Fertilizer should not be used when planting amaryllis bulbs indoors or outdoors. The Cooperative Extension of Clemson University recommends fertilizing amaryllis plants only after the bulb begins to grow and produce leaves.
Fertilizing an amaryllis bulb before it produces leaves can kill the plant's roots. After growth commences, the bulb should be fertilized twice monthly with a water soluble fertilizer formulated for potted plants or a low nitrogen, bulb booster type fertilizer for outdoor plants. Fertilizer should be applied according to the product's instruction label. After the bulb flowers, fertilization and watering should be stopped for 8 to 10 weeks to encourage the bulb to reflower.