Introduction

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.

Onion Best Practices

Best Management Practices (BMPs) are the best recommended agronomic practices for growing a specified crop. These practices are based on research and experience and apply to onions under the specified agro-ecological conditions.
The recommended BMPs are not the only way to grow onions but are the best way determined by Netafim. The BMPs may change as additional proven research becomes available.

Agro-ecological situation
Climate
Conditions: Temperate, tropical and subtropical; cool season crop
Day length: For short day varieties less than 12 hours, for intermediate day varieties between 12 to 14 hours and for long day varieties more than 14 hours of light for bulbing
Altitude: up to 2500 m above sea level
Rainfall: 500 to 600 mm/annum
Relative humidity: 60 to 70%
Optimum ambient temperature: Seedling growth 20 – 25 ºC, vegetative growth 13 – 24 ºC, before bulbing 15 – 21 ºC and for bulb development 20 – 25 ºC

Soil
Soil suitability: Fertile, light, deep friable well drained fine sandy, loamy and alluvial
Optimum soil pH: 5.8 to 6.5
Soil bulk density: 1.3 – 1.4 Mg/m3 favors better root penetration and bulb development and soil water air relations
Groundwater table: Below 2.0 m
Critical soil salinity level (ECe): 1.2 dS/m above which yield decreases
Soil to avoid: Waterlogged, alkaline and saline soils.

Improved varieties
Several varieties with great differences in pungency, sugar content, disease resistance, seed stem formation, double centers, bulb shape, size, colour, duration and yield are available depending on the country.

Planting material
Healthy 8 – 10 weeks old transplants having 2 – 3 leaves, small dry bulbs and pickling onions.

Spacing
Inter row spacing: 0.10 to 0.40 m
Intra-row spacing: 0.05 to 0.10 m
Optimum plant density: 0.25 to 1.0 million plants/ha. However, 80 bulbs/m2 is a reasonable density.

Seeding rates   
Varies with cultivar, market, moisture, planting date, spacing and cost of production.
Transplanting depth: 3 to 4 cm
Seeding depth: 1.3 to 1.9 cm
Direct seeded crop with mechanical seeders requires 5 to 8 kg seed/ha

Land prepration
Clod free seedbed with good tilth to express its bulb yield potential, SDI installation and optimal soil water air relations.
Destroy the hard pan, if any, using either chisel plough or a subsoiler.
Primary tillage by mould board plough or disc plough and secondary tillage by disc harrows, tyned harrows or rotavator to achieve proper tilth.
Broad bed (0.8 m to 1.5 m) and furrow (0.30 m) system depending on the number of rows per bed.
Compost: 25 – 30 tons/ha

Crop rotation
A 5-year rotation adequate to check weeds, diseases & pests and avoids yield losses.
Without proper rotation yield losses are up to 40 – 50% due to soil borne diseases.
Cereal grains such as wheat & oats; corn, sugarcane, rice & forages are the best rotational crops.

Weed control
Managing weeds is critical for successful onion production.
Weeds compete for light, water &nutrients and reduce bulb yield by 42 to 83% depending on the weed intensity.
Critical crop: Weed competition period is initial 4 – 6 weeks.
Integrated weed control program involving crop rotation, manual weeding, stale seedbed, soil fumigation, soil solarization, maintenance of optimum plant population, mechanical intercultivation and herbicide chemical applications.

Herbicides:
Paraquat (Gramoxone): 0.70 – 1.1 kg a.i./ha
Glyphosate (Round up): 1.1 – 4.5 kg a.i./ha as pre-plant herbicides
DCPA (Dacthal 75 WP): 5.1 – 11.3 kg a.i./ha at planning
Bensulide (Prefar 4E): 4.5 kg a.i./ha
Pendimethalin (Prowl 3.3 EC): 0.57 – 1.7 kg a.i./ha
Oxyfluorfen (Goal 2XL): 0.13 – 0.28 kg a.i./ha
Bromoxynil (Buctril): 1.37 – 0.43 kg a.i./ha as post-planting herbicides

Irrigation system
Drip version: Surface or subsurface drip irrigation combined with fertigation
Drip product: DripNet PC, Super typhoon, Dripline 17009
Bed size: 0.8 to 1.5 m bed width, 0.3 m furrow with 0.15 m bed height
Number of rows per bed: 2 to 6
Dripline spacing: 0.6 m with 1 to 3 driplines per bed and 1 dripline for 2 to 3 crop rows
Dripline installation depth under SDI: 15 cm
Emitter spacing: 0.30 m
Emitter flow rate: 0.6 LPH, 1.0 LPH, 1.6 LPH and 2.0 LPH depending on soil texture

Crop water requirement & irrigation scheduling
Estimate crop water requirements as a product of daily reference crop evapotranspiration by Penman-Monteith method and crop coefficient for a given day according to the plant developmental stages.  Begin with 0.5 Kc of daily ETo in the initial period, raise it to 0.8 at vegetative, 1.0 at beginning of end of bulbification and decrease it to 0.7 at dry leaf harvest period of onions.
Daily crop water requirement: 5 to 7.25 mm/day.
Seasonal crop water requirement: 400 to 775 mm under range of environments.
Irrigation scheduling at 10 to 20 centibars using tensiometers installed at 20cm soil depth maximizes total, marketable and colossal bulb yield of onion.

