Everything a healthy mature bulb needs to produce a flowering plant is in the dormant bulb following its harvest by a reputable grower. However fertilizer may be necessary for the future success of bulbs that are meant to perennialize (return for several years), depending on your soil fertility.
It is a good idea before planting bulbs, or any other plants, to get your soil tested to determine your soil type and pH and inform you of which amendments may be needed and how much to apply. This information can also help you make the right decision when choosing bulb species, as certain bulbs require specific soil conditions. Your Local Extension Service can provide you with information about soil testing.
Fertilizing Bulbs
The application of fertilizer to bulb plantings is best done as a top dressing. To avoid burning a bulb or its roots, never mix fertilizer or fresh manure into the planting hole with the bulb. The high concentration of salts in some forms of nitrogen can dessicate plant tissue if it makes direct contact. The newly emerging roots on a bulb are very sensitive to these salts; death of the roots will result in bulb failure.
Although fertilizer suggestions for Dutch bulbs vary between authorities, current recommendations are to mix slow release bulb fertilizer into the top few inches of soil once a year. The NFBIC recommends fertilizing in the Fall when the bulbs are putting out new roots which will readily absorb nutrients. If fertilizer must be applied over bulbs in the spring, try to make the application as shoots emerge (after the soil has thawed). It is not as desirable to fertilize Spring flowering bulbs too close to flowering time or too long after bulbs are finished flowering. As Spring flowering bulbs go dormant for Summer their roots die back and cease nutrient uptake until the soil cools in the Fall and starts the cycle over.
There are several formulations of bulb fertilizer on the market, going by any number of names like Bulb Food, Bulb Booster or Bulb Tone. Typically, bulb fertilizer is slow release, with a nutrient formulation like 9-9-6, 4-10-6, 5-10-20 or 10-10-20. The common formulation 9-9-6 is ideal for most types of bulbs including garden lilies, tulips, etc.. Daffodil experts recommend using slow release 5-10-20 or 10-10-20 for daffodils, if it is available. Whichever product you choose, follow the recommended application rates on the package. A fall top dressing of well rotted manure or compost annually is also beneficial.
Garden lilies (Lilium species) should be fertilized in the spring as shoots are emerging, with a standard garden formulation like 5-10-10 or 10-20-20 or something similar, with a lower first number. A second application of fertilizer is recommended just before flowering, especially if lilies are planted in sandy soil. As with Dutch bulbs, slow release fertilizer is preferable.
Organic gardeners may choose to apply cottonseed or blood meal for nitrogen, greensand for potash (potassium) and bone meal for phosphorous. Bone meal is not a complete bulb food and modern processing methods leave it low in nutrients, so it is not recommended for use solely as bulb fertilizer.
