Onion soil prep

What is the truth about soil prep for areas where onions are to be planted? I have read for example that you should not dig in manure because sometimes the onions will rot ?? but you should add general fertiliser at 4oz per m2. Now farmyard manure is about 0.5% N max and debatable if that is all available so a bucket load over one m2 would be equal to about 12.5g N  which is almost the same as the  general fertiliser I have?? So why not add the manure as it has other obvious benefits re water retention and soil structure?

I prep my onion bed with a good helping of wood ash then a general fertilizer and my onions always follow last years potato crop the ground having been manured the previous autumn prior to planting the potato's the next spring