Onions are easy to grow, but they're very slow to mature. Fortunately, a well-planned garden can give you enough onions to last the whole year.
In the Southern Hemisphere we plant in March and harvest in October.
Onions will not grow well in acidic soil or heavy clay, but your average onion simply adores a well-drained sandy soil. If your soil is too acidic, put on lime.
Remember not to plant onions in a plot that had legumes (beans) of any kind the previous year.

Fertilizer
Half a wheelbarrow of manure or compost per m2.

Planting There are two ways of planting onions:

Direct Sowing will get you more onions, more quickly, but takes a lot more effort because you will have to prepare the soil very well with a rake - onions do not do well in rough soil.
For direct sowing you will need to water well every day, weed very well, and mulch thoroughly.
Make the rows 40cm apart and, when the onions start to grow, thin out the small ones until the plants are about 8cm apart.

Sowing in a Seedbed takes longer, but is not as much work.
Sow the onion seeds in rows, making the rows 10cm apart. Water and weed well.
The onions will be ready to transplant at about 8 weeks, when they are as thick as a pencil.
To transplant, make rows 40cm apart. Plant the onions 8cm apart in the rows, as deepas they were in the seedbed.
Cut off the ends of the roots and the tops of the leaves to make your onions bigger.

Harvesting

Onions will be ready for harvest about 8months ater planting. The easiest way to tell is that the leaves will become yellow and fall over.
Hold your onions by the leaves and pull the onions out of the ground. Leave them to dry for 3 days, covering each onion with the leaves of another onion so that they don't get sunburnt.
Tie them together in bunches by their leaves and hang them up in a cool dry place.

Pests and Diseases
Thrips are tiny insects that eat the leaves of onions and make little silver marks.
Downy Mildew is a disease which makes pale green and sometimes purple sopts in the leaves. The leaves can turn yellow and die.