Origin of Onion
Onion (Allium cepa L.) Family: Alliaceae) is by far the most important bulb crop.
Its origin is not definitely known but it is believed to have come from the eastern Mediterranean region and central Asia including Iran and Pakistan.
Onions were also cultivated in Egypt at least 4800 years ago as evidenced by their pictures carved on Egyptian monuments.
Onion is not known in the wild state. Onions were unknown to the American Indians and were taken to America by the Spaniards.
Now the cultivation has been expended to all parts of the world.
Relatively high levels of organic materials are required for optimum growth and development; sandy loam soils are generally suitable.
A pH of 5.8-6.8 is considered favorable but crops may also be grown on relatively acid peat soils.
They succeed best in a mild climate without excessive rainfall or great extremes of heat and cold.
Onions are not suited to regions with heavy rainfall in the lowland humid tropics.
Cool conditions with adequate moisture supply are most suitable for early growth.
Also, warm, drier conditions are necessary for maturation, harvesting, and curing.
Thus, a relatively high temperature, as well as a long photoperiod are desirable for bulb formation.
Under a very short photoperiod, the plants form new leaves indefinitely without bulbing.
Onions generally require 14-16hrs of day length for bulb formation.
Cultivars have been selected which will form bulbs at low elevations but the highest yields are normally obtained from crops grown at elevations above 1000m.
The Botany of Onion
Onion is a biennial crop, storing food in the bulb during the first season and flowering in the second season.
The root system of onions is shallow and fibrous.
The leaf consists of two main parts – a sheathing leaf base and a hollow, linear, cylindrical or flattened blade.
Both are being separated by a short membranous ligule.
The outer leaf bases are thin, fibrous and dry, forming a protective covering around the inner fleshing ones, which are laden with food.
A very short flattened stem is produced at the base of the plant which increases in diameter as growth continues.
New leaves are produced by the apical meristem.
Towards the end of the first season’s growth, the apical meristem or shoot apex grows to produce a leafless flowering stalk, the ‘scape’.
It is 0.6-0.9m tall, and pushes through the center of the pseudostem formed by the sheathing leaf bases.
The scape is hollow, cylindrical, swollen near the middle and tapering towards the end.
The developing inflorescence is protected by a membranous spathe, which at maturity splits to form two to three persistent bracts.
Numerous greenish-white flowers are arranged in an umbellate cymose manner.
The flowers are protandrous; the inner stamens shed their pollens first, followed by the outer stamens.
The whole process takes 2-3 days, after which the style reaches its full length and becomes receptive.
The fruit is a globular capsule and the seeds are smooth, black and wrinkled when dry.
Cultural Practices of Onions
Seeds are down either directly in the field or in seedbeds from where the young plants are transplanted to the field.
This is done after 6-8 weeks when seedlings are 8-12cm tall.
The normal spacing is 7.5-10cm apart in rows 30-40cm apart.
Seeds can be sown in drills – 30cm apart and thinned to 10-15cm between plants, depending on the vigor of the cultivars.
Bulbs from the previous season are sometimes replanted as setts.
A good size onion is cut horizontally and the bottom half with roots is planted in a well-prepared seedbed.
Three crops of onion are produced annually under northern conditions.
Seeds of the first crop are down in the nursery late in April, transplanted in mid-June and harvested between August and September.
For the second crop, the seeds are down in June, transplanted between July and August and harvested between November and December.
This second crop invariably requires supplemental irrigation.
The third crop is grown entirely during the dry season under irrigation.
The seedlings are raised between September and October, transplanted between November and December and the crop is ready for harvest in March/April.
Weeds can be controlled by herbicides, eg. Oxadiazon at 1.0 kg AI/ha applied pre-plant.
Also by hand at 3 and 7 weeks after planting but excessive and deep cultivation should be avoided to prevent disturbance around the developing bulbs and the superficial root system.
Onions give a good response to organic manures.
Phosphate and potash may be applied at or before planting, with a later application of nitrogen as a side dressing.
Nitrogen could also be applied to field plots in two equal splits at 2-3 weeks and at 6-7 weeks after planting.
In Nigeria, recommended fertilizer rates are 45kg P2O¿5, 50kg N and 30kg K¿2O per hectare. Compound fertilizer, NPK 15.15.15, at 200-300kg/ha can also be applied at 3 weeks.
Harvesting and Storage
The crop matures in 90-150 days after planting, depending upon the cultivar and the type of planting material.
At maturity, the tops droop, fall over and begin to die back.
As all the plants do not mature at the same time, harvesting is done when about 25 percent of the tops have fallen.
If onions are not harvested on time, bulbs are used to regenerate new growth, flowers and subsequently seeds are set.
Harvesting entails pulling, picking up and drying (curing).
Well-cured onions should be hard and the neck form and not easily dented by pressing with the thumb.
Cured onions should be stored under dry conditions with good air circulation.
Thereafter, they can be stored for a period of up to six months.
Intact onions are odorless, the odor being only formed after cutting, bruising or other injuries due to the release of organic sulfur components, mostly n-propyl sulfide.
Expected Yield from Onion Farm
Yield varies greatly, but 7.5-10 ton/ha can be obtained fairly easily in the tropics.
Yields are highest with dry season crop where 25 tons/ha of bulbs are easily possible.
Uses of Onion
The uses of onions are many. The immature and mature bulbs are eaten raw as a salad or they may be cooked and eaten as a vegetable.
They are used extensively for flavoring soups, ketchup, canned meat products, sausages, stews, sauces and a variety of other savory dishes.
Small bulbs are pickled in vinegar or brine.
Onion and garlic extracts have antibacterial properties.
Diseases and Pests of Onions
Downy mildew – caused by Peronospora destructor is widely distributed and is particularly prevalent during periods of high humidity.
The leaves become covered with a violet mold.
Purple blotch – caused by Alternaria porri can cause heavy losses during growth and storage.
White sunken spots are produced on the leaves, which enlarge to become purple with a yellow halo.
Pink and white rot are also common and are soil-borne.
The yellow dwarf virus produces short yellow streaks on the leaves.
It is transmitted mechanically by vegetative propagation and by aphids, but not through the seeds.
Crop rotation and plant sanitation assist in the control and various fungicides have been recommended.
The most serious pest of onions is the thrip, Thrips tabaci, which causes considerable losses.
It results in the appearance of silvery blotches on the leaves.
Malathion may be used for control. The stem and bulb nematode, Ditylenchus dipsaci, attacks onions and related species.