Acacia nilotica (PROSEA)
Acacia nilotica (L.) Willd. ex Del.
- Protologue: Fl. Aegypt. Ill.: 79 (1813).
- Family: Leguminosae
- Chromosome number: 2n= 52, 104
- Acacia arabica (Lamk) Willd. (1806).
- Babul acacia, Egyptian thorn (En).
Origin and geographic distribution
Babul acacia is a native of tropical Africa, from Egypt south to Mozambique and Natal, extending to south-western Asia (Iran, Pakistan), and east to India. It is widely distributed in India, wild, cultivated, as well as naturalized, and it was introduced to Sri Lanka, Burma, Indonesia (Java in 1850, Lesser Sunda Islands) and tropical Australia.
The bark produces tannin which in India is used for tanning and dyeing leather black or various shades of brown. Babul acacia bark is the most commonly used tanning material in northern India. The tannin produces a heavy leather which is firm and durable but hard. If combined with myrobalans (from Terminalia spp.) the tannin produces excellent leather. Dried mature pods are used in local tanneries in Sudan and rarely in India to produce a pinkish-white leather of good quality.
The wood produces Indian gum which is sweet and of poorer quality than gum arabic obtained from Acacia senegal Willd. Indian gum is used for printing and dyeing calico, as a sizing material for cotton and silks, and also in the manufacture of paper. It is also useful as an emulsifying and suspending agent. The gum from the pods is used for dyes and inks in India. The young pods, young leaves and shoots are used as vegetables and as fodder. The seeds are used as cattle feed. In eastern Java sprouted seeds are consumed as a vegetable, and well-roasted seeds are mixed with coffee.
The trees make good hedges, e.g. to protect plantations against grazing animals. They are also used as fire-breaks, e.g. in the Baluran National Park in East Java. In Sudan they are used to afforest inundated areas. Babul acacia is a valuable species for reclamation of waste lands, especially on alkaline soils. In some places in India babul acacia serves as a host for lac insects.
The bark, gum, leaves and pods are used in various traditional medicines. An extract of the root is a potential inhibitor of tobacco mosaic virus.
The timber is harder than teak and is used for making agricultural implements, boat handles, brake blocks, cart-wheels, planks, tent pegs, etc. The wood is valuable as fuel and for the production of charcoal. The wood shavings are used as raw material for paper. Young bark is used as fibre for toothbrushes.
Production and international trade
No recent statistics are available on production and trade of this species. The various products made from the tree are used locally but do not enter international trade.
The tannin content of the bark varies with the age of the plant. Older trees have more tannin than younger ones. The content of tannin in the bark varies from 7% to 23%, with an average of 12%, on a dry weight basis. In India the tannin content of mature pods is 12-19%, and 18-27% in de-seeded pods. However, de-seeded pods in Sudan may contain 40% tannin.
Besides tannin, the bark also contains colouring matter. Babul acacia bark tans slowly and gives a dark-coloured leather. These undesirable properties may be modified by the use of myrobalans or by the use of modern methods such as controlling the pH of the tan liquor. The tannin of the bark belongs to the proanthocyanidin type.
The tanning material from the pods has the disadvantage that it tends to ferment easily. The percentage of sugar-like components and soluble non-tannins is too high to be of any value for making tannin extracts. The tan-stuff is a mixture of several tannins of the group of gallotannins and ellagitannins. Tannins from fallen pods penetrate the soil and are hydrolized, producing gallic and ellagic acids which are allelopathic to grasses and other herbs.
The pods provide an excellent tanning material, producing a light-coloured leather, especially when young pods are used. The leather is durable and has a high reputation for book binding. In Africa the pods are used in tanning, whereas in India the bark is used and little attention is paid to the pods.
On a dry weight basis the pods contain 11-16% crude protein, and the leaves 14-20%.
The gum of babul acacia varies in colour from pale yellow to reddish-brown or almost black depending on the age of the tree and climate at harvesting. It is soluble in water and its aqueous solutions are very viscous. The darker gum contains tannin and is less soluble in water. It has a moisture content of ca. 13% and is slightly dextrorotatory. It is composed of galactoaraban which hydrolysizes to give 1-arabinose and d-galactose. The quality and composition of the gum depends on climate and methods of collection.
Babul acacia timber is strong and fairly heavy with an air-dry weight of 650-850 kg/m3.
The sapwood is soft, yellowish-white, decays rapidly and is soon destroyed by insects. The heartwood is pale red, mottled with darker streaks, turning to reddish-brown when exposed to light, and it is not readily attacked by insects. The timber is very durable if it is well-seasoned. However, during the dry season it is liable to split. The energy value of the wood varies from 20 160-20 790 kJ/kg.
On a dry weight basis the seeds contain 4% lipid, 39.5% triglyceride, and 9.6% hydrocarbon-wax-ester. Seed weight is small, 1 kg containing 7000-11 000 seeds.
- A shrub or small to medium-sized, spiny and evergreen tree, usually less than 10 m tall, but sometimes attaining 20 m tall; bole short, straight or bent, with diameter up to 60(-80) cm, crown umbrella-shaped; bark dark brown, longitudinally deeply fissured, 1-1.5 cm thick; branches ascending, branchlets smooth, densely grey pubescent when young, at each leaf-base provided with 2 stipular spines 1-5 cm long, straight, sharp-pointed and white; sometimes spines are absent.
