The potato (Solanum tuberosum) is an herbaceous annual that grows up to 100 cm (40inches) tall and produces a tuber - also called potato - so rich in starch that it ranks as the world's fourth most important food crop, after maize, wheat and rice. The potato belongs to the Solanaceae - or "nightshade"- family of flowering plants, and shares the genus Solanum with at least 1,000 other species, including tomato and eggplant. S. tuberosum is divided into two, only slightly different, subspecies: andigena, which is adapted to short day conditions and is mainly grown in the Andes, and tuberosum, the potato now cultivated around the world, which is believed to be descended from a small introduction to Europe of andigena potatoes that later adapted to longer day lengths. [Drawing: © CIP]
The tuber
As the potato plant grows, its compound leaves manufacture starch that is transferred to the ends of its underground stems (or stolons). The stems thicken to form a few or as many as 20 tubers close to the soil surface. The number of tubers that actually reach maturity depends on available moisture and soil nutrients. Tubers may vary in shape and size, and normally weigh up to 300 g (10.5 oz) each. 
At the end of the growing season, the plant's leaves and stems die down to the soil level and its new tubers detach from their stolons. The tubers then serve as a nutrient store that allows the plant to survive the cold and later regrow and reproduce. Each tuber has from two to as many as 10 buds (or "eyes"), arranged in a spiral pattern around its surface. The buds generate shoots that grow into new plants when conditions are again favourable.
Andean heritage
The potato's story begins about 8,000 years ago near Lake Titicaca, which sits at 3,800 m (12,500 ft) above sea level in the Andes mountain range of South America, on the border between Bolivia and Peru. There, research indicates, communities of hunters and gatherers who had first entered the South American continent at least 7,000 years before began domesticating wild potato plants that grew around the lake in abundance.
Some 200 species of wild potatoes are found in the Americas. But it was in the Central Andes that farmers succeeded in selecting and improving the first of what was to become, over the following millennia, a staggering range of tuber crops. In fact, what we know as "the potato" (Solanum species tuberosum) contains just a fragment of the genetic diversity found in the seven recognized potato species and 5,000 potato varieties still grown in the Andes.
Although Andean farmers cultivated many food crops - including tomatoes, beans and maize - their potato varieties proved particularly suited to the quechua or "valley" zone, which extends at altitudes of from 3,100 to 3,500 m (10,200 - 11,500 ft) along the slopes of the Central Andes (among Andean peoples, the quechua was known as the zone of "civilization"). But farmers also developed a frost-resistant potato species that survives on the alpine tundra of the puna zone at 4,300 m (14,100 ft).
The food security provided by maize and potato - consolidated by the development of irrigation and terracing - allowed the emergence around 500 AD of the Huari civilization in the highland Ayacucho basin. Around the same time, the city state of Tiahuanacu rose near Lake Titicaca, thanks largely to its sophisticated "raised field" technology - elevated soil beds lined with water canals - which produced potato yields estimated at 10 tonnes per hectare (4.4 tons per acre). At its height, around 800 AD, Tiahuanacu and neighbouring valleys are believed to have sustained a population of 500,000 or more.
Meteoric rise. The collapse of Huari and Tiahuanacu between 1000 and 1200 led to a period of turmoil that ended with the meteoric rise of the Incas in the Cusco valley around 1400. In less than 100 years, they created the largest state in pre-Columbian America, extending from present-day Argentina to Colombia.
The Incas adopted and improved the agricultural advances of previous highland cultures, and gave special importance to maize production. But the potato was fundamental to their empire's food security: in the Incas' vast network of state storehouses, potato - especially a freeze-dried potato product called chuño - was one of the main food items, used to feed officials, soldiers and corvée labourers and as an emergency stock after crop failures.
The Spanish invasion, in 1532, spelt the end of the Incas - but not of the potato. For, throughout Andean history, the potato - in all its forms - was profoundly a "people's food", playing a central role the Andean vision of the world (time, for example, was measured by how long it took to cook a pot of potatoes).
