Nitrogen fixing trees are the safest solution to the problem of nitrogen deficiency in present day’s agriculture. Nitrogen fixing tress can solve the problem by accumulating Nitrogen from the atmosphere and storing that within their body to use that themselves and release that into the soil for other neighbors.
Though these trees don’t possess any danger for us like chemical fertilizers do, they may bring a total collapse to an invasion-resistant ecosystem if they are introduced unwisely. If you want to use them properly then always go for native NFTs which will bring you the best result without threatening anything. Here are 9 trees, with their images, characteristics, functions, beneficial impacts to the soil etc., those are great soil companions for three different climate types- Tropical, Hot and Arid, and Mediterranean.
Great Soil Companions in Tropical Climates:
A Tropical climate is a non-arid climate and has an almost constant temperature throughout the year. Some areas outside the Tropics are also known to have tropical climate. Trees that are very good nitrogen fixers and soil restorers in this climate are discussed below….
Leucaena leucocephala is an aggressive, rapid-growing small, variably shrubby and branched to medium-sized tree. This tree is evergreen, deep rooted and is able to bear flowers, immature pods and mature pods at the same time. This is a native variety to Central America. In tropical regions like South-East Asia, Africa, South America etc. this tree has been used in agro forestry.
Common names: Jumbay, river tamarind, white leadtree, Subabul, white popinac etc.
Trunk: This tree has clear trunk to 5 m and upright angular branching; the diameter of its trunk is 10-50 cm.
Leaf: This tree has leaves those are slightly asymmetric, acute at tip and obtuse at base.
Flower: Its flower heads are 12-21 mm in diameter which contains 100-180 flowers each. The flowers are white or pale cream-white in color.
Fruit: The seed pods are 11-19 cm in length and 15-21 mm in width
Why it is a great soil companion: Leucaena leucocephala is a great nitrogen fixing variety in tropical climate; as 5 to 6 annual pruning of some Leucaena hedge rows added between 180 and 250 kg/ha/yr Nitrogen(N) to the soil (Kang et al. 1981a, b). Apart from utilizing Leucaena pruning as a source of N, they can be used as mulch or incorporated in the soil before planting to add high amount of N.
Casuarina equisetifolia or Casuarina is a tall, pine-like tree with wispy appearance; this aggressive evergreen tree can grow up to 20-30m.
Common names: Casuarina, ironwood, beefwood, Australian Pine, she oak etc.
Trunk: Single straight and rough-barked trunk; open and irregular crown of branches.
Leaf: Minute scale-leaves occur in whorls of 6-8 leaves per whorl.
Flower: Its unisexual flowers are tiny, wind-pollinated and brown in color.
Fruit: Small, round and cone-like fruits.
Why it is a great soil companion: Casuarina trees are effective when used in land reclamation and sand dune stabilization. The tree can fix N at a rate of 60 to 95 kg N/ha/yr in stands mixed with Eucalyptus. When they are planted purely they can fix 80-95 kg N/ha/yr; 50-70% of that fixed N is taken from the atmosphere.
Acacia mearnsii or Black wattle is a vigorous, fast growing, highly invasive leguminous tree which is native to Australia. They are evergreen, 6-25 m high, single-stemmed or multi-branched trees.
Common name: Black wattle, Australian acacia, green wattle, late black wattle, mimosa etc.
Trunk: Straight trunk having a diameter up to 50 cm.
Leaf: They have 8-15 cm long, dark green leaves with 8-21 pairs of pinnae. The leaves are feathery with very soft hair.
Flower: Grow many sweet scented, tiny flowers composed of many stalked, pale yellow balls.
Fruit: Narrowly oblong pods with minute hairs contain 1-14 seeds. The pods are fairly straight, 5-15 cm long and 4-9 mm wide.
Why it is a great soil companion: Acacia mearnsii is a very good nitrogen-fixer; fix 250 kg/ha N per year. Its pruning adds N to the soil and it is a good source of green manure. It is a very good soil companion in Tropics and can restore and regenerate soils.
Great soil companions in Hot and Arid Climate:
Arid climate covers around 33% of Earth’s land. Legumes are champion that are drought tolerant are highly abundant in the hot arid climates. In arid climate where the decomposition of organic matter (OM) is slow, plants cannot take their required N from the decomposition of OM. There are some legumes that perform as great N fixers in the hot deserts. These N fixing legumes gather great amount of nitrogen compounds in their tissues than non-fixing trees and thus act as a great resource to the soil and neighboring trees.
Olneya tesota or Desert Ironwood is a perennial tree variety, which is native to hot and arid climate regions like Sonoran Desert. This species is long living, evergreen, and grows up to 10 meters.
Common names: Desert Ironwood, Arizona Ironwood, Palo-de-Hierro, Palo-de-Fierro etc.
Trunk: Grey trunk, more fissured with age; contains thorns on twigs. Average trunk diameter is 60 cm.
Leaf: Leaves are bluish-green in color and are arranged on a petiole. Leaves are 15 cm long with 6-9 leaflets.
Flower: Flowers are magenta-red, medium purple, or white to pale pink in color; having 5 unequal petals.