- Leaves alternate, bipinnately compound with 3-6 pairs of pinnae, rachises 3-10 cm long, pubescent, glandless or with several glands; leaflets (8-)10-15(-20) pairs per pinna, membranous, elliptic or narrowly oblong, 3-6 mm × 1-2 mm, rounded and oblique at base, obtuse at apex, entire, usually glabrous, subsessile.
- Flowers very small, 5-merous and arranged in 1-3 cm long peduncled, globose heads; heads 1-2 cm in diameter, 1-6 per leaf axil, with ca. 50 flowers per head, dark yellow and sweet-scented; subtending leaves of upper nodes reduced or not yet developed.
- Fruit a usually dehiscent oblong-linear and flattened pod, 7.5-15 cm × 1-2 cm, constricted between the seeds, distinctly stalked, densely tomentellous, becoming black when ripe, 5-12 seeded.
- Seeds ovoid-circular, flattened, ca. 5 mm × 4 mm, black.
- Germination epigeal, seedling with petiolate circular-ovate cotyledons and very short epicotyl; first leaves alternate, sometimes opposite.
Growth and development
Seedlings and young plants thrive best in open places with sufficient moisture. They prefer loose soils and absence of grasses and weeds.
Babul acacia is fast-growing, but the productivity varies according to environment. In India, trees in plantations along canals grow much faster than trees in natural stands. The mean heights of trees in plantations 5 and 10 years after planting are about 5 m and 25 m respectively, whereas in natural stands of the same ages, mean heights of 3 m and 5.5 m respectively, have been reported. In savanna land in Baluran National Park in East Java 17-year-old trees reach an average height of 6 m. Here the species, formerly planted as a hedge crop and firebreak in 1969, has become a noxious weed colonizing a large part of the grazing area of the savanna.
Other botanical information
Acacia nilotica is an extremely variable species. It has been divided into a considerable number of subspecies and varieties. The specimens planted and naturalized in Java are usually considered representative of subspecies indica (Benth.) Brenan.
Babul acacia occurs from sea-level to 1300 m altitude. It thrives in areas with an annual rainfall of 400-2300 mm. It will tolerate drought or flooded conditions for several months. Natural stands often occur along river banks which are subject to periodic inundation. Babul acacia is reported to tolerate annual mean temperatures of 19-28°C, but it can grow at extreme conditions of temperature and in soil with various characteristics, including heavy clay soils and saline sites with pH 5.0-8.0. Babul acacia prefers alluvial soil. It does not tolerate frost or shade when young. This plant is suitable for marginal lands with extremely high or low temperatures.
Propagation and planting
Babul acacia is propagated by seed. The seeds can be sown directly in the field, or they can first be sown in nurseries and the seedlings transplanted to the field later. The seeds should be scarified and soaked in warm water for several hours before sowing to obtain good germination. For direct sowing, ridge-sowing is recommended, with a sowing rate of 1 kg per ha. Seeds collected from goat and sheep dung germinate more easily.
When planted for the production of tannin and gum, babul acacia plants should have sufficient space (4 m × 4 m) so that each tree receives enough light. Thinning is necessary to maintain optimum growth of the stand. In India thinning is started at the age of 10 years and is repeated at intervals of 5-6 years. Plants tolerate pruning well, which makes them useful as hedge plants. Since in a suitable environment the dispersal of the species may be very fast, regular monitoring of the stands is necessary.
Diseases and pests
Two beetle species have been recorded as the most destructive insect pests of babul acacia in India, i.e. Coelasterna scabrata, a root-borer beetle, and Psiloptera fastuosa, which strips the bark from shoots and branches. Some plant parasites such as Dendrophthoe falcata (L.f.) Ettingsh. and Loranthus spp. have been reported on babul acacia. Damping-off in seedling stands, root rot, and heartwood rot caused by fungi have been reported.
For harvesting 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.
In harvesting gum, 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.
In India a plantation of about 600 plants per ha produced ca. 12 t of bark after 15 years of planting. In Sudan a babul acacia tree yields about 18 kg of de-seeded pods per year, and the yield of gum is up to 0.9 kg/year but usually much less. The yield of gum decreases as a tree gets older.
For many years babul acacia has been used for various purposes in India. It has potential in South-East Asia as a source of tannin, gum, timber, fodder and fuel. The species is fast-growing, easy to propagate, and tolerant of dry conditions and poor soils. Babul acacia is one of the species worth considering for reclamation of wasteland, especially in areas where supply of fuel is critical.
- Alikodra, H.S., 1987. Tanaman eksotik akasia (Acacia nilotica) dan masalahnya bagi ekosistem savanna di Taman Nasional Baluran [The exotic plantation of Acacia nilotica and its problems on the ecosystem of savanna of Baluran National Park]. Duta Rimba 13(79-80): 30-34.
- Bhatnagar, S.S. (Editor), 1948. The wealth of India. Raw materials. Vol. 1. Delhi, India. pp. 5-9.
- Duke, J.A., 1981. Handbook of legumes of world economic importance. Plenum Press, New York, USA. pp. 9-11, fig. 3.
- Hadipurnomo, 1981. Mengenal tanaman pagar Acacia arabica [Acacia arabica as hedge crop]. Duta Rimba 7(46): 13-15.
N. Wulijarni-Soetjipto & R.H.M.J. Lemmens