Farmers in some parts of the high Andes still measure land in topo, the area a family needs to grow their potato supply - a topo is larger at higher altitudes, where plots need to lie fallow for longer. They classify potatoes not only by species and variety, but by the ecological niche where the tubers grow best, and it is not unusual to find four or five species cultivated on a single, small plot of land.
Planting tubers remains the most important activity of the farming year near Lake Titicaca, where the potato is known as Mama Jatha, or mother of growth. The potato remains the seed of Andean society.
Dawn of agriculture
Incan myths relate that the Creator, Viracocha, caused the sun, moon and stars to emerge from Lake Titicaca. He also created agriculture when he sent his two sons to the human realm to study and classify the plants that grew there. They taught the people how to sow crops and how to use them so that they would never lack food.
Diffusion
The diffusion of the potato from the Andes to the rest of the globe reads like an adventure story, but it began with a tragedy. The Spanish conquest of Peru between 1532 and 1572 destroyed the Inca civilization and caused the deaths - from war, disease and despair - of at least half the population.
The conquistadores came in search of gold, but the real treasure they took back to Europe was Solanum tuberosum. The first evidence of potato growing in Europe dates from 1565, on Spain's Canary Islands. By 1573, potato was cultivated on the Spanish mainland. Soon, tubers were being sent around Europe as exotic gifts - from the Spanish court to the Pope in Rome, from Rome to the papal ambassador in Mons, and from there to a botanist in Vienna. Potatoes were grown in London in 1597 and reached France and the Netherlands soon after.
But once the plant had been added to botanical gardens and herbalists' encyclopaedias, interest waned. European aristocracy admired its flowers, but the tubers were considered fit only for pigs and the destitute. Superstitious peasants believed the potato was poisonous. At the same time, however, Europe's "Age of Discovery" had begun, and among the first to appreciate potatoes as food were sailors who took tubers to consume on ocean voyages. That is how the potato reached India, China and Japan early in the 17th century.
The potato also received an unusually warm welcome in Ireland, where it proved suited to the cool air and moist soils. Irish immigrants took the tuber - and the name, "Irish potato" - to North America in the early 1700s.
Long summer days. The widespread adoption of the potato as a food crop in the northern hemisphere was delayed not only by entrenched eating habits, but by the challenge of adapting a plant grown for millennia in the Andes to the north's temperate climate. Only a drop of the rich potato gene pool had left South America, and it took 150 years before varieties suited to long summer days began to appear.
Those varieties arrived at a crucial time. In the 1770s, much of continental Europe was devastated by famines, and the potato's value as a food security crop was suddenly recognized. Prussia's Frederick the Great ordered his subjects to grow potatoes as insurance against cereal crop failure, while the French scientist Parmentier succeeded in having the potato declared "edible" (around the same time, on the other side of the Atlantic, US President Thomas Jefferson served french fries to White House guests).
After initial hesitation, European farmers - even those in Russia, where the potato was called the "devil's apple" - began growing potatoes on a large scale. Potato became Europe's food reserve during the Napoleonic wars, and by 1815 it had become a staple crop across northern Europe. By then, the Industrial Revolution was transforming agrarian society in the United Kingdom, displacing millions of rural people into crowded cities. In the new urban environment, the potato became the first modern "convenience food" - energy-rich, nutritious, easy to grow on small plots, cheap to purchase, and ready to cook without expensive processing.
Increased potato consumption during the 19th century is credited with helping to reduce the scourge of diseases such as scurvy and measles, contributing to higher birth rates and the population explosion in Europe, the US and the British Empire.
"Potato famine". But the potato's success proved a double-edged sword. For the tubers that were being cloned and cultivated across North America and Europe belonged to a few, genetically similar varieties. That meant they were highly vulnerable: a pest or disease that struck one plant could spread quickly to the rest.
The first sign of impending disaster came in 1844-45, when a mould disease, late blight, ravaged potato fields across continental Europe, from Belgium to Russia. But the worst came in Ireland, where potato supplied 80 percent of calorie intake. Between 1845 and 1848, late blight destroyed three potato crops, leading to famine that caused the deaths of one million people.
