Acacia nilotica (acacia)

The flowers are bright yellow and borne on globe-shaped flower heads. The flowers are sweetly smelling and appear near the beginning of the rainy season. Flowering is prolific, and can occur a number of times in a season. Often only about 0.1% of flowers set pods.

The nutritious pods retain their seeds at maturity and are dispersed by animals. The pods are compressed, slightly curved, and vary from slightly constricted to almost rosary-like (like a string of beads). The pods are smooth or covered with fine hairs. A mature tree can produce 2,000 to 3,000 pods in a good fruiting season, each with eight to sixteen seeds, yielding 5,000 to 16,000 seeds per kg, depending on the subspecies.

Important synonyms: Mimosa arabica Lam.; Mimosa scorpioides L.; Mimosa nilotica L.; Acacia scorpioides W. Wight, Vachellia nilotica (L.) P.J.H. Hurter & Mabb.

A wide range of pests and diseases affect this species. The stem borer Cerostema scabrator is a pest of economic importance on young plantations in India. Euproctis lunata and E. subnotata occasionally defoliate patches of forest in Sukkur and Hyderabad. Bruchid beetles attack the seeds, destroying up to 70 % of them. Buprestid beetles cause a dieback disease in Sudan. Fungal rots (Fomes papianus and F. badius) attack unhealthy trees, and powder post beetles (Sibixylon anale and Lyctus africanus) attack the sapwood of felled timber. Many wild mammals feed on seed pods and a large number of insect species attack the mature seed.

Acacia nilotica inflorescences (Image: Jonathan Timberlake)

Acacia nilotica wood burns without too much smoke and provides good charcoal. The flowers provide both pollen and nectar for bees. The species is suitable for live fencing, mine timber, railway sleepers, boat building, wheels, water wells and so on as its wood is durable and resistant to borers and termites. The sap-wood and heart-wood was used in ancient Egypt for house beams, furniture, panelling and statues as it was regarded as impervious to insect and fungus attack. The bark contains tannins and has been used to preserve and soften leather. Photochemical analysis has shown the presence of two types of tannin (gallotannins and catechins) which explain its therapeutic action as well as its use in tanning hides.

Gum is present in the bark but tends to be dark in colour. This species may indeed have been the original source of the true gum arabic which is now obtained commercially from Senegalia senegal. The Acacia nilotica gum, samogh or samuk (arabic) is sold in boles and it is commercially of inferior quality. It has been used as an emulsifying agent and emollient. It is edible and is used to relieve throat and chest complains.

Babul (subspecies indica) is a popular farm tree of the central plains of India. More recently interest has centred on the fastigiate form (subspecies cupressiformis). This subspecies makes an ideal windbreak to surround fields as its narrow crown shades less than other windbreak species.

In shamanism Acacia nilotica has been used to drive away evil spirits.

Acacia nilotica seed pod (Image: RBG Kew)

Extracts of the pod have been found to be active against two fresh-water snails (Bulinus truncatus and Biomphalaria pfeifferi), which are vectors of the parasitic disease schistosomiasis in Sudan. A pilot scheme to produce ‘tan’ (a vegetable molluscicide) has been set up. This is a water extract in which tannins amount to over 56% of the product, and are probably the active ingredient.

In Senegal Acacia nilotica is considered an important tree in the forest reserves along the banks of the Senegal River. In Sudan and the Upper Nile region it has been planted as part of the reforestation of the areas alongside river banks, which are subject to flooding.

In India this species is used extensively on degraded saline/alkaline soils, growing on soils up to pH 9, with a soluble salt content below 3%. It also grows well when irrigated with tannery effluent, and colonises waste heaps from coal mines. Over 50,000 hectares of the Indian Chambal ravines have been rehabilitated with A. nilotica by aerial seeding (it is one of the three most frequently used trees for this purpose). In India, too, A. nilotica is an important tree for lac insects  - lac being used as a dye and as shellac, a glazing agent.

Description of seeds

Subcircular in outline, 6-9 x 5-8 mm, with areole (pleurogram) 5-7x 4-7 mm. The seed is free of tannins, but contains mucilage and sugars. They also contain a dark yellow fixed oil at a concentration of 5.46 %.

Seed from natural populations of some subspecies are available from India and some Sahelian countries. A broader range of germplasm and Rhizobium inoculum, is available from the Oxford Forestry Institute (Oxford OX1 3RB UK) for field trials.

In irrigated plantations in the Sind and Punjab, 10-15 seeds are spot sown at 2 x 3 m spacing on the tops of trenches. They are thinned to three to four seedlings after three to four months. Further thinning occurs at five year intervals. Rotations are 20-25 years. In the Thal desert, Pakistan (where there is 250 mm annual rainfall), promising growth resulted from irrigation of ten day intervals. Growth rates varied considerably depending on the sites, with maximum mean annual increment of 13 m3 /ha at 20 years old and 10.5 m3 /ha at 30 years recorded.