Peanut, also known as groundnut and goober (Arachis hypogaea), is a crop of global importance. It is widely grown in the tropics and subtropics, being important to both smallholder and large commercial producers. It is classified as both a grain legume, and, because of its high oil content, an oil crop. World annual production is about 46 million tonnes per year. Peanut pods develop under the ground, which is very unusual among crop plants.
As a legume, peanut belongs to the botanical family Fabaceae (also known as Leguminosae, and commonly known as the bean or pea family). Like most other legumes, peanuts harbor symbiotic nitrogen-fixing bacteria in root nodules. This capacity to fix nitrogen means peanuts require less nitrogen-containing fertilizer and improve soil fertility, making them valuable in crop rotations.
Peanuts are similar in taste and nutritional profile to tree nuts such as walnuts and almonds, and are often served in similar ways in Western cuisines. The botanical definition of a "nut" is a fruit whose ovary wall becomes very hard at maturity. Using this criterion, the peanut is not a nut, but rather a legume. However, for culinary purposes and in common English language usage, peanuts are usually referred to as nuts.
History:
Cultivated peanut (A. hypogaea) has two sets of chromosomes from two different species, thought to be A. duranensis and A. ipaensis. The two species' chromosomes combined by hybridization and doubling, to form what is termed an amphidiploid or allotetraploid. Genetic analysis suggests this hybridization event probably occurred only once and gave rise to A. monticola, a wild form of peanut that occurs in a few restricted locations in northwestern Argentina, and by artificial selection to A. hypogaea. The process of domestication through artificial selection made A. hypogaea dramatically different from its wild relatives. The domesticated plants are more bushy and compact, and have a different pod structure and larger seeds. The initial domestication may have taken place in northwestern Argentina, or in southeastern Bolivia, where the peanut landraces with the most wild-like features are grown today. From this primary center of origin, cultivation spread and formed secondary and tertiary centers of diversity in Peru, Ecuador, Brazil, Paraguay, and Uruguay. Over time, thousands of peanut landraces evolved; these are classified into six botanical varieties and two subspecies (as listed in the peanut scientific classification table). Subspecies A. h. fastigiata types are more upright in their growth habit and have shorter crop cycles. Subspecies A. h. hypogaea types spread more on the ground and have longer crop cycles.
The oldest known archeological remains of pods have been dated at about 7,600 years old. These may be pods from a wild species that was in cultivation, or A. hypogaea in the early phase of domestication. They were found in Peru, where dry climatic conditions are favorable to the preservation of organic material. Almost certainly, peanut cultivation antedated this at the center of origin where the climate is moister. Many pre-Columbian cultures, such as the Moche, depicted peanuts in their art. Cultivation was well established in Mesoamerica before the Spanish arrived. There, the conquistadors found the tlalcacahuatl (the plant's Nahuatl name, whence Mexican Spanish cacahuate, Castillian Spanish cacahuete, and French cacahuète) being offered for sale in the marketplace of Tenochtitlan. The peanut was later spread worldwide by European traders, and cultivation is now very widespread in tropical and subtropical regions. In West Africa, it substantially replaced a crop plant from the same family, the Bambara groundnut, whose seed pods also develop underground. In Asia, it became an agricultural mainstay and this region is now the largest producer in the world.
In the English-speaking world, peanut growing is most important in the United States. Although it was mainly a garden crop for much of the colonial period, it was mostly used as animal feed stock until the 1930s. The US Department of Agriculture initiated a program to encourage agricultural production and human consumption of peanuts in the late 19th and early 20th centuries. George Washington Carver developed hundreds of recipes for peanuts during his tenure in the program.
Botany:
Peanut is an annual herbaceous plant growing 30 to 50 cm (1.0 to 1.6 ft) tall. As a legume, it belongs to the botanical family Fabaceae (also known as Leguminosae, and commonly known as the bean or pea family). Like most other legumes, peanuts harbor symbiotic nitrogen-fixing bacteria in their root nodules. The leaves are opposite and pinnate with four leaflets (two opposite pairs; no terminal leaflet); each leaflet is 1 to 7 cm (⅜ to 2¾ in) long and 1 to 3 cm (⅜ to 1 in) across. Like many other legumes, the leaves are nyctinastic, that is, they have "sleep" movements, closing at night.
