Nitrogen-Fixing Trees

What are nitrogen-fixing trees?

Every plant needs nitrogen to grow, yet almost none can use the vast supply of it in the air. Nitrogen-fixing trees solve this quietly, through a partnership millions of years old. In small nodules on their roots they host bacteria that take nitrogen straight from the atmosphere and turn it into forms the plant can absorb. In exchange the tree feeds the bacteria sugars. As leaves, roots and pruned branches from these trees decompose, the captured nitrogen enters the soil — and the plants growing nearby.

This is why nitrogen-fixing trees are often called support species or fertility trees. They do for free, and continuously, what a bag of synthetic fertiliser does once, at a cost, and with a heavy industrial footprint. A well-designed planting can build its own soil fertility from the air and the sun.

Among the best-known are the legumes — Robinia (black locust), acacias, the tree lucerne (tagasaste), and many shrubs — alongside non-legumes such as alder and sea buckthorn, which partner with a different bacterium called Frankia.

How Terra Humana understands them

For Terra Humana, nitrogen-fixing trees are the clearest proof that restoration and abundance need not depend on external inputs. A degraded Mediterranean hillside, poor and sun-baked, can begin to rebuild its own fertility the moment the right support species take root. They embody a principle at the heart of our work: that healthy land is a partnership, not a supply chain.

They also matter economically. Fertiliser is one of the largest recurring costs — and largest environmental burdens — of conventional growing. Land that fertilises itself is land that can heal and feed people without depending on volatile global markets.

Nitrogen-fixing trees in practice

At Sa Medada, near Jerzu in Sardinia, support species are planted alongside the fruit, nut and timber trees of Project Eden rather than in separate plots. As they grow they are pruned back, and the cut material is left on the ground as mulch — releasing nitrogen exactly where the neighbouring trees can reach it. This "chop and drop" rhythm turns fast-growing support trees into a renewable source of fertility.

The approach is drawn from syntropic agriculture and permaculture, where support species are not a side note but the engine of the system. It is early work, measured in growing seasons, and part of what Terra Humana wants to document: how quickly a poor Mediterranean soil can be rebuilt when the trees themselves are put to work.