Vigna subterranea
Vigna subterranea is a member of the family Fabaceae. Its name is derived from the Bambara ethnic group. It reproduces via geocarpy, ripening its pods underground, much like the peanut.
The plant originated in West Africa. As a food and source of income, the Bambara groundnut is considered to be the third most important leguminous crop in those African countries where it is grown, after peanut and cowpea. The crop is mainly cultivated, sold and processed by women, and is, thus, particularly valuable for female subsistence farmers.
Bambara groundnut represents the third most important grain legume in semi-arid Africa. It is resistant to high temperatures and is suitable for marginal soils where other leguminous crops cannot be grown. It is a low-impact crop. The entire plant is known for soil improvement because of nitrogen fixation.
The pods can be eaten fresh or boiled after drying, and can be ground either fresh or dry to make puddings.
Biology
Bambara groundnut is a herbaceous, intermediate, annual plant, with creeping stems at ground level. It is a small legume plant that grows to a height of with compound leaves of three leaflets having stipules about 3 mm long. The flowers have a tube calyx about 1 mm long and 5 lobes about 1 mm long as well as a whitish yellow corolla, 4–7 mm long. The fruit is an indehiscent pod almost globose about 2.5 cm in diameter. The plant generally looks like bunched leaves arising from branched stems, which form a crown on the soil surface.Bambara is considered as a fast-growing crop. The growth cycle is between 90–170 days and under optimal conditions the cycle is about 120–150 days to pod maturity. Flowers appear 40–60 days after planting. After pollination, the pod reaches maturity, and during another 55 days, the seeds fully develop, producing again every 30 days.
Generative reproduction is for the Bambara groundnut autogamous,, and. After self-fertilization, pale yellow flowers are borne on the freely growing branching stems; these stems then grow downwards into the soil, taking the developing seed within the pods, which makes breeding and development of new cultivars for the traits of interest difficult. The seeds will form pods encasing seeds just below the soil. The pods are round, wrinkled and each contains one or two seeds that are round, smooth and very hard when dried.The seeds may be cream colored, brown, red, mottled or black eyed and their size is about 8.5–15 mm × 6.5–10 mm × 5.5–9 mm.
Several factors are essential for promoting cross-pollination in Bambara and these include a proper nursery habitat, short day lengths, an average temperature of 26 °C, which is necessary for optimum flowering and pod formation and a relative humidity of 90%. The strict photoperiod requirement of Bambara also limits its productivity in countries further away from the equator. In some accessions, long days negatively affect pod-setting, resulting in crop failure.
The genus Vigna, which includes about 80 species, is found throughout the tropics. There are considerable morphological differences between wild and domesticated types of Bambara groundnuts. Long runners are produced by wild Bambara groundnut, and the seeds are smaller and more uniform in size. The pods are thin and do not wrinkle when drying. Domesticated versions are more compact, have fleshy pods that wrinkle as they dry, with longer, less slender, and more erect petioles, and larger seeds. Wild and domesticated types are sometimes distinguished as var. spontanea Hepper and var. subterranea.
Origins and domestication
The Bambara groundnut is thought to have originated in West Africa, particularly in areas now known as Nigeria, Cameroon, and the Central African Republic. It was domesticated by local communities and has been cultivated for centuries as an important source of nutrition and sustenance. Its cultural significance is notable among the Bambara people of Mali, from whom its common name is derived.Distribution
The spread of V. subterranea beyond its native range is closely linked to the interplay of human migration, trade, colonialism and agricultural practices.The Bambara groundnut is predominantly cultivated across Sub-Saharan Africa, encompassing several regions. In West Africa, it is grown in countries such as Benin, Ghana, Togo, Nigeria, Ivory Coast, Mali, Burkina Faso, Guinea-Bissau, and Senegal. In Central Africa, it is found in Cameroon, Chad, the Central African Republic, and the Democratic Republic of the Congo. Nations in East Africa like Kenya, Uganda, Tanzania, Ethiopia, and Sudan also cultivate it. In Southern Africa, it is grown in countries such as Angola, Namibia, Mozambique, Zambia, and Eswatini.
