Test the soil where you intend to grow multiplying onions and correct the soil's pH balance to between 6.0 and 6.5. Applications of agricultural lime raise the pH and acidic fertilizers lower the pH.
Till the soil deeply with the garden tiller. Work in rotted vegetable matter or finished compost to improve soil quality and soil drainage.
Dig a trench 4 inches deep and 6 inches wide and the length of the proposed onion row. Scatter 1/2 lb. of 10-10-10 fertilizer along the bottom of a 100-foot trench.
Rake soil over the top of the fertilized trench to prepare the site for the onion row. Onions grow feeder roots only 6 inches long and grow best with a band of fertilizer laid below the planting row, according to University of Hawaii Department of Tropical Plant and Soil Sciences.
Plant multiplying onion seed 1/4 inch deep over the fertilized trench, spaced 1 1/2 inches apart. Cover the seed with fine soil.
Cultivate the rows to suppress weeds or weed by hand to reduce competition with the onions in the row.
Apply 1 lb. 10-10-10 fertilizer as a side dressing, five to six weeks after plants emerge. Thin onion plants to 3 inches apart to encourage bulb growth or harvest green onions when stalks reach pencil thickness.