Gardeners who desire healthy and robust onions must include fertilizer in their list of onion maintenance chores. When you take the time to fertilize your onion plants, you will have larger onions and a greater number of onions to harvest. Prior to planting onions, distribute straight phosphorus in the soil beneath the onions. This vital step will pay back rich dividends when it is time to harvest onions.
Prepare the growing area for onion planting. Cultivate the soil with the garden spade and then rake the soil surface smooth.
Dig a three-inch-deep trench along the planned row of onions.
Spread ½-cup of granular phosphorus fertilizer into the prepared trench for each 10 feet of planting row.
Cover the fertilizer with 2.5 inches of soil.
Plant the onion seeds approximately three inches apart directly over the trench. Cover the seeds with ½-inch of soil.
Water the newly planted onion seeds generously. Keep the soil evenly moist while the onions establish.
Wait until the onion plants are at least three inches high. Sprinkle the ammonium sulfate along the planting rows, using ½-cup of fertilizer for every 10 feet of planting row. Work the fertilizer into the soil gently with the hand rake. Water the onions generously.
Fertilize the onions with ammonium sulfate once per month until you harvest the onions.