If you’ve ever bought fertilizer of any kind, you’ve probably seen the 3 large numbers, such as 24-4-8 or 10-5-8, prominently displayed on the package. Always displayed in the same order, these numbers represent the percentage in weight of nitrogen, phosphorus (P2O5), and potassium (K2O), or collectively seen as (N-P-K). So, a 15-12-5 fertilizer has 15% nitrogen, 12% phosphorus and 5% potassium.
Each of three ingredients has a specific purpose as being a beneficial nutrient needed for plant health and growth. These nutrients are usually missing from the soil because large quantities are needed for healthy plant growth and survival. Take note that there is a wide variety of these weighted percentage formulations. Knowing when to use what particular formulation is important and will depend on your specific application. 
The first number, nitrogen, is the primary nutrient responsible for plant growth. It is also needed in order to produce chlorophyll, which gives our plants the green tint.
The second number, phosphorus, aids in root growth and blooming. You’ve probably seen packaging promoting Bloom power fertilizer (see photo at right). High phosphorus fertilizer would benefit applications such as a newly transplanted tree or aiding in the blooms and the overall establishment of the plants in your garden.
The third number, potassium, helps with the overall health and vigor of the plant. It also helps the plant withstand stress due to large temperature changes and drought. Fall fertilizers have high potassium percentages, which help to overwinter the yard.
Before applying any fertilizer to your soil, it’s wise to have your soil tested. That will give you a baseline of the current condition of your soil. There are several options for testing your soil: You can purchase a store-bought soil test kit from your local hardware store or contact your local Cooperative Extension Service.
