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Onion (Allium cepa )
Chromosome Number: 32
Taxonomic Classification
Class : Liliopsida
Order : Asparagales
Family : Alliaceae
Genus : Allium
Onion is one of the most important commercial vegetables. It is grown in western, northern as well as in southern India. Maharashtra, Gujarat, Uttar Pradesh, Orissa, Karnataka, Tamil Nadu, Madhya Pradesh, Andhra Pradesh and Bihar are major onion-growing states in India.
Climate and soil  
Onion is a cool season crop. However, it can be grown under a wide range of climatic conditions. It grows well under mild climate without extreme heat or cold or excessive rainfall. In areas where average annual rainfall exceeds 75–100cm in the monsoon periods, it can be grown only as a summer crop. The ideal temperature requirement of the onion crop is 12.8°–21°C before bulbing and 15.5°–25°C for bulb development. Very low temperature in the early stages favours bolting, whereas a sudden rise in temperature favours early maturity in rabi, resulting in small-sized bulbs. Onion can be grown on all types of soils. However, deep friable loam and alluvial soils are best for its successful production. Free drainage, freedom from weeds and presence of organic matter favour production of good crop. It cannot be grown in alkaline or lowlying marshy lands. The optimum pH is 5.8–6.5.
A number of varieties have been developed in India. Onion varieties can be broadly classified into 2 groups—common onion and multiplier onion. Common onion can be further grouped into 3 sub-groups based on colour of skin—red, yellow and white.
Planting Common onion can be grown by raising seedlings in a nursery and transplanting them in field, planting bulbs directly in the field, broadcasting or drilling seeds directly in the field and planting sets for production of kharif onions Transplanting It is more commonly practised for an irrigated crop, resulting in a high yield with large-sized bulbs. Seedlings are first raised in the nursery. For an early kharif crop in south India, March–April is optimum time of sowing. August–September is most suitable time for a kharif crop. For a rabi crop in plains, it is October—November. For high altitude hilly areas, March–April is recommended as sowing time. A nursery bed of 3m × 0.6m size may be raised up to 15–25cm with a distance of 70cm between the beds to facilitate intercultural operations. Generally sandy loam soils are preferred for nursery beds. The edge and top of the bed should be quite firm to avoid pit formation during rains or at the time of irrigation. The top surface up to 2–3cm should be enriched with fine, seived and decomposed farmyard manure or compost after sowing. Seeds are sown in lines 4–5cm apart in rabi and 5–7cm in kharif. Sowingshould not be done more than 2–3cm deep. After sowing, the beds are mulched with dry grass or straw or any other material to maintain and preserve the required soil moisture. As and when there is lack of moisture in the bed, it should be watered by the sprinkling can. After the germination dry grass and mulching material should be removed. As and when needed nursery should be protected from heavy rains in kharif season. The continuous covering of nursery favours attack of fungal diseases due to risk of high humidity and heat. Therefore, the seedlings should be covered only when there is rise in damage by heavy rains. To protect young seedlings from heat of direct sun rays, partial shade should also be provided. About 10–12kg seed in rabi and 12–15kg seed in kharif is required to raise seedlings for planting in one hectare. The seedlings are ready for transplanting 6–7 weeks and 8–9 weeks after sowing during kharif and rabi respectively. Seedlings 20–25cm in height are ready for transplanting. Over-aged seedlings result in bolting, taking longer time to start new growth, whereas under-aged seedlings do not establish well after transplanting. The spacing varies from variety-to-variety on the basis of size of the bulbs. Planting by bulbs This is practised to meet the demand of green onion for salad in early winter. Bulbs are dibbled 15cm on the side of 45cm wide ridges or in beds. Another practice is to dibble bulbs in furrows made with country plough. The field is irrigated after forming beds and channels. For planting one hectare, 750kg of medium-sized bulbs are required. Larger-sized bulbs tend to flower early resulting in low yield. In northern India, bulbs from the previous harvest are planted in August or September. The secondary bulbs developing from the mother bulbs find a ready market as green onions early in the season (October–December). Direct sowing Onions can also be grown by direct seeding. Soil is thoroughly pulverized and made free of clods. Varieties with bigger-sized bulbs are grown in rows 30cm apart. Seedlings 6–8 weeks old, may be thinned. For smaller-sized bulbs, sowing can be done by broadcasting and later thinned 5cm each way. A light irrigation is given immediately after sowing. Interculture and irrigation may be repeated after every 10 days. Planting by sets Sets are small-sized onions produced by the seedlings, to mature in the nursery beds as such instead of transplanting them. These are used to get an early crop of onion in some parts of Gujarat, Maharashtra and Rajasthan. Mahsera and Talaza in Gujarat are the areas where sets are raised for local use as well as for supply to other states. For planting one hectare, 5–8kg seed is sufficient to raise enough number of sets in 200m2 area. Nursery is prepared as usual. Mid-January–February is ideal time of sowing. Let the plants remain in the nursery till their leaves fall (till April). By this time plants form small sets due to lesser spacing. Later on dig out the sets. About 1.5–2cm sized, disease-free sets are selected. Very small sets result in more splits, doubles and bolters. Larger-sized sets increase cost of production. The graded sets are stored in hessian cloth bags or in shallow baskets or in racks under properly ventilated conditions. Generally, 10q of sets of 1.5–2.0cm diameter are required for planting one hectare crop. The sets are planted 10cm apart in rows on both