To begin preparing a new onion bed you need to first choose a sunny site with good drainage but the real key to growing onions in a permanent bed in is to build up the soil fertility first. There is a down side to this however as you can also encourage the build up of diseases so it is advisable to rotate your onion bed rotate with the rest of the vegetable garden. Try to avoid soils that have been planted with onions within the past three years, and because onions are shallow rooted and poor competitors with other plants, avoid sites with a history of perennial weeds. .

Soil sterilisation
It is possible to grow onions on the same bed year after year but in order to maintain successful and healthy cropping a strict health routine must be followed. If there are any onions that you suspect are harbouring any kind of disease then remove not only the plant but also a small amount of soil from where the onion was growing. Hopefully this will eliminate any unwanted bacteria in the soil. It is also worth watering the bed with a dilution of Jey's Fluid once the crop has been harvested - this again will help to kill any unwanted bacteria or fungi. There are onion beds that were started over 140 years ago that are still in production today using this method!

If you can, start preparing your onion bed in the autumn by digging in plenty of well-rotted farm manure. This will give the ground a chance to settle over the winter period and allow frosts to break down the soil clods. If you soil is to acidic – below pH 5.5 – you will need to add lime to the bed according to manufactures recommendations. In general, onions prefer a pH of between 6 and 7.5 and a fine tilth to be sown into. Weather permitting, the frosts will do a good job of this.
You can sow main-crop onion seeds as soon as your soil will allow you to which can be any time form late February, but you can steal a march here by picking a dry day a few weeks before sowing time and raking the soil to a fine tilth. Onions like a firm bed so tread over the area you have just raked.

Try adding a general fertiliser like growmore for extra fertility, and for an even earlier crop you can sow onion seed under glass or cloches in January. (There is an advantage that can be gained by setting up cloches before planting. If cloches are placed over the ground prior to planting, the ground has some time to warm up, reducing the chances of a check in growth). The soil may require some watering to achieve a uniform moisture before seeding onions, but try to avoid sowing into a dry seed bed.
Micronutrients are also important in onion production - in particular boron and zinc - so look at giving your onions a periodic liquid feed of seaweed based fertiliser. However if your onion are clearly growing well then this will probably be unnecessary.

For the adventurous, try this old recipe for giant, exhibition onions care of W.Robinson's home of the Mammoth Onion.

Dig out a trench 1 ½ ft deep, further forking over the bottom of the trench if the base ground is solid. Then, into every four square yards of the bed work the following: 4 Forkfuls of pea, bean or tomato haulms 1 garden barrow of well rotted farmyard manure 5oz Bonemeal 6oz Sulphate of Potash and every 5/7 years only 2oz Sulphate of Iron.
First scatter the haulms at the bottom of the trench, and then mix the fertiliser into the bed. It is essential that the greater proportion of the manure should be near the top of the bed within 3 inches of the soil surface. This will enable the roots to come into contact with the manure during the early stages of growth. This work is best done in the Late Autumn or Early Winter when the bed is reasonably dry, this can then be left to the elements over the winter period. Come March, work the top of the bed into a fine tilth adding the following to an area of 4sq yards. 2oz Superphosphates 1oz Hydrated Lime or 1.5lbs Calcified Seaweed.
For further information click onto:
How to Sow and Grow Spring Onions from Seed
Onion Soup
ROUND HEADED LEEK - Allium sphaerocephalon
Planting Onion Sets
What is Crop Rotation?
What is a Seed?
Which Vegetable Seeds can be Sown in August?

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