Choose nitrogen-fixing companions from plants that really form root nodules: clover, vetch and lupine, bean and pea relatives, black locust, alder, autumn olive and Siberian pea shrub. Do not count honey locust, which is a legume that does not fix nitrogen even though it is often listed as a fixer. A fixer feeds its neighbours mostly when its leaves, roots and nodules die or are cut and laid down as mulch, so the supply arrives slowly and depends on how much fixer you grow and how often you cut it.
Nitrogen-fixing companions are plants whose root nodules host bacteria that turn nitrogen gas from the air into a form plants can use, planted among food trees to raise the nitrogen available to them.
Not Every Legume Fixes Nitrogen, And Honey Locust Is The Common Exception
Two different groups of bacteria do the fixing. Rhizobia live in nodules on the roots of most legumes. A second group, Frankia, lives in nodules on non-legume woody plants such as alder and the Elaeagnus group, which includes autumn olive. A plant that does not host either bacterium fixes nothing, whatever family it belongs to.
Black locust (Robinia pseudoacacia) forms nodules and fixes nitrogen. Honey locust (Gleditsia triacanthos) is also a legume but does not nodulate, and does not fix nitrogen. The two are often confused in permaculture lists. The carrier guide, Food Forest & Permaculture Design, lists honey locust only as a dual-purpose canopy tree (pods for animal feed), and puts black locust, alder, autumn olive and Siberian pea shrub in its fixer lists, which is correct. Note that black locust becomes a tall tree, not only a sub-canopy one.
For the underlying chemistry and the soil bacteria, see Nitrogen Fixation and Legumes.
Fixed Nitrogen Reaches Neighbours Through Decay More Than Through Roots
Fixation costs the host plant sugar, so it fixes less when the soil already supplies plenty of nitrogen. Nitrogen the fixer has captured becomes available to its neighbours mainly when nodules, roots and leaves die and decompose, or when you cut the top growth and lay it on the ground (chop-and-drop). Nitrogen leaking directly from living roots is generally a small share by comparison. The practical consequences: a fixer needs to be cut or to shed leaves to feed anything, and a heavily cut herbaceous fixer feeds its neighbours faster than a slow tree does.
What You Need
- A fixer matched to the layer. Ground and herb layers: white clover, vetch, lupine. Shrub layer: Siberian pea shrub (the carrier lists it as a nitrogen fixer and chicken feed). Tree layer: black locust, alder (wet ground).
- The right bacteria in the soil. A legume planted where its rhizobia have never lived may fail to nodulate. Dig a plant and cut a nodule: a pink or red interior shows it is working; white, green or grey does not.
- A cutting tool for chop-and-drop.
Method: Place, Establish, Cut
Step 1: Place by function. Put clover and vetch in the ground layer under and between trees, and keep tree fixers on the sunny side, where their shade falls outside the planting.
Step 2: Check nodulation in the first season. If nodules are absent or inactive, the fixer is not feeding anything; replace it or inoculate with soil from a nodulating stand of the same species.
Step 3: Cut on a schedule. Cut herbaceous fixers a few times through the growing season and leave the material in place. Pollard or coppice shrubs and small trees if you want a larger amount of leaf.
Step 4: Watch the neighbours. Compare the leaf colour and growth of trees with and without fixers nearby, not just the fixer’s own health.
Diagnosing A Fixer That Is Not Delivering
- No nodules — the right bacteria are missing, or soil acidity is too extreme for them.
- Nodules but yellow neighbours — the fixer may be small, unfed (fixation needs phosphorus and other minerals) or never cut.
- Fixer thriving, nothing else benefiting — decay has not happened yet; wait for the cut or shed material to break down.
Common Mistakes
- Counting honey locust as a fixer — it provides pods and shade, not nitrogen.
- Planting a fixer and never cutting it — a standing fixer feeds slowly.
- Planting a fixer in already-rich soil — it fixes less when nitrogen is available.
Safety
Several common fixers are toxic or invasive
Black locust bark, seeds and leaves are toxic to people and livestock (the flowers are the edible part); it suckers and is invasive outside its native range. Autumn olive is invasive in many regions. Bitter lupine seed is poisonous if eaten. Check regional invasive-species lists before planting a tree fixer, and keep horses and livestock away from black locust foliage and bark.
What’s Next
- Learn next: Pest-Confusing Polycultures — the next role in a guild
- Combine with: Cover Crops — fixers used on open ground