To a new grower, these numbers may mean nothing, but they would very soon learn that those numbers are a crucial factor in construction and application of any fertilizer.
The numbers on the label of a bag of soil or fertilizer represent the percentage of three different chemicals. In the 1800’s, a man named Justus Von Liebig found that Nitrogen, Phosphorus and Potassium are essential for healthy plant growth. Since then, almost all fertilizers and nutrients for cultivation have included these chemicals.
Represented by their periodic element labels, N, P and K, their letters are replaced by the numbers you see on fertilizer bags and containers, in a consistent order on every package (N-P-K). Each number represents how much Nitrogen, Phosphorus and Potassium is present in a percentage by volume (e.g. 16-8-6 means 16% N, 8% P and 6% K) in that product.
All fertilizers will contain one or more of these components. When one is missing from a fertilizer it is replaced by a 0 in the NPK ratio (e.g. 15-0-0). This means that you can buy separated fertilizers that have more Nitrogen but lack Phosphorus and Potassium because most soils are already rich in them. These are known as complete and incomplete fertilizers. A complete fertilizer will have all three, while an incomplete fertilizer may only have one or two.
Most general purpose fertilizers will contain an equal ratio of NPK (e.g. 12-12-12) or a slightly higher amount of Nitrogen compared to Potassium and Phosphorus. These fertilizers are designed to meet most general needs of a plant throughout its growing season.
You can also purchase specialized fertilizers. These fertilizers are specifically designed for certain plants and growth requirements. Some contain much more N than P or K to promote active growth in early stages, while a fertilizer with more P and K than N would be useful in stimulating root growth and plant vigor. If you know exactly what you want to grow, there are even specialized fertilizers for specific plants, from citrus trees to rhododendrons.