What Do Fertilizer Labels Mean?
Commercial fertilizers, by standard, contain at least one of the three primary nutrients (N), phosphorous (P), and potassium (K). Phosphorous and potassium in their elemental form cannot be absorbed by the plants. In fertilizers they come in a form that is useable by plants. Phosphorous comes in the form of phosphate (P2O5) while potassium comes as potash (K2O). Fertilizer qualifies to be a commercial grade only when it contains more than five percent of N, P2O5 or K2O.
Every bag of commercial fertilizer is labeled with three numbers separated by dashes as shown in the above picture. These numbers reflect the percentage of the three primary nutrients in the fertilizer, the first number being nitrogen (N) - the second number is phosphate (P2O5) - the third number is % potash (K2O). The label always lists the nutrients in this order.
16-20-0 is a fertilizer containing 16% nitrogen, 20% phosphate, and 0% potash. Mixed fertilizers (fertilizers that contain two or more of the primary nutrients) either complete or incomplete, are made by combining different single-element sources to form a desired blend. Because of this combination, the resulting fertilizer mix may have particles of varying colors. In as much as plants require different proportions of the primary nutrients, manufacturers produce different grades.
How to determine the nutrient content of fertilizers