The Irish catastrophe led to concerted efforts to develop more productive and disease-resistant varieties. Breeders in Europe and North America, drawing on new potato germplasm from Chile, produced many of the modern varieties that laid the foundation for massive potato production in both regions for most of the 20th century.
Meanwhile, European colonialism and emigration were taking the potato to all corners of the globe. Colonial governors, missionaries and settlers introduced potato growing to the floodplains of Bengal and Egypt's Nile delta, the Atlas Mountains of Morocco, and the Jos plateau in Nigeria. Emigrant farmers took the potato to Australia and even to South America, establishing the potato in Argentina and Brazil.
In the Asian heartland, the tuber moved along more ancient routes, finding its way from the Caucasus to Turkey's Anatolian plateau, from Russia to western China, and from China to the Korean Peninsula. In the mountain valleys of Tajikistan, some potato types have been grown long enough to be considered "old local varieties".
The 20th century saw the potato finally emerge as a truly global food. The Soviet Union's annual potato harvest reached 100 million tonnes. In the years following the Second World War, huge areas of arable land in Germany and Britain were dedicated to potato, and countries like Belarus and Poland produced - and still do - more potatoes than cereals.
The potato came into its own as a snack food. The invention in the 1920s of the mechanical potato peeler helped make potato crisps America's top-selling snack. A restaurant chain founded by the McDonald brothers in the US in 1957 spent millions of dollars to "perfect the french fry". A Canadian firm, McCain, that began making frozen french fries in 1957, expanded to open 55 production facilities on six continents and now supplies one third of all french fried potatoes produced internationally.
Exploding demand. From the 1960s, cultivation potato of began expanding in the developing world. In India and China alone, total production rose from 16 million tonnes in 1960 to almost 100 million in 2006. In Bangladesh, potato has become a valuable winter cash crop, while potato farmers in southeast Asia have tapped into exploding demand from food industries. In sub-Saharan Africa, potato is a preferred food in many urban areas, and an important crop in the highlands of Cameroon, Kenya, Malawi and Rwanda.
The potato has an extraordinarily rich past, and a bright future. While production in Europe - the potato's "second home" for four centuries - is declining, the potato has ample room for expansion in the developing world, where its consumption is less than quarter that of developed countries.
Today in mountainous Lesotho, many farmers are shifting from maize to potato, assisted by an FAO project for production of virus-free seed tubers. In China, agriculture experts have proposed that potato become the major food crop on 60 percent of the country's arable land, and say a staggering 30 percent increase in potato yields is within reach.
And in Andes, where it all began, the Government of Peru created in July 2008 a national register of Peruvian native potato varieties, to help conserve the country's rich potato heritage. That genetic diversity, the building blocks of new varieties adapted to the world's evolving needs, will help write future chapters in the story of Solanum tuberosum.
Papa, patata, potato...
While the Incas called it papa (as do modern-day Latin Americans), Spaniards called the potato patata, apparently confusing it with another New World crop, the sweet potato (known as batata). In 1797, the English herbalist Gerard referred to the sweet potato as "common potatoes", and for many years S. tuberosum was known as the "Virginia potato" or "Irish potato" before finally displacing batata as the potato.
Cultivation
Potato is grown in more than 100 countries, under temperate, subtropical and tropical conditions. It is essentially a "cool weather crop", with temperature being the main limiting factor on production: tuber growth is sharply inhibited in temperatures below 10°C (50°F) and above 30°C (86°F), while optimum yields are obtained where mean daily temperatures are in the 18 to 20°C (64 to 68°F) range.
For that reason, potato is planted in early spring in temperate zones and late winter in warmer regions, and grown during the coolest months of the year in hot tropical climates. In some sub-tropical highlands, mild temperatures and high solar radiation allow farmers to grow potatoes throughout the year, and harvest tubers within 90 days of planting (in temperate climates, such in northern Europe, it can take up to 150 days).
The potato is a very accommodating and adaptable plant, and will produce well without ideal soil and growing conditions. However, it is also subject to number of pests and diseases. To prevent the build-up of pathogens in the soil, farmers avoid growing potato on the same land from year to year. Instead, they grow potato in rotations of three or more years, alternating with other, dissimilar crops, such as maize, beans and alfalfa. Crops susceptible to the same pathogens as potato (e.g. tomato) are avoided in order to break potato pests' development cycle.
