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= Oxalis tuberosa =

Oxalis tuberosa is an annual plant that overwinters as underground stem tubers. These tubers are known as oca, oka, or New Zealand Yam. The plant was brought into cultivation in the central and southern Andes for its tubers, which are used as a root vegetable. The plant is not known in the wild, but populations of wild Oxalis species that bear smaller tubers are known from four areas of the central Andean region. Introduced to Europe in 1830 as a competitor to the potato and to New Zealand as early as 1860, it has become popular in that country under the name New Zealand yam and is now a common table vegetable. .

Edibility
The stem tubers of oca form in the ground in the autumn. These are commonly boiled before eating although they can also be eaten raw. The leaves and young shoots can be eaten as a green vegetable.

The flavour is slightly tangy, and texture ranges from crunchy (like a carrot) when undercooked, to starchy or mealy when fully cooked. Though the original Andean varieties are widely variable in colour from purple to yellow, the usual New Zealand variety is a fleshy pink.

Oca can be boiled, baked or fried. In the Andes it is used in stews and soups, served like potatoes or can be served as a sweet. Oca is eaten raw in Mexico with salt, lemon, and hot pepper.

Oca are fairly high in oxalates, concentrated in the skin. The traditional Andean preparation methods were geared towards reducing the oxalate level of the harvested vegetable. This is done by exposure to sunlight which decreases the organic acid content and thereby increases the sweet taste of the oca. The bioavailability of oxalate appears to be similar to spinach. Recent oca cultivars have a lower oxalate content, and have also been selected for more flexibility in day lengths.

Cultivation
Oca is one of the important staple crops of the Andean highlands, second only to the potato due to its easy propagation, and tolerance for poor soil, high altitude and harsh climates.

Distribution
Outside of the Andean region Oxalis tuberosa is cultivated commercially in Mexico and New Zealand. Compared to potatoes, which are now cultivated in 130 countries around the world, these tubers are essentially still unknown outside of the Andean region.

Propagation
Oca is usually propagated vegetatively by planting of whole tubers. Further is propagation by planting of cuttings, which is sometimes applied, or by seed propagation, which is never used in practice.

Climate requirements
Oca need a long growing season, and are day length dependent, forming tubers when the day length shortens in autumn. In areas with harsh winter climates, early frosts may cut back the foliage before the tubers have a chance to form. In tropical areas where the days are unchanging in length, oca will not set a crop successfully.

Soil requirements
The crop grows with very low production inputs and in generall on plots of marginal soil quality and tolerates acidities between about pH 5.3 and 7.8.

Cropping factors
The cultural practice is similar to potatoes. Planting is done in rows or hills 80-100 cm apart with plants, spaced 40-60 cm apart in the rows. .). Monoculture predominates, but interplanting of several tuber species in one field is common in Andean production. In many instances Andean farmers will allow a mixture containing two or three tuber crops to be planted in one field. Often this mixture consists of several different clones of each species. Such mixed fields may later be sorted into tuber types during harvest or before cooking. Mixed planting with legumes or grain crops is occasionally practiced.

Pest and Diseases
Crops in the Andes are usually infected with viruses, causing chronic yield depression. Adequate techniques to remove viruses have to be applied before the varieties can be used outside the Andean region. Cultivation is also constrained by the oca weevil (Ariodistus tuberculatus), Andean potato weevil (Premnotrypes spp) and the ulluco weevil (Cylydrorhinus spp), beetles often destroying entire crops. Further notable pests are nematodes.

Yields
Yields vary with the cultural method. Annals from Andean countries report about 7-10 tons per hectare for Oxalis tuberosa production. But with adequate inputs and virus free propagation material, oca production can range from 35 to 55 tons per hectare.

Limitations
As already mentioned, viruses, day length restrictions and the precence of oxalates can be considered as limiting factors. Scientists work with specific breeding, selection and virus cleaning programs on these purposes.

Agricultural potential
Potential distribution to other suitable ecogeographical zones of for example Asia and Africa is possible. The cultivation and use of Oxalis tuberosa in New Zealand already indicates a wider utilization and agricultural interest than has been previously recognized.

Alternative names

 * Apilla in Bolivia
 * Hibia in Colombia
 * Yam  in many other places, such as Polynesia and New Zealand.

The Dioscorea vegetables known elsewhere as yams are generally very uncommon in these countries. Yams are commonly confused with Sweet potatoes, although they are not closely related. In New Zealand Sweet potatoes are commonly referred to by their Māori name of kūmara.