Onions require frequent, regular applications of fertilizer to produce the highest yields and the largest, best quality onions. In addition to their fertilization needs, onions require moist, weed-free soil with a pH level between 6.2 and 6.8. For best results, gardeners who live in areas with limited sunlight should grow short-day varieties, while gardeners who live in areas where sunlight is abundant should plant long-day varieties.
Soil test kits to reveal soil nutrient deficiencies are sold through garden centers; local laboratories that test soil for a fee may be found through university extension schools. Ideally, soil will feature approximately 15 parts per million (ppm) or higher of phosphorous, 100 ppm of potassium, and greater than 5 ppm of sulfur. Soil should also feature a concentration of zinc greater than 0.6 ppm. Soils that are found to be deficient in these areas may be amended. A super phosphate, or a 0-20-0 fertilizer will rectify a phosphorous deficiency. A complete 10-20-20 fertilizer is indicated for a deficiency of phosphorous and potassium. Potassium sulfate is used for a deficiency of sulfur or potassium and zinc fertilizer for zinc deficiency.
A general application of 10-20-10 fertilizer at the time of planting is always a good idea. This is done before placing the onion seedlings in the ground by amending the soil with 10-20-10 fertilizer. In a 2- to 3-inch trench that is dug 2 to 3 inches deeper than the planting depth of the seedlings and as long as the row of onions to be planted, 1.5 cups of fertilizer is distributed. The trench is then filled with garden soil. This fertilizer will aid in the development of the young seedlings.
Three weeks after planting the seedlings in the ground, the gardener may begin applications of nitrogen. Nitrogen is useful in all circumstances regardless of the fertility of the soil. Nitrogen-based fertilizers, such as ammonium sulfate or ammonium nitrate, may be applied to the soil at a rate of 1 cup per 20-foot row, scattered near the seedlings in a narrow band located 2 or 3 inches from the plant base. Applications of this fertilizer should continue until the seedling necks are soft.
Deep watering is important after applying fertilizer to the soil. Fertilization should stop after the necks of the seedlings have become soft, usually after mid-summer, as fertilizing after this point reduces the storage potential of the onions. Some gardeners may turn to high quality composted manure for their nitrogen needs. Manure should be worked into the soil in a band 2 or 3 inches from the seedlings, then watered just like other fertilizers.
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