Classification of the genus Acacia (in the wider sense) has been subject to considerable debate. It is generally agreed that there are valid reasons for breaking it up into several distinct genera, but there has been disagreement over the way this should be done. As of 2017, it is widely (but not completely) accepted that the section that includes the majority of the Australian species should retain the name Acacia, whilst other sections of the genus should be transferred to other genera. This species is transferred to Vachellia[].

Common Name: Gum Arabic Tree

General Information

Vachellia nilotica is a fast-growing though small, spiny, evergreen tree with a broad, rounded (rarely umbrella-shaped) crown. It can range in height from about 2.5 metres on poorer soils to a tree 14 metres or more tall[, , ]. The straight bole can be 100cm in diameter[].
One of the most important multipurpose tree species of dryland Africa and South Asia, it is a source of food, medicine and many other commodities for the local people[, ]. The tree is also exploited commercially for the resin that exudes from the trunk[], and also for the tannins obtained from the bark and seedpods[]. It is the most commonly used source of tannins in northern India[]. The tree is often planted, especially in India as a street tree and ornamental[].

Known Hazards

Especially in times of drought, many Acacia species can concentrate high levels of the toxin Hydrogen cyanide in their foliage, making them dangerous for herbivores to eat.

Botanical References

Range

Drier areas of Africa, Arabian Peninsula, western Asia to northern India.

Habitat

Woodlands of various sorts, wooded grasslands, scrub and thickets[]. Often found along river banks which are subject to periodic inundation[]. Found at elevations from sea level to 1,300 metres[].

Properties

Weed Potential	Yes
Edibility Rating
Medicinal Rating
Other Uses Rating
Habit	Evergreen Tree
Height	10.00 m
Growth Rate	Fast
Pollinators	Bees, Insects
Cultivation Status	Cultivated, Ornamental, Wild

Cultivation Details

Gum arabic succeeds in subtropical to tropical lowland areas and at elevations up to 1,300 metres[]. It thrives in areas with an annual rainfall in the range of 400 - 2,300mm[]. It is reported to tolerate an annual mean temperature in the range of 19 - 28°c, though it can grow at extreme conditions of temperature[]. It does not tolerate frost when young[].
Requires a sunny position[]. Succeeds in a range of soils, including heavy clay soils and saline conditions[]. It tolerates a pH ranging from 5.0 - 8.0[]. It will tolerate drought or flooded conditions for several months of the year and can be grown in marginal land[].
The tree has escaped from cultivation and become established in the wild in many areas outside its native range. It is classed as 'Invasive' in several areas, including Australia and some Pacific Islands[]. In a suitable environment, plants can spread very quickly by means of their seeds[]. Regular monitoring of any stands is necessary to prevent the plant becoming a nuisance[]. The plant is considered to be a serious weed in S. Africa[].
Trees flower and produce seedpods abundantly by the time they are 5 - 7 years old[].
In India, a plantation of about 600 plants per hectare produced around 12 tonnes of bark for tannins after 15 years of planting[]. In Sudan, a tree yields about 18 kg of de-seeded pods per year, and the yield of gum can be up to 0.9 kg/year, though it is usually much less[]. The yield of gum decreases as a tree gets older[].
The tree has a deep and extensive root system[].
This species has a symbiotic relationship with certain soil bacteria, these bacteria form nodules on the roots and fix atmospheric nitrogen. Some of this nitrogen is utilized by the growing plant but some can also be used by other plants growing nearby[].

Edible Uses

The young pods, young leaves and shoots are used as vegetables[, , ].

The sprouted seeds are consumed as a vegetable[]. The seeds can be mixed with dates and then fermented into an alcoholic beverage[].

The flowers are made into fritters[].

The well-roasted seeds are ground into a powder and used as an adulterant mixed with coffee[].
The roasted seeds are used as a condiment[].

The gum obtained from the stems is eaten mixed with sesame seeds, fried in ghee, or used in the preparation of sweetmeats and candied flowers[].

A type of wine, known as 'sak', is made from the bark[].

Medicinal

The bark, gum, leaves and pods are used in various traditional medicines[]. The bark, leaves and pods are rich in tannins and so are astringent[]. Extracts of the plant have been shown to be inhibitory to at least four species of pathogenic fungi[].

The bark is used to treat a wide variety of ailments in traditional medicine, both internally in the form of a decoction, and externally as a wash[]. In particular, its astringency makes it an excellent treatment for diarrhoea and dysentery, whilst it is also used as a nerve stimulant and in treating conditions such as leprosy, coughs, and intestinal pains[].
Both the gum and the bark have been used for treating cancers and/or tumours (of ear, eye, or testicles); chest problems including colds, congestion, coughs and tuberculosis; indurations of liver and spleen; fevers, gallbladder problems, haemorrhage, haemorrhoids, leucorrhoea, ophthalmia, sclerosis and smallpox[].