With good agricultural practices, including irrigation when necessary, a hectare of potato in the temperate climates of northern Europe and North America can yield more than 40 tonnes of fresh tubers within four months of planting. In most developing countries, however, average yields are much lower - ranging from as little as five tonnes to 25 tonnes - owing to lack of high quality seed and improved cultivars, lower rates of fertilizer use and irrigation, and pest and disease problems.
Soil and land preparation
The potato can be grown almost on any type of soil, except saline and alkaline soils. Naturally loose soils, which offer the least resistance to enlargement of the tubers, are preferred, and loamy and sandy loam soils that are rich in organic matter, with good drainage and aeration, are the most suitable. Soil with a pH range of 5.2-6.4 is considered ideal.
Growing potatoes involves extensive ground preparation. The soil needs to be harrowed until completely free of weed roots. In most cases, three ploughings, along with frequent harrowing and rolling, are needed before the soil reaches a suitable condition: soft, well-drained and well-aerated.
Planting
The potato crop is usually grown not from seed but from "seed potatoes" - small tubers or pieces of tuber sown to a depth of 5 to 10 cm. Purity of the cultivars and healthy seed tubers are essential for a successful crop. Tuber seed should be disease-free, well-sprouted and from 30 to 40 g each in weight. Use of good quality commercial seed can increase yields by 30 to 50 percent, compared to farmers' own seed, but expected profits must offset the higher cost.
The planting density of a row of potatoes depends on the size of the tubers chosen, while the inter-row spacing must allow for ridging of the crop (see below). Usually, about two tonnes of seed potatoes are sown per hectare. For rainfed production in dry areas, planting on flat soil gives higher yields (thanks to better soil water conservation), while irrigated crops are mainly grown on ridges.
Stages in crop development
1. Planted seed tuber
2. Vegetative growth
3. Tuber initiation
4. Tuber bulking
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Crop care
During the development of the potato canopy, which takes about four weeks, weeds must be controlled in order to give the crop a "competitive advantage". If the weeds are large, they must be removed before ridging operations begin. Ridging (or "earthing up") consists of mounding the soil from between the rows around the main stem of the potato plant. Ridging keeps the plants upright and the soil loose, prevents insect pests such a tuber moth from reaching the tubers; and helps prevent the growth of weeds.
After earthing up, weeds between the growing plants and at the top of the ridge are removed mechanically or using herbicides. Ridging should be done two or three times at an interval of 15 to 20 days. The first should be done when the plants are about 15-25 cm high; the second is often done to cover the growing tubers.
Manuring and fertilization
The use of chemical fertilizer depends on the level of available soil nutrients - volcanic soils, for example, are typically deficient in phosphorus - and in irrigated commercial production, fertilizer requirements are relatively high. However, potato can benefit from application of organic manure at the start of a new rotation - it provides a good nutrient balance and maintains the structure to the soil. Crop fertilization requirements need to be correctly estimated according to the expected yield, the potential of the variety and the intended use of the harvested crop.
Water supply
The soil moisture content must be maintained at a relatively high level. For best yields, a 120 to 150 day crop requires from 500 to 700 mm (20 to 27.5 inches) of water. In general, water deficits in the middle to late part of the growing period tend to reduce yield more than those in the early part. Where supply is limited, water is directed towards maximizing yield per hectare rather than being applied over a larger area.
Because the potato has a shallow root system, yield response to frequent irrigation is considerable, and very high yields are obtained with mechanized sprinkler systems that replenish evapotranspiration losses every one or two days. Under irrigation in temperate and subtropical climates, a crop of about 120 days can produce yields of 25 to 35 tonnes/ha (11 to 15.6 tons per acre), falling to 15 to 25 tonnes/ha (6.6 to 15.6 tons per acre) in tropical areas.
