Fertilizer for flowers is like food to humans. It is the nutrient source that creates healthy plants. Flowers draw nutrients from sun, water, soil and from fertilizer. The three best, and most important, fertilizers, are nitrogen, phosphorus and potassium. These are the nutrients needed in the largest amounts by plants. Each of these minerals, when used by the flowering plant, assists in different growing functions of the plant.
Fertilizer containers usually show three numbers (NKP numbers) on the packaging, such as 20-15-15. These numbers always refer to the three best (main) fertilizers used by plants. The first number refers to nitrogen. A fertilizer with a higher first number would have a higher ratio of nitrogen. This fertilizer should be used at the beginning cycle of a flower plant's life. It promotes greenery. However, if you were to use a high nitrogen fertilizer through the life of the flower plant, you may get no, or very little, flower budding. Nitrogen is taken from the atmosphere and added with hydrogen to produce a form that can be used by plants: anhydrous ammonia.
The second number on the fertilizer packaging is phosphorus. It promotes strong root growth, as well as flower bud increase. Although you might use a higher ratio of nitrogen as the flower plant is young, you would also include some phosphorus. However, as the season progresses, before any flower buds are seen on the plant, you would use a higher phosphorus fertilizer. This will help to create more flowers on your plant. The ratio of phosphorus will be higher, such as 15-20-15. The phosphorus solution is produced from marine life fossils and sulfuric acid.
As the buds appear on the flower plant, switch to a higher potassium fertilizer, such as 15-15-20 (higher third number). Potassium is considered the big booster for plant blooms. Potassium regulates water levels within the flower plant, affecting the color and size of the flower. Potassium also acts as an overall plant strengthener. This nutrient, for fertilizers, is formed from a compound known as potash, taken from natural ore deposits.