Beyond Africa, Bambara groundnut has been introduced to other continents. On the Indian Ocean Islands, it is found in Madagascar, where it is known locally as voanjobory or voanjabory and remains an important component of traditional cuisine. It is also cultivated in Mauritius and the Comoro Islands. In Asia, it is cultivated in India, particularly in the state of Karnataka, and in Indonesia, especially on the island of Java. In Australasia, it is grown in Papua New Guinea.
In the Caribbean, it is grown in Dominican Republic where it is known as "manicongo", it was once common in local markets but has since declined in prevalence over the past decades. This particular lineage, descends from seeds that were carried to the Americas by enslaved people during the Atlantic slave trade and the Age of Discovery.
Ecology
Biological nitrogen fixation
Like many other legumes, Bambara groundnut fixes atmospheric nitrogen through the process of biological nitrogen fixation. Its potential to be used as an alternative to chemical fertilizer in agriculture has been investigated for many years. The process is also very important to improve soil fertility and to supply other non-leguminous crops with the nitrogen left in the soil after the legume is harvested.Cultivation
Soil requirements
Optimal soils for Bambara groundnut production are sandy soils to prevent waterlogging. Well-drained soils make the harvest easier and prevent rotting of the pods. Stony areas are typically avoided to prevent damage of the pods.Optimal soil depth is between 50 and 100 cm, with a light soil texture. Soil fertility should be low and soil pH is best suited between 5 and 6.5 and should not be lower than 4.3 or higher than 7. Bambara groundnut is tolerant to salinity, but high sodium chloride concentration in the soil will result in yield losses.
Climate requirements
The production is best suited between a latitude of 20° and 30°, i.e. the tropical wet and dry and the subtropical dry summer climate zones. Optimal temperature is between 19 °C and 30 °C. Temperatures below 16 °C and above 38 °C are not suited for the production of Bambara groundnut.The Bambara groundnut is very drought-resistant. The minimal annual rainfall requirement is about 300 mm and optimal annual rainfall is between 750 mm and 1400 mm and should not exceed 3000 mm. Bambara groundnut can tolerate heavy rainfall, but it will result in yield losses if they happen at harvest.
Seedbed requirements and sowing
Before sowing, the seeds can be treated with pesticides to prevent insect and fungal attack, and being eaten by bush fowl. Priming the seeds with water by soaking them overnight and then drying them before sowing improves seedling emergence, vigour and yield.Sowing is usually performed manually by peasant farmers in tropical Africa, but it can also be done mechanically on industrial farms using modified soya bean planters. Manual sowing is generally done using a hoe or a cutlass to open the soil. One seed is placed in each hole which are then closed.
Seedbed type doesn't seem to affect yield or biomass production of Bambara groundnut. The crop can thus be planted on flat terrain, although it is also planted in ridges. Studies show that increased sowing density has a positive effect on production calculated on a per-area basis, but has a negative effect on per-plant yield. It is assumed that at higher sowing densities, increased competition between plants is the cause of lower pod and seed number per plant.
Cropping system and fertilization
The cropping system is semi-permanent and the Bambara groundnut can be cultivated as single crop or as intercrop. Best suited intercrops are sorghum, millet, maize, peanut, yams and cassava. Bambara groundnut is mainly cultivated as intercrop, however the planting density varies between 6 and 29 plants per square meter. For woodland savannas of Côte d'Ivoire, the highest yield is attainable with a plant density of 25 plants per square meter. Despite its suitability for intercropping systems due to its ability to fix atmospheric nitrogen, a non-negligible proportion of farmers grow the Bambara groundnut in monoculture and report that its performance is better as a single crop. Cultivation is mainly performed manually and is comparable to the production of peanut.The Bambara groundnut can grow on soils with low fertility and is even reported to do better on these soils than on fertile ones. Nevertheless, phosphate fertilization can have a beneficial effect. For example, the application of superphosphate can improve the yield of the Bambara groundnut. Moreover, fertilization with phosphorus enhances the crop's nitrogen fixation and increases its nitrogen content.