Soil should be well-drained, loose, and rich in nitrogen; compact soil affects bulb development. A neutral PH is recommended.
Onions are heavy feeders and need constant nourishment to produce big bulbs. If you remember, add aged manure or fertilizer the fall before planting.
At planting time, mix in some nitrogen fertilizer, such as Gaia 4-4-4 too, and side dress every few weeks with compost until the bulbing process begins.
Plant onion sets as soon as the ground can be worked in the spring, usually late March or April. Make sure temperature doesn’t go below 20 degrees F.
Plant the smaller sets 1 inch deep, with 4 to 5 inches between each plant and in rows 12 to 18 inches apart.
Think of onions as a leaf crop, not a root crop. When planting onion sets, don’t bury them more than one inch under the soil; if more than the bottom third of the bulb is underground, bulb growth can be restricted. Practice crop rotation with onions to avoid disease.