Fruit: Produces 5-8 cm long seedpods which are light reddish brown in color.
Why it is a great soil companion: This tree has nitrogen-fixing nodules on its root system to fix huge amount of N to the soil. It adds nitrogen-rich leaf litter fertilizers to the surrounding soil. This species is known as “nurse plant” as it provides safe place for seed germination and protection to seedlings of other trees from extreme cold.
Prosopis velutina or Velvet Mesquite is one of the most common native trees in North American southwestern deserts. It is a small to medium-sized perennial legume which plays a great role in the ecology of hot deserts.
Common names: Velvet Mesquite, Common Mesquite, Arizona Mesquite etc.
Trunk: It has gnarled and twisted branches and joint twigs.
Leaf: Dark to dusty green leaves with short gray hairs; each leaf has 14-30 pairs of 1/3 to 1/2 inch long leaflet.
Flower: 2-3 inches long white or pale yellow flowers, which blooms in early summer.
Fruit: 3-9 inches long and 1/2-1/4 inches wide edible slender brown pod.
Why it is a great soil companion: These trees can restore nitrogen to the soil through nitrogen fixation. They are known as “nurse trees” as they feed other young seedlings with their nitrogen-rich soil, provide shelter and shade to other trees and provide cover to large animals.
Prosopis glandulosa or Honey mesquite is a North American native tree reaches 25-30 feet in height and forms a rounded canopy that spread about 25-35 feet. This is a highly aggressive variety with a great beneficial impact on the soil.
Common names: Western honey mesquite, Honey mesquite, Mesquite, Algoroba.
Trunk: Typically multi-trunked, no thorns and droop branched trunk.
Leaf: They have leaves with 7-18 sets of paired leaflets.
Flower: Pale yellow flowers with elongated spikes; blooms from March to November.
Fruit: Pod or pod-like, 3-6 inches, showy, brown colored edible fruits.
Why it is a great soil companion: They are a great resource to the local people as they provide food, drink, alcohol, fuel, medicine, and fertilizer. These trees can restore nitrogen to the soil through nitrogen fixation. They grow very rapidly, provides edible seed pod in abundance and provide very dense shade.
Great soil companions in Mediterranean Climate:
Mediterranean climate, a very mild climate which has warm to hot, dry summers and mild to cool, wet winters, is common on the western sides of continents. Some trees those are great soil companions with their many beneficial effects to soil are discussed below-
This very fast growing leguminous species is native to Africa and Middle East; it can grow up to 30 m tall. It has deep-penetrating tap roots that make it enable to resist droughts. It is an ideal companion for soils of Mediterranean climate.
Common names: Ana tree, Balanzan tree, Apple-ring acacia, Anaboom, Mogabo etc.
Trunk: Bigger trunk with 2 m diameter.
Leaf: Doubly pinnate leaves which has bluish-green leaflets.
Flower: Flowers occur in long spikes; they are yellowish to cream in color.
Fruit: Fruits are capsules or pods, up to 10 cm long, twisted into a ring shape and rosy red in color.
Why it is a great soil companion: These trees are very efficient nitrogen fixer and add a huge quantity of nitrogen to enrich the soil. They are also used for erosion control and for food, drink and medicine. Pruning of this tree also adds nitrogen and double the yields in maize crops, as its leaves are high in nitrogen.
Robinia pseudoacacia or Black Locust is a medium-sized tree that grows up to 80 feet tall but usually stays between 30 and 50 feet in height with a canopy of 30 feet width. This aggressive variety is native to the southeastern region of United States.
Common names: Black Locust, False Acacia
Trunk: Generally 0.8 m in diameter covered with thick, deeply furrowed blackish bark.
Leaf: 10-25 cm long leaves; a pair of short spines are present at the base of each leaf. The tree has a parallel and compound pinnate which has 9-19 oval leaflets.
Flower: Flowering occurs at May or June. Flowers are fragrant, nectar bearing and cream-white in color.
Fruit: Legume two-valved, 3-4 inches long, smooth fruits, those contain 4 to 8 seeds each.
Why it is a great soil companion: This tree is an efficient nitrogen fixing species and is able to produce a great quantity of nitrogen to use for own purpose and to supply neighboring plants. Black locust can restore the poor condition of the soils of Mediterranean region. These trees provide very high quality timbers. They also produce edible flowers and fuel wood.
Acacia boormanii or Snowy River Wattle is a small tree or shrub with smooth, grey bark. This multi-stemmed, evergreen, frost-hardy shrub grows up to 4.5 m tall. These small trees are native to south eastern Australia. These trees are very good soil companion in Mediterranean climate as it can fix nitrogen to the soil.
Common names: Snowy River Wattle.
Trunk: Thin stem.
Width: About 6-12 feet.
Leaf: Small, grey-green leaves.
Flower: Bright yellow colored flowers. Flowering time Winter-spring.
Fruit: In the form of seed pods.
Why it is a great soil companion: This species is vigorous and rapid growing, thus capable of adding nitrogen at pruning. It has a shallow and spreading root system that fixes nitrogen to the soil. This tree is effective in erosion control for clayey sites or sandy sites.