Compound fertilizer

Fertilizers are usually labeled with three numbers, as in 18-20-10, indicating the relative content of the primary macronutrients nitrogen (N), phosphorus (P), and potassium (K), respectively.

More precisely, the first number ("N value") is the percentage of elemental nitrogen by weight in the fertilizer; that is, the mass fraction of nitrogen times 100. The second number ("P value") is the percentage by weight of phosphorus pentoxide P2O5 in a fertilizer with the same amount of phosphorus that gets all of its phosphorus from P2O5. The third number ("K value") is analogous, based on the equivalent content of potassium oxide K