Olive
- Russian Olive
- Sea Buckthorn
Nitrogen Fixing Trees
Similarly to shrubs, nitrogen fixing trees can for part of a permaculture garden, providing nitrogen for other plants nearby. Trees can fill in the canopy layer, providing shade to the garden, and vertical structure for some climbing plants. Some trees, such as Alder, can be coppiced. This means they can be cut down every few years and will re-grow multiple new stems. Coppicing can provide a useful source of wood while maintaining the health of the tree. Examples of nitrogen fixing trees are:
- Scottish Laburnum