The root has been used to treat tuberculosis and is also said to cure impotence[].

The bruised leaves are used as a poultice on ulcers[].

The wood has been used to treat smallpox[].

Agroforestry Uses:

A valuable species for the reclamation of waste lands, especially on alkaline soils and where the supply of wood for fuel is critical[].
Vachellia nilotica is a pioneer species. A deep and extensive root system is formed on dry sites, the taproot developing first and then the laterals, which become compact and massive with age. On flooded sites however, the root system is largely lateral[]. The trees are used in reforestation projects, especially on inundated land[, ].
The trees tolerate pruning well, which makes them useful as hedge plants. They are used to protect plantations against grazing animals[]. They are also used as fire-breaks[].

Other Uses

A sticky red resin, known as Indian Gum Arabic, exudes from the bark[]. It is sweet, but of poorer quality than the gum arabic obtained from Acacia senegal[]. It is used for printing and dyeing calico, as a sizing material for cotton and silks, and also in the manufacture of paper[]. The gum is still used in making candles, inks, matches, and paints[]. It is also useful as an emulsifying and suspending agent[].
To harvest the gum, the trees are wounded by removing a part of the bark and bruising the surrounding bark[]. Good-quality gum is reddish in colour, almost completely soluble in water and tasteless[]. Usually it is traded in ball form[].

The gum obtained from the pods is used for dyes and inks in India[].

The bark and the seedpods are rich in tannin[, ]. The inner bark contains 18 - 23% tannin[]. The tannin content of entire pods can vary from 12 - 19% and from 18 - 27% after removal of the seeds. In Sudan the de-seeded pods can reach tannin contents of up to 50%. Seeds are usually removed because of their high content of sugar-like components which tend to cause the tannin liquor to ferment[].
The bark is used for tanning and dyeing material black or various shades of brown[].
Dried mature pods are used in local tanneries in Sudan and rarely in India to produce a pinkish-white colour[]. The tannins are used for dyeing clothes a yellow colour[]. The tannins are used as a source of khaki-to-brown dyes if used without a mordant, or grey and black dyes for cotton if combined with a mordant of iron-rich mud[].
For harvesting the bark for tanning, the trees are felled and the bark is separated from the logs by beating them with wooden mallets. The strips obtained are then sun-dried, chopped into small chips and sent to tanneries. The bark is often only a by-product; the trees are primarily felled for timber and fuel[].

A brownish oil is obtained from the seeds[]. No uses are given.

An extract of the root is a potential inhibitor of tobacco mosaic virus[].
The aqueous extract of the fruit, rich in tannin (18 - 23%) has shown algicidal activity against Chroccoccus, Closteruim, Coelastrum, Cosmarium, Cyclotella, Euglena, Microcystis, Oscillatoria, Pediastrum, Rivularia, Spirogyra, and Spirulina[].
In a trial, a methane extract of the seedpods at a concentration of 10,000ppm in distilled water reduced the egg hatching of root nematodes by 95% after 21 days compared with a control[].

A fibre obtained from the bark of slender branches is used to make coarse ropes and paper[, , ]. These slender branches and their fibres are also used for toothbrushes[, ].

The twigs are esteemed for tooth brushes (chewsticks)[].

The heartwood is pale red, red-brown to deep red, often darkening upon exposure; it is distinctly demarcated from the yellowish white sapwood. The wood is strong, heavy, hard, durable, close-grained and resistant to water and ants[]. Very shock-resistant and harder than teak, it is used for making agricultural implements, sugar and oil presses, boat handles, brake blocks, cart-wheels, planks, tent pegs, etc[, , ]. The wood shavings are used as raw material for pulping to make paper[].
The wood is valuable as a fuel and also for the production of charcoal[].

Propagation

Seed - best sown when fresh and still moist. Seeds can be extracted from the pods, while seeds that are ejected by sheep during rumination or those from cattle and goat droppings may also be collected. These latter seeds germinate easily due to fermentation and moistening which soften the seedcoat[].
If not sown when moist, the seed develops a hard seedcoat and may benefit from scarification before sowing to speed up germination. This can usually be done by pouring a small amount of nearly boiling water on the seeds (being careful not to cook them!) and then soaking them for 12 - 24 hours in warm water. By this time they should have imbibed moisture and swollen - if they have not, then carefully make a nick in the seedcoat (being careful not to damage the embryo) and soak for a further 12 hours before sowing.
The germination rate of moist or treated seed generally varies from 50 - 90%, it usually starts 1 - 3 weeks after sowing and is mostly complete in one month[]. Seeds can be sown directly in the field, or they can first be sown in nurseries and the seedlings transplanted to the field later. For direct sowing, ridge-sowing is recommended, with a sowing rate of 1 kg per ha[].

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