Pests and diseases
Against diseases, a few basic precautions – crop rotation, using tolerant varieties and healthy, certified seed tubers - can help avoid great losses. There is no chemical control for bacterial and viral diseases but they can be controlled by regular monitoring (and when necessary, spraying) of their aphid vectors. The severity of fungal diseases such as late blight depends, after the first infection, mainly on the weather - persistence of favourable conditions, without chemical spraying, can quickly spread the disease.
Insect pests can wreak havoc in a potato patch. Recommended control measures include regular monitoring and steps to protect the pests' natural enemies. Even damage caused by the Colorado Potato Beetle, a major pest, can be reduced by destroying beetles, eggs and larvae that appear early in the season, while sanitation, crop rotations and use of resistant potato varieties help prevent the spread of nematodes.
Harvesting
Yellowing of the potato plant's leaves and easy separation of the tubers from their stolons indicate that the crop has reached maturity. If the potatoes are to be stored rather than consumed immediately, they are left in the soil to allow their skins to thicken - thick skins prevent storage diseases and shrinkage due to water loss. However, leaving tubers for too long in the ground increases their exposure to a fungal incrustation called black scurf.
To facilitate harvesting, the potato vines should be removed two weeks before the potatoes are dug up. Depending on the scale of production, potatoes are harvested using a spading fork, a plough or commercial potato harvesters that unearth the plant and shake or blow the soil from the tubers. During harvesting, it is important to avoid bruising or other injury, which provide entry points for storage diseases.
Storage
Since the newly harvested tubers are living tissue – and therefore subject to deterioration - proper storage is essential, both to prevent post-harvest losses of potatoes destined for fresh consumption or processing, and to guarantee an adequate supply of seed tubers for the next cropping season.
For ware and processing potatoes, storage aims at preventing "greening" (the build up of chlorophyll beneath the peel, which is associated with solanine, a potentially toxic alkaloid) and losses in weight and quality. The tubers should be kept at a temperature of 6 to 8°C degrees, in a dark, well-ventilated environment with high relative humidity (85 to 90 percent). Seed tubers are stored, instead, under diffused light in order to maintain their germination capacity and encourage development of vigorous sprouts. In regions, such as northern Europe, with only one cropping season and where storage of tubers from one season to the next is difficult without the use of costly refrigeration, off-season planting may offer a solution.
Potato varieties
Although the potato cultivated worldwide belongs to just one botanical species, Solanum tuberosum, the tubers come in thousands of varieties with great differences in size, shape, colour, texture, cooking characteristics and taste. Here's a small sample of potato diversity.
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1. Atahualpa
Bred in Peru, a high yielding variety good for both baking and frying | 2. Nicola
Widely grown Dutch variety, one of the best for boiling, also good in salads | 3. Russet Burbank
The classic American potato, excellent for baking and french fries |
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4. Lapin puikula
Grown in Finland for centuries, in fields bathed in the light of the midnight sun | 5. Yukon Gold
A Canadian potato with buttery yellow flesh suitable for frying, boiling, mashing | 6. Tubira
CIP-bred variety grown in West Africa. White flesh, pink skin, and good yielding |
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7. Vitelotte
A gourmet French variety prized for its deep blue skin and violet flesh | 8. Royal Jersey
From the Isle of Jersey: the only UK vegetable with an EU designation-of-origin | 9. Kipfler
Hails from Germany. Elongated with cream flesh, popular in salads |
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10. Papa colorada
Brought to the Canary Islands by passing Spanish ships in 1567 | 11. Maris Bard
Bred in the UK, a white variety with a soft waxy texture good for boiling | 12. Désirée
Red-skinned, with yellow flesh and a distinctive flavour. |
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13. Spunta
Another popular commercial tuber, good for boiling and roasting | 14. Mondial
A Dutch potato with smooth good looks. Boils and mashes well | 15. Unknown
From Chile, one of more than 5 000 native varieties still grown in the Andes |
Uses of potato
Once harvested, potatoes are used for a variety of purposes, and not only as a vegetable for cooking at home. In fact, it is likely that less than 50 percent of potatoes grown worldwide are consumed fresh. The rest are processed into potato food products and food ingredients, fed to cattle, pigs and chickens, processed into starch for industry, and re-used as seed tubers for growing the next season's potato crop.
