Onion (Allium cepa L.), the most used flavoring vegetable for centuries, is believed to have originated in the tropical central or Western Asia. Onions come in three colors – yellow, red, and white. Approximately 88% of the crop is devoted to yellow onion production, with about 7% red onions and 5% white onions.
The world production is about 64 million tons of bulbs from 3.45 million ha (FAO, 2007). Major onion producing countries include China, India, USA, Pakistan, Turkey, Russia, Iran, Brazil, Mexico & Spain. China is the biggest onion producer while 44.5% of all onions are harvested in China and India. Productivity is highest in Ireland at 58 tons/ha. Demand for onion is growing as both fresh and processed food and a decreasing availability of land for area expansion means that yields will have to be improved. Critical to achieving improved bulb yields will be access to an adequate water supply, including more efficient use of scarce water and costly fertilizer inputs.
Onion is grown in about 175 countries under temperate, subtropical and tropical conditions. The onion is basically a cool season crop. Yields are affected significantly by temperature and optimum ambient temperature are for seedling growth 20 – 25 ºC, vegetative growth 13 – 24 ºC, before bulbing 15 – 21 ºC and for bulb development 20 – 25 ºC. Day length for short day varieties is less than 12 hours, for intermediate day varieties day length is between 12 to 14 hours and for long day varieties it is more than 14 hours of light for bulbing.
Onion requires fertile, light, deep friable and well-drained soils. Furthermore, suitable soils can be loamy & alluvial with a pH of 5.8 to 6.5. Compacted soils (> 1.6 to 1.7 g/cm3) affect bulb enlargement. The crop is moderately sensitive to soil salinity with yield decrease at different levels of critical soil salinity (ECe). The crop is usually sown in the nursery and transplanted after 30 to 35 days. Direct seeding in the field is also practiced. The crop is usually planted in rows or on raised beds with two or more rows in a bed, with spacing of between rows and 0.05 m to 0.10 m between plants. Optimum soil temperature for germination is 15 to 25 °C. For bulb production the plant should not flower since flowering adversely affects yields. Cultivation during the growing period must avoid damage to roots and bulbs, and in temperate climates ridges are earthed up to avoid greening of bulbs. Proper crop variety selection is essential, particularly in relation to the day length requirements; for example, a long day temperate variety in tropical zones with short days will produce vegetative growth only without forming the bulb. The length of the growing period varies with climate but in general 130 to 175 days are required from sowing to harvest.
Adoption of drip irrigation and fertigation in onion proved to be technically feasible and economically viable and beneficial in many ways both in developed and developing regions of the world. Drip irrigation in many diverse agro-ecological situations registered higher yield (15 to 20 tons/ha) besides saving in water (30 to 40%), saving of fertilizers (25%) and improving grade of bulbs in comparison to conventional furrow & overhead irrigation methods. Under Indian & Philippines conditions drip irrigated onion registered 66 to 80% higher yield over conventional furrow method with a Net Present Value (NPV) of 3064 USD/ha & 3725 US$/ha, respectively, and a payback period of one year.
For high bulb yields, the seasonal crop water requirements were estimated to be 400 to 775 mm under range of climatic conditions and varying length of growing season with a daily evapotranspiration rate of 5 to 7.25 mm/day. Irrigation scheduling using tensiometers at a soil water tension of 20 – 25 centibars enabled efficient use of water, fertilizer and energy inputs. Onion is a heavy feeder of nutrients. Root system is shallow and fibrous, hence fertigation is recommended for higher nutrient availability and use efficiency. The aim of the fertigation program is to cover the difference between crop demand and supply. The nutrient requirements of drip irrigated potato per hectare are relatively high: 175 to 400 kg N, 75 to 150 kg P2O5, 200 to 300 kg K2O, 20 to 40 kg MgO. The nutrient uptake by onion was estimated to be 160 kg N, 76 kg P2O5, 115 kg K2O, 16.6 kg MgO and 128 kg CaO/ha for a crop yield of 40 tons of bulb yield/ha. Other best management practices include earthing up, protection of crop from pests and diseases, weed management, harvesting and post harvesting operations to minimize losses.