The onion is cultivated all around the world and it is one of the oldest vegetables. The world production is of 67 millions tons of bulbs (FAO, 2008). The countries of higher production of the world include United States, India, China, Pakistan, Turkish, Russia, Iran, Spain and Mexico. The onion is cultivated in 175 countries in mild, tropical and subtropical climatic conditions. The onion is basically a crop of fresh season.
Climate and soil requirements
The production depends on the temperature and the optimum environment to the Blooming is of 25ºC, while the vegetative growing varies from 15ºC to 25ºC. The optimum temperature before the bulbs growing is from 15ºC to 20ºC and to the development of these from 20ºC to 25ºC.
It prefers deep soils, rich in organic material, of average consistence and not calcareous. It is not convenient to cultivate onions in sandy soils.
It is affected by an excess of damp and the sudden changes can produce the cracking of the bulbs. Once the plant has started the growing, the dump of the soil should be maintained above the 50% in the first 30cm. of the soil. The excess of dump at the end of the growing affects negatively its preservation. It is recommended the soil to have good dump retention in the 15-25 cm. of the soil.
The onions must be planted in flat and high surfaces of 15 bulbs by meter and a separation of 15-20 cm between the lines of the same surface. The size of the bulb depends on the density of the plants, although too much density produces foliar illnesses.
Irrigation must be done after plantation. The onion requires frequent application of water and the best system is drip irrigation, because sprinkling cleans the fungicides of the leaves and increases the risk of foliar illnesses.
The commonest is to divide the irrigation in two stages: germination and development. The number of risks is higher to the second sows, due to its vegetation is in spring or summer, while the sows of the end of summer and autumn are developed during winter and spring. The hydric shortage in the last period of vegetation favours the preservation of the bulb, but gives a sourer taste. The irrigation must be stopped from 15 to 30 days before the harvest.