@Benjamin as you plant to transplant your onions land preparation begins one month before date of planting this enable you to:-
-Eradicate aggressive weeds e.g couch grass
- help to achieve fine tilth ( minimum fine soil particles as possible)- bearing in mind that onions are annual root bulb crops which require loose soil to develop bigger bulbs
- you may raise beds if water isn't an issue or sunken beds to harvest more water in areas where rainfall is insufficient.
- Do soil test to establish the PH values which very much helpful in determining right fertilizer requirements. Onions do well in PH range of 6.0-6.8
-If soils have hard pans then it's crucial to do ripping so to make the soil loose for better results
- If available add 3-4tonnes of well decomposed animal/compost manure
-If irrigation is to be used (drip method) then set the system before planting and water the field to capacity a day before planting.
................ Any other question please ask.....
Thanks