Worldwide onion production amounts to about 64 million tonnes of bulbs from 3.45 million hectares. The main onions producers are China, India, USA, Pakistan, Turkey, Russia, Iran, Brazil, Mexico and Spain. Onions demand is on the rise both as fresh food and as a processed product, and a lower availability of agricultural land for expansion areas indicates that production needs to be improved. It will be crucial, for better bulb performance, to have access to an adequate water resource which includes more efficient use of poor water resources and the introduction of expensive fertilizers.
Onions are grown in about 175 countries with temperate, subtropical and tropical climates. The onion is basically a cold season crop. Cultivation is discreetly sensitive to soil salinity, resulting in a decrease in production due to the different levels of critical salinity of the soil. Usually the crop is sown in a controlled environment and transplanted after 30 to 35 days. Direct sowing is also performed in the field. The adoption of drop irrigation and fertilization-irrigation on onions has proved to be technically feasible, economically viable and beneficial both in developed and developing world regions. Drop irrigation in many different agro-ecological situations has led to an increase in production (15-20 tonnes / ha) as well as to water saving (from 30 to 40%); a better saving of fertilizers (25%) and bulb quality compared to conventional sprinkling and targeted irrigation. In India and the Philippines, drop irrigated onions increased production from 66 to 80% compared to targeted irrigation with a one year ROI (return of investment) period.
As far as large bulb productions are concerned, seasonal crop water requirements have been estimated between 400 and 775 mm/ha in a range of climatic conditions and variable length of the growing season with a daily evaporation-transpiration rate between 5 and 7.25 mm/day. Irrigation scheduling, using tensiometers at 20-25 centibars of soil water tension, allows optimum use of water, fertilizer and energy. Onions are a great consumer of nutrients. The root system is shallow and fibrous, therefore fertilization-irrigation is recommended for increased nutrients availability and efficiency of use. The purpose of the fertilization-irrigation program is to bridge the gap between crop demand and supply. The per hectare nutrients requirement for drop irrigated onions is relatively high: from 175 to 400 kg N, 75 to 150 kg P2O5, 200 to 300 kg K2O, 20 to 40 kg MgO. The nutrients quantity absorbed by onions was estimated to be worth 160 kg N, 76 kg P2O5, 115 kg K2O, 16.6 kg MgO and 128 kg CaO/ha for a crop production equal to 40 tonnes of bulbs per hectare. Other optimum management techniques include dirt covering, pest and disease protection, weeds removal, harvest and post harvest operations for minimizing losses.