What fertilizer or blend of fertilizers is needed to grow large onions. My soil is sandy with a fair amount of organic material (grass clippings and leaves) worked in.
Your onions do not need heavy fertilization. It is best to do half strength all-purpose fertilizer. Onions grow bigger by having more leaves. The more leaves they produced, the bigger the onions will be. If you feel you need a little boost, then go with a higher nitrogen fertilizer.