NPK fertilizers are three-component fertilizers. They provide sources of potassium as well as nitrogen and phosphorus. Such NPK fertilizers are 70-75% soluble in water.
NPK rating is a government mandated rating system describing the amount of nitrogen, phosphorus, and potassium. NPK ratings consist of three numbers separated by dashes (e.g., 10-10-10 or 16-4-8) describes the chemical content of fertilizers. The first number represents the percentage of nitrogen in the product; the second number, P2O5 consists of 56.4% oxygen and 43.6% elemental phosphorus; the third, K2O consists of 17% oxygen and 83% elemental potassium. The generalized form is N-P-K. A 50-pound bag of fertilizer labeled 16-4-8 contains 8 pounds of nitrogen (16% of the 50 pounds), 2 pounds of P2O5 (4% of 50 pounds), and 4 pounds of K2O (8% of 50 pounds). Australian convention adds a fourth number for Sulphur. Although unimportant in a practical sense, fertilizers do not contain P2O5 or K2O per se. These concise formulas are shorthand for the nature of the phosphorus (phosphate) or potassium (K+) sources.