Food uses: fresh, "frozen", dehydrated
FAO estimates that just over two-thirds of the 320 million tonnes of potatoes produced in 2005 were consumed by people as food, in one form or another. Home-grown or purchased in markets, fresh potatoes are baked, boiled or fried and used in a staggering range of recipes: mashed potatoes, potato pancakes, potato dumplings, twice-baked potatoes, potato soup, potato salad and potatoes au gratin, to name a few.
But global consumption of potato as food is shifting from fresh potatoes to added-value, processed food products. One of the main items in that category goes by the unappetizing name of frozen potatoes, but includes most of the french fries ("chips" in the UK) served in restaurants and fast food chains worldwide. The production process is fairly simple: peeled potatoes are shot through cutting blades, parboiled, air dried, par fried, frozen and packaged. The world's appetite for factory-made french fries has been put at more than 11 million tonnes a year.
Another processed product, the potato crisp ("chips" in the US), is the long-standing king of snack foods in many developed countries. Made from thin slices of deep-fried or baked potato, they come in a variety of flavours - from simple salted to "gourmet" varieties tasting of roast beef and Thai chili. Some crisps are produced using a dough made from dehydrated potato flakes.
Dehydrated potato flakes and granules are made by drying a mash of cooked potatoes to a moisture level of 5 to 8 percent. Flakes are used in retail mashed potato products, as ingredients in snacks, and even as food aid: potato flakes have been distributed as part of US international food assistance to more than 600,000 people. Another dehydrated product, potato flour, is ground from cooked, whole potatoes and retains a distinct potato taste. Gluten-free and rich in starch, potato flour is used by the food industry to bind meat mixtures and thicken gravies and soups.
Modern starch processing can retrieve as much as 96 per cent of the starch found in raw potatoes. A fine, tasteless powder with "excellent mouth-feel", potato starch provides higher viscosity than wheat and maize starches, and delivers a more tasty product. It is used as a thickener for sauces and stews, and as a binding agent in cake mixes, dough, biscuits and ice-cream.
Finally, in eastern Europe and Scandinavia, crushed potatoes are heated to convert their starch to fermentable sugars that are used in the distillation of alcoholic beverages such as vodka and akvavit.
Non-food uses: Glue, animal feed and fuel-grade ethanol
Potato starch is also widely used by the pharmaceutical, textile, wood and paper industries as an adhesive, binder, texture agent and filler, and by oil drilling firms to wash boreholes. Potato starch is a 100% biodegradable substitute for polystyrene and other plastics and used, for example, in disposable plates, dishes and knives.
Potato peel and other "zero value" wastes from potato processing are rich in starch that can be liquefied and fermented to produce fuel-grade ethanol. A study in Canada's potato-growing province of New Brunswick estimated that 44,000 tonnes of processing waste could produce 4 to 5 million litres of ethanol.
One of the first widespread uses of the potato in Europe was as farm animal feed. In the Russian Federation and other east European countries, as much as half of the potato harvest is still used for that purpose. Cattle can be fed up to 20 kg of raw potatoes a day, while pigs fatten quickly on a daily diet of 6 kg of boiled potatoes. Chopped up and added to silage, the tubers cook in the heat of fermentation.
Seed potatoes: renewing the cycle...
Unlike other major field crops, potatoes are reproduced vegetatively, from other potatoes. Therefore, a part of each year's crop - ranging from 5 to 15 percent, depending on the quality of the harvested tubers - is set aside for re-use in the next planting season. Most farmers in developing countries select and store their own seed tubers. In developed countries, farmers are more likely to purchase disease-free "certified seed" from dedicated suppliers. More than 13 per cent of France's potato growing area is used to produce seed potatoes, and the Netherlands exports some 700 000 tonnes of certified seed a year.
Potato calculations
In 2005, FAO estimates, the world produced 314,375,535 tonnes of potatoes and consumed 218,129,000 tonnes as food. How did it arrive at that figure? FAO's statistics division has developed a simple formula for the calculation: consumption equals production, imports and beginning stocks, minus exports, potatoes used as feed and seed, waste, other non-food uses and ending stocks.
