When onions grow to 6″ and again at 12″ tall, it is time to fertilize them. Avoid fertilizing within a month of harvesting.
Apply 1/2 pound (or 1 1/4 cups) of 21-0-0 (Ammonium Sulfate) to 100 square feet
Square Foot gardens
Apply 1/2 cup of 21-0-0 (Ammonium Sulfate) to 12 squares
Apply blood meal or soybean meal.