Posted: 01/01/2015 at 10:40
I think a lot can depend on the weather. I live in Dordogne where it can be very hot in summer. However, I always manure the site in autumn and fertilise with organic granules when I prepare for planting. I rotovate the soil too, so that it is quite crumbly, my soil was clay but is much better now after years of manure. I know some on this site don't agree with rotovating, but my plot is big and it works well for me. I also water well in the growing period. My onions are big.
I had a friend who grew onions for shows in the UK and she used amazing amounts of fertiliser and water. I think if your plot is sloping with sandy free draining soil then there just aren't enough nutrients left in it. I know brassicas like firm soil, but I didn't think onions did.