Trees and Nitrogen

Trees and other plants form symbiotic relationships with micro-organisms in the soil that fix N from the air. This happens in nodules that may be seen on the roots. There are two main sorts of micro-organisms involved:

1. RHIZOBIUM.
This operates with Pea and Bean family. It is most effective in well drained soils with full sun and a slightly acid to alkaline pH, with the notable exception of Gorse which prefers acid soils. There are also many herbaceous pIants ~ in this family which fix N – Peas, Beans, Clovers, Vetches, Trefoils, Alf-alfa, etc.

Plant Species:

There are no native trees in this family but these do well in most soils;

Black locust – Robinia pseudacacia
Labumum (poisonous, not recommended for most sites)

Larger Shrubs – native:
Gorse – UIex species
Broom – Cytisus scoparius

Shrubs – introduced:
Tree Lupin – Lupinus arboreus
Pea Trees – Caragana spp.
Ornamental Brooms – Genista and cytisus spp.

2. FRANKIA.
This operates with woody plants belonging to several families, and includes some which do well in shade.

Plant Species:
The Alders (Alnus) make the largest trees in this group. There are species suited to wet and dry soils, alpine and lowland sites. They are generally fast growing and wind-firm, so good for windbreaks etc. Tolerate air pollution, poor soils and coastal conditions.

Black Alder (native) – A. glutinosa Needs moist/wet soil. Important in wetland ecologies.
Italian Alder – A cordata. Happy in dry or wet soil, maybe the most useful N-fixing tree we have.
Grey Alder – A. incana. Alpine and arctic, wet or dry soil, used for reclaiming spoil heaps, tough.

Eleagnus. Large shrubs, fast growing, wind resistant and good on coasts. Deciduous and evergreen species.Edible fruit with medecinal qualities. Good companion plants with larger tree species. Evergreen species good inshade. Many ornamental with fragrant blossom. Good for informal hedges and beneath trees in windbreaks.
Deciduous species:
E. angustifolia – Oleaster, Russian Olive. To 7m high, may be cut back by hard winters, stands hard pruning. Fruit eaten raw or cooked, seed usually removed, fibrous. Makes sherbet. Bee plant.
E. commutata – Silverberry. To 3m high. As above, good on chalk, suckers.
E. umbellata – Autumn Olive. To 4.5m high. As above, coastal dune stabilizer.
Evergreen species:
E x ebbingei. Garden hybrid, to 5m. Tolerates deep shade. Very fragrant flowers in autumn, good quality fruit produced in April / May. Excellent potential as a new fruit crop. Bee plant.
E. pungens – Thorny Eleagnus. As above, slightly less hardy.

Hippophae rhamnoides – Sea Buckthorn. Native shrub, suits well drained soils, Inc. coastal or alpine. Fruit very rich in vitamins.Plants either male or female, plant in groups. Needs full sun. Silvery leaves, orange berries.

Cool Temperate 1-2-98