The specific name, hypogaea means "under the earth", because peanut pods develop underground, a feature known as geocarpy. The flowers are 1.0 to 1.5 cm (0.4 to 0.6 in) across, and yellowish orange with reddish veining. They are borne in axillary clusters on the stems above ground and last for just one day. In structure, they appear superficially similar to the flowers of peas and beans. However, intriguing differences are seen. The ovary is not positioned where expected, but is at the base of what appears to be the flower stem (in fact, this "stem" is a highly elongated floral cup). After fertilization, a short stalk at the base of the ovary (termed a pedicel) elongates to form a thread-like structure known as a "peg". This pushes the ovary down into the soil, where it develops into a mature peanut pod.[19] Pods are 3 to 7 cm (1.2 to 2.8 in) long, normally containing one to four seeds.
Cultivation:
Peanuts grow best in light, sandy loam soil with a pH of 5.9–7. Their capacity to fix nitrogen means that, providing they nodulate properly, peanuts benefit little or not at all from nitrogen-containing fertilizer, and they improve soil fertility. Therefore, they are valuable in crop rotations. Also, the yield of the peanut crop itself is increased in rotations, through reduced diseases, pests and weeds. For instance, in Texas, peanuts in a three-year rotation with corn yield 50% more than nonrotated peanuts. Adequate levels of phosphorus, potassium, calcium, magnesium, and micronutrients are also necessary for good yields. To develop well, peanuts need warm weather throughout the growing season. They can be grown with as little as 350 mm of water, but for best yields need at least 500 mm. Depending on growing conditions and the cultivar of peanut, harvest is usually 90 to 130 days after planting for subspecies A. h. fastigiata types, and 120 to 150 days after planting for subspecies A. h. hypogaea types. Subspecies A. h. hypogaea types yield more, and are usually preferred where the growing seasons are long enough.
Peanut plants continue to produce flowers when pods are developing, therefore even when they are ready for harvest, some pods are immature. The timing of harvest is an important decision to maximize yield. If it is too early, too many pods will be unripe. If too late, the pods will snap off at the stalk, and will remain in the soil. For harvesting, the entire plant, including most of the roots, is removed from the soil. The fruits have wrinkled shells that are constricted between pairs of the one to four (usually two) seeds per pod.
Harvesting occurs in two stages: In mechanized systems, a machine is used to cut off the main root of the peanut plant by cutting through the soil just below the level of the peanut pods. The machine lifts the "bush" from the ground and shakes it, then inverts the bush, leaving the plant upside down on the ground to keep the peanuts out of the soil. This allows the peanuts to dry slowly to a little less than a third of their original moisture level over a period of three to four days. Traditionally, peanuts were pulled and inverted by hand.
After the peanuts have dried sufficiently, they are threshed, removing the peanut pods from the rest of the bush. It is particularly important that peanuts are dried properly and stored in dry conditions. If they are too high in moisture, or if storage conditions are poor, they may become infected by the mold fungus Aspergillus flavus. The fungus releases a toxic and highly carcinogenic substance aflatoxin.
Production and trade:
Peanut is widely produced in tropical and subtropical regions of the world. China accounts for 37% of world production, Africa for 25%, India for 21%, the Americas for 8% and Oceania for 6% (for major producing countries see table). Major exporters are India, which accounts for 37% of world exports, Argentina for 13%, the United States for 10%, China for 8% and Malawi for 5%. Major importers are the Netherlands, which accounts for 17% of world imports (most being shipped on to other countries in the European Union), Indonesia that accounts for 10%, Mexico for 7%, Germany for 6% and Russia for 5%.
In the United States, Georgia is the leading peanut-producing state, followed by Texas and Alabama, respectively. About half of all peanuts produced in the US are grown within a 100 mi (160 km) radius of Dothan, Alabama. Dothan is home to the National Peanut Festival established in 1938 and held each fall to honor peanut growers and celebrate the harvest.
