In addition to proper fertilising and watering techniques, onion variety selection is an important factor in growing large onions. Long-day onions grow well in Northern regions, while short-day onions produce large bulbs in the South. The number and size of the leaves at maturity predicts how large the onions will be. More leaves means larger onions.
Before planting onion transplants or sets, amend the soil with 3 inches of compost, which improves soil texture and provides some slow-release nutrients. Add 1/2 cup balanced fertiliser per 10 feet of row and till the amendments to a depth of 8 inches. Use 1/2 cup super phosphate fertiliser instead of balanced fertiliser if planting onion seed. Super phosphate acts as a starter fertiliser, encouraging strong, early roots.
Onions need a steady supply of nitrogen to form large bulbs. Side dress growing plants in early and midsummer with 1/2 cup nitrogen-based fertiliser. Use ammonium sulphate if your soil is alkaline because this fertiliser lowers the pH slightly. Ammonium nitrate works well for gardens with acidic soil.
Spread two or three shovelfuls of compost among the onions instead of fertiliser and dig it lightly into the soil. Take care not to nick the bulbs with the shovel. Read package labels carefully and choose a high-quality compost made from manure and composted vegetable material. Avoid composts that contain biosolids or sludge in the vegetable garden.
Don't apply fertiliser after midsummer. Late applications of fertiliser encourage new, soft growth and the onions won't keep as well. In addition to adequate fertiliser, onions need evenly moist soil to grow large. Mulch the ground with untreated grass clippings to conserve moisture. Spread 1/4 inch grass clippings over the soil each week, allowing the clippings to dry before adding more.