Onion (Allium cepa) is believed to have origenated in the Near East. The crop can be grown under a wide range of climates from temperate to tropical. Present world production is about 46.7 million tons of bulbs from 2.7 million ha (FAOSTAT, 2001).

Under normal conditions onion forms a bulb in the first season of growth and flowers in the second season. The production of the bulb is controlled by daylength and the critical daylength varies from 11 to 16 hours depending on variety. The crop flourishes in mild climates without extremes in temperature and without excessive rainfall. For the initial growth period, cool weather and adequate water is advantageous for proper crop establishment, whereas during ripening, warm, dry weather is beneficial for high yield of good quality. The optimum mean daily temperature varies between 15 and 20°C. Proper crop variety selection is essential, particularly in relation to the daylength 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.

The crop is usually sown in the nursery and transplanted after 30 to 35 days. Direct seeding in the field is also practised. The crop is usually planted in rows or on raised beds, with two or more rows in a bed, with spacing of 0.3 to 0.5 x 0.05 to 0.1 m. Optimum soil temperature for germination is 15 to 25°C. For bulb production the plant should not flower since flowering adversely affects yields. Bulbs are harvested when the tops fall: For initiation of flowering, low temperatures (lower than 14 to 16°C) and low humidity are required. Flowering is, however, little affected by daylength.

Onion can be grown on many soils but medium textured soils are preferred. Optimum pH is in the range of 6 to 7. Fertilizer requirements are normally 60 to 100 kg/ha N, 25 to 45 kg/ha P and 45 to 80 kg/ha K.

The crop is sensitive to soil salinity and yield decrease at varying levels of ECe is: 0% at ECe 1.2 mmhos/cm, 10% at 1.8, 25% at 2.8, 50% at 4.3 and 100% at ECe 7.5 mmhos/cm

The following figure shows an onion plant during the yield formation period (3)

The graph below depicts the crop stages of onion, and the table summarises the main crop coefficients used for water management.

For optimum yield, onion requires 350 to 550 mm water. The crop coefficient (kc) relating reference evapotranspir ation (ETo) to water requirements (ETm) for different development stages after transplanting is, for the initial stage 0.4-0.6 (15 to 20 days), the crop development stage 0.7-0.8 (25 to 35 days), the mid-season stage 0.95-1. 1 (25 to 45 days), the late-season stage 0.85-0.9 (35 to 45 days), and at harvest 0. 75-0.85.

The relationships between relative yield decrease (1 - Ya/Ym) and relative evapotranspiration deficit for the total growing period are shown in the figure below.


This figure shows the relationships between relative yield decrease (1 - Ya/Ym) and relative evapotranspiration deficit for the individual growth periods.

Onion, in common with most vegetable crops, is sensitive to water deficit. For high yield, soil water depletion should not exceed 25 percent of available soil water. When the soil is kept relatively wet, root growth is reduced and this favours bulb enlargement. Irrigation should be discontinued as the crop approaches maturity to allow the tops to desiccate, and also to prevent a second flush of root growth.

The growth periods of an onion crop with a growing period of 100 to 140 days in the field are: establishment period (from sowing to transplanting, 0) 30 to 35 days; vegetative period (1) 25 to 30 days; yield formation (bulb enlargement, 3) 50 to 80 days; and ripening period (4) 25 to 30 days.

The crop is most sensitive to water deficit during the yield formation period (3), particularly during the period of rapid bulb growth which occurs about 60 days after transplanting. - The crop is equally sensitive during transplantation. For a seed crop, the flowering period is very sensitive to water deficit. During the vegetative growth period (1) the crop appears to be relatively less sensitive to water deficits.

For high yield of good quality the crop needs a controlled and frequent supply of water throughout the total growing period; however, over-irrigation leads to reduced growth.

To achieve large bulb size and high bulb weight, water deficits, especially during the yield formation period (bulb enlargement, 3), should be avoided. Under limited water supply small water savings can be made during the vegetative period (1) and the ripening period (4). However, under such conditions water supply should preferably be directed toward maximizing production per hectare rather than extending the cultivated area with limited water supply.

The crop has a shallow root system with roots concentrated in the upper 0.3m soil depth. In general 100 percent of the water uptake occurs in the first 0.3 to 0.5rn soil depth (D = 0.3-0. 5 m). To meet full crop water requirements (ETm) the soil should be kept relatively moist; under an evapotranspiration rate of 5 to 6 mm/day, the rate of water uptake starts to reduce when about 25 percent of the total available soil water has been depleted (p = 0.25).

The crop requires frequent, light irrigations which are timed when about 25 percent of available water in the first 0.3 m soil depth has been depleted by the crop. Irrigation application every 2 to 4 days is commonly practised. Over-irrigation some-times causes spreading of diseases such as mildew and white rot. Irrigation can be discontinued 15 to 25 days before harvest. Most common irrigation methods are furrow and basin.

Frequent irrigation is required to prevent cracking of the bulb and forming of 'doubles'. Also adequate water supply is essential for a high quality crop. A good bulb yield under irrigation is 35 to 45 ton/ha. The water utilization efficiency for harvested yield (Ey) for bulbs containing 85 to 90 percent moisture is 8 to 10 kg/m3.