Somehow, years ago, gardeners became convinced fertilizers high in phosphorus, especially bone meal and superphosphate, must be used on fall-planted bulbs such as tulips, daffodils, hyacinths and lilies.
“Perhaps this idea arose because phosphorous is essential in the transfer of energy from the leaves to the bulb where it is stored,” said Dr. Charles Mitchell, an agronomist with the Alabama Cooperative Extension System. Phosphorus was low in most soils before fertilizer use became widespread.
Roots of fall-planted bulbs are weak and slow-growing. They are less able to use soil phosphorus than crops growing in the summer. Therefore, most fertilizer grades sold for flowering bulbs, such as 4-8-6, 5-30-10, 8-24-24, 4-12-12 or 5-10-15, have a high rate of phosphate (the middle number) compared to nitrogen (the first number). Some gardeners insist on using bone meal (0-10-0), rock phosphate (0-20-0) or concentrated phosphate (0-46-0) when planting bulbs. They reason it promotes more quality flowers.
“While it is true all plants need phosphorus to complete their life cycle, many soils have become high in phosphorus from continued use of high phosphorus fertilizers. Even 13-13-13 or 8-8-8, will build soil phosphorus over several years. Composts, manure and other organic fertilizers will build up soil phosphorus as well, added Mitchell.
An Auburn University soil test summary indicated almost half of Alabama gardens don’t need additional phosphorus. Adding more probably won’t hurt, but it is a waste of a natural resource and could damage the environment. Ask for fertilizers containing a low middle number such as 15-0-15; use it sparingly to avoid too much nitrogen. A good rule is about 1 pint per 100 square feet of bed (20 x 5 foot bed).
Excessive nitrogen is often a reason flowers fail to bloom properly. The more nitrogen you add, the more green leafy growth you’ll get at the expense of flowers.