Question: What do the numbers on a fertilizer mean? For example, a 12-36-14 formula?
Answer: The three numbers appearing on a fertilizer’s label represent the percentage contained of the three major elements” nitrogen, phosphorus, and potassium. In addition, most fertilizers contain various secondary and trace elements, which may or may not be listed on the package. We’ll limit ourselves to the original question, since a more detailed answer would require a very lengthy explanation.
As simply as possible–nitrogen promotes healthy foliage development, phosphorus promotes root, bud, and blossom development, and potassium promotes general plant health. For example, fertilizers referred to a “blossom boosters” have relatively high phosphorus content. An example would be one having a 1-6-5 formula. On the other hand, fertilizers like fish emulsion (5-1-1) are considered to be very good for promoting lush, green, foliage. A fertilizer such as “Root-n-Bloom” (5-50-17), encourages rapid root and blossom development.
Each different fertilizer serves its own purpose. Many exhibitors will change fertilizer formulas depending upon what kind of growth they would like to encourage. A high nitrogen fertilizer, for example, might be used while a showplant is disbudded, to encourage greater foliage growth. Once disbudding is stopped, switching to a high phosphorus fertilizer would encourage production of flower buds and blossoms for show. Unfortunately, what formulas to use, and when, is often learned only by experience growing under your specific cultural conditions. If it works, stick with it.
As a rule, however, African violets prefer a relatively “balanced” formula, one having moderate levels of all three major elements. Using extreme formulas such as 1-6-5 or 5-1-1 exclusively, can produce plants with yellowed foliage or lack of bloom, respectively. Also, remember that your soil mix also provides some nutrients (though “soilless” mixes provide very little), as does the water used.