I will try to answer your question although exact amount I do not know.
Fertilize the soil before you plant the onions. Either get a time release balanced granular fertilizer or use a couple inch thick layer of compost. Use only nitrate based fertilizer and not sulfate (makes onions more sweet and mild).
Once you planted the onion, you do not have to fertilize for another year.
A few more details to add to Paprika’s answer! Onions like a good boost and can tolerate some extra nutrients better than most other vegetables. Adding phosphorus-based fertilizers will encourage root and bulb formation during the onions’ early times.
As Paprika mentioned, you probably don’t have to use commercial fertilizers throughout the year–but that doesn’t mean that your onions won’t appreciate a constant supply of compost. Compost is preferable to commercial fertilizers because you won’t be in danger of burning their green tops.
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