Fertigation
Apply mineral fertilizers based on the targeted yield, leaf analysis results, fertilizer experiment results, leaf deficiency symptoms, nutrient uptake, soil analysis results, nutrient recycling etc.
Nutrient uptake:
160 kg N
76 kg P2O5
115 kg K2O
16.6 kg MgO
128 kg CaO/ha for a crop yield of 40 tons of bulb yield/ha

Optimum leaf nutrient levels at bulb enlargement stage in young mature leaf are as follows:
3.1% N,
0.33% P
3.3% K
0.27% Mg
0.5% Ca
9 ppm B
0.73 ppm Cu
27 ppm Mn
10 ppm Fe
7 ppm Zn of dry matter

Recommended nutrient dose:
175 to 400 kg N
75 to 150 kg P2O5
200 to 300 kg K2O
20 to 40 kg MgO/ha
For fertigation use water soluble fertilizers such as: urea (46% N), potassium nitrate (13% N & 46% K2O), monoammonium phosphate (12% N & 61% P2O5), ammonium nitrate (34% N), etc.

Pests
Important pests include onion maggots, cut worms, wire worms, leaf minor and stem nematodes.
Important diseases include bacterial soft rots, botrytis leaf spot, botrytis neck & bulb rot, downy mildew, basal rot, blue mold rot, black mold, purple blotch, stemphylium leaf blight, pink root, fusarium basel rot , rust, sour skin, white rot & onion yellow dwarf
Detect outbreaks and identify problem areas by routine patrols.
Monitor economic threshold levels and take up appropriate plant protection measures.

Physiological disorders
Bolting: Production of flower stalks at the expense of bulb formation and enlargement.
Greening of onions: Bulbs exposed to sunlight produce chlorophyll and result in greening of bulbs.
Pinking: Bruising during harvest or handling.
Sunscald: Exposure of bulbs to direct sunlight during high temperatures.

Harvesting
Harvesting depends on its type and purpose such as greens, immature bulb & mature bulb.
Optimum time of harvesting is one week after 50% of leaves have fallen.
Maturity symptoms include yellowing of leaves and dry at the top; bulbs turn red and attain their optimum varietal size.
Bulbs continue to grow even after the foliage has fallen down.
Harvesting is done by pulling the bulbs. On large scale mechanical harvesters are used
Ideal conditions for storage, include low humidity (< 80% RH) and low temperature (2 – 10 ºC).

Tuber yield
Under drip irrigation and fertigation a good commercial tuber yield is 60 – 100 tons/ha depending on length of growing season, climate & variety.
Water utilization efficiency is varying between 12 and 15 kg/m3 of water.

FAQ’S

Why measure soil water tension and what is the recommended level of soil water tension for irrigation of onions?

Soil water tension is a measure of how strongly water is held in the soil. Onion yield and grade are related to the amount of energy needed for plants to remove water from the soil. Soil water tension also provides information on soil saturation which can help growers avoid saturating the soil, thereby maintaining aeration of plant roots and reducing water losses and leaching of nutrients. These factors make irrigation by soil water tension using tensiometers environmentally and economically important. Recommended soil water tension for commencing irrigation for drip irrigated onion is 20 – 25 centibars measured at 20 cm depth.

How should I irrigate my onions?

Onions have a shallow root system and therefore yield and grade respond sensitively to irrigation management. Drip irrigation of onions facilitates application of water in the root zone at controlled water regime without fluctuations at low intensity and high frequency. Smaller amount of water application at high frequency by drip minimizes negative environmental consequences due to leaching and runoff. Further drip irrigation in onions proved to be technically feasible and economically viable over other methods of irrigation under range of environments besides increasing bulb yields, preventing splitting of bulbs and larger proportion of Grade A bulbs. Drip technology also allows significant saving in water, labor and energy required for pumping water. Thus, in the long run financial analysis showed that drip is the most appropriate system for modern potato growing with higher economic returns.

When are onions ready to harvest?

The correct harvest stage is one week after the observation that 50% of the leaves have fallen.

Why have I got a poor onion plant stand?

A poor plant stand can be caused by poor quality seed, incorrect planting depths, poor quality irrigation water or incorrect irrigation management after planting.

How important is water quality for onions?

Water quality is more important in the young seedling stage and depends on the environmental conditions present at that stage. Water conductivity (salt content) should be less than 1.2 dS/m while levels of up to 1.8 dS/m can be tolerated after seedling establishment

I harvested my onions in late summer and they began to rot by fall. Why?

Onions may rot at either the base or neck. Rotting at the base may be caused by soil-borne fungi or carelessness in harvesting and handling, but it is usually caused by damage from root maggots. If onion bulbs rot at the neck, either they have been cured insufficiently before storage or the leaves have been severely infected by fungi during the growing season. Onions that were “hilled” or covered with soil before harvest often start to rot soon after harvest. Many of the “sweet” varieties are very poor keepers and should be enjoyed fresh soon after harvest, as no method of storage keeps them from rotting for very long.

How much a drip irrigation system costs per hectare for onions?

This is very variable and depends on the following factors: 1. Conveyance of water from source to the filed: Normally this is the most expensive component of the irrigation system. It depends on the distance and elevation the water has to be conveyed by the pipelines. 2. Peak crop water demand: Amount of water we need to apply to meet the peak crop evapotranspiration requirements during the crop peak demand. This is a function of prevailing climate conditions, crop canopy cover and efficiency of the irrigation system. 3. Other considerations: The land topography (flat or undulated) of the design area; the soil texture which determines the emitter spacing, for example sandy soils require closer emitter spacing and clayey soil require wider emitter spacing that will have a significant impact on the system cost per unit area.

What kind of increased crop production can I expect from converting furrow irrigated onion to drip irrigation?

From our field experience in a range of environments, it depends on variety used, climatic conditions, length of growing season, management capability etc. As a rule-of-thumb bulb yields increase by 20 – 50% over traditional furrow irrigation besides 40 to 45% saving in water under optimal soil tilth, timely planting, correct fertigation, weed control and other farming practices.