sides of ridges spaced at 35–45cm spacing. Sets are normally planted by mid-August to get an early crop by mid-November. The planting, however, can be continued up to mid-September so as to stagger the availability of produce up to mid-December. A light irrigation is given immediately after planting the sets. In multiplier onion, bulbs are first separated and then bigger-sized bulbs are used for planting. Manuring and fertilization Onion needs a heavy dose of fertilizers for a good yield. However, fertilizer requirements depend upon soil type and type of crop. Farmyard manure @ 20–25 tonnes/ha may be incorporated into the soil one month before transplanting. Full amount of P and K and half of N is to be added just before transplanting. Rest of the N is to be given as a single dose 20–25 days after transplanting in case of sowing by sets and multiplier onions, whereas it is given into 2 doses in seedling transplanted onion, first 30 and second 45–60 days after transplanting. In addition to the chemical fertilizers, some other nutrients are also useful for improving onion quality. Zinc application (1–3ppm) increases yield as well as improves quality. Cultural operations As plants are less spaced and roots are less penetrating, it is essential to keep the crop weed-free, especially at the initial stage of plant growth. In onion, hand-weeding in generally practised but once the vegetative growth of the plants is complete, hand-weeding becomes very difficult and expensive also. Therefore, chemical weed control along with a hand-weeding after 45 days is recommended. Baseline @ 1 litre/ha or Stomp @ 3.5 litres/ha immediately after transplanting are quite effective to control weeds. Irrigation Water requirement in general, depends upon plant, its growth stage, soil type and climatic conditions. Onion is a shallow-rooted crop with roots generally confined within 8cm of the soil surface. It is very rare that they go up to 15cm . Water requirement of onion is therefore, less in the beginning. During rainy or kharif season, one irrigation is given immediately after the transplanting, especially in north India. Delay in irrigation may result in seedling mortality up to 80–90%. In kharif crop 8–10 irrigations are sufficient. A late kharif crop requires 12–15 irrigations, whereas rabi crop 15–20. Water is a critical requirement at the time of bulb formation. Its scarcity at this stage can affect the yield drastically. However, excessive moisture favours incidence of purple blotch especially when the vegetative growth is over. Do not let the field remain dry for a longer period, otherwise bulbs may split and crop may have a forced early maturity. Frequent light irrigations at weekly intervals promote proper bulb development and good yield. Sprinkler and drip irrigation systems are also recommended. In rabi season, stop the irrigation when leaves start falling. In kharif crop, stop the irrigation 10 days before digging.
Harvesting & Postharvest management
Onion is ready for harvesting in 3–5 months and 2–3 months after transplanting for dry and green onions respectively. Green onions are harvested when they are just ready for earthing. Plants are uprooted by hand and their roots are cut. They are washed and bundled as per market requirement. The optimum time for harvesting bulbs for sale or for storage from a rabi crop is when leaves start falling. The exact time is one week after 50% leaves have fallen. So optimum harvesting time is when leaves start turning yellow and becoming dry at the top. Bulbs turn red and attain their optimum varietal size. Multiplier onions are harvested when 50–70% leaves have fallen. Before they dry fully, leaves are cut 2–2.5cm above the bulb. After harvesting, a kharif crop should be left in the field for a few days for drying of leaves. Common onion varieties yield up to 250–300q/ha, small-sized pickling type varieties 160–200q/ha, and multiplier onions 150–180q/ha. Onion bulbs that are to be stored in crates are usually thoroughly cured before being placed in storage. The purpose of curing (drying) is to remove excess moisture from the outer skin and neck of onion which helps reduce the infection of disease. This also helps minimize shrinkage and development of skin colour. Therefore, onions should be adequately cured. Time required for curing largely depends upon the prevailing weather conditions. For a kharif crop in north India, when temperature is quite low, curing is done for 2–3 weeks along with the leaves under the sun. If temperature is very low then use drier, blowing hot air at 46°C for 16hr. A rabi crop is cured by covering bulbs with their leaves in the rows in field for 2–3 days, then cutting the leaves 2–2.5cm above the neck and curing under shade for 7–10 days. Kharif and late kharif crops in Maharashtra are cured in the same way as rabi crop except that curing in field is extended by 1–2 days. Small and multiplier onions are also cured similarly. Onions are cured when neck is tight and the dried scales rustle. After curing, onions are graded and classified by grading machine and hand. Before storage, doubled, broken, rotten bulbs and bulbs having undesirable features are removed. To get a good price, grading is practised even after storage also. Outer dry scales generally are removed during grading thereby making bulbs look attractive. Grading and classification are necessary for both local market as well as for export. Delhi market prefers big-sized bulbs; Calcutta, Patna and Lucknow prefer medium-sized bulbs and Bangalore, Bhopal, Jabalpur and Hyderabad markets have demand for medium to big-sized bulbs. Markets of Bhubaneswar, Guwahati and other north-eastern states prefer small-sized onions. Jute bags are used for onion packaging for sending to distant markets by trucks, trains or even by air. Generally 40kg capacity jute bags are used for transport within country, whereas for export, jute bags of 8–25kg capacity are used. Onions should be packed in 14–15kg capacity cane baskets for export purpose.
Nutritional Value
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Date: 2015-12-11; view: 802

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