At least 25 percent of the nitrogen applied should be in the nitrate form because high rates of ammonium-containing fertilizers can be toxic to onions. Nitrate nitrogen is readily leached by heavy rains, so apply it in small amounts in regular sidedressings. It is not leachable and is usually found in ample amounts in many soils. Availability is limited, though, in cool soils, so it is best to band at least part of the phosphorus 2 inches below and 2 inches to the side of the seed. As the plant uses nitrogen, it also uses sulfur to make proteins.
Sulfur is also important in forming pungent compounds.
Apply 50 to 80 pounds of sulfur per acre at planting. Micronutrients are also important in onion production. In one method of fertilization, nitrogen may be applied in a split application with all of the phosphorus and potassium applied preplant. Sidedress with a nitrogen fertilizer such as calcium nitrate or sodium nitrate when plants are 3 to 4 inches tall. A second sidedress may be needed three to five weeks later depending on when the onions will be harvested or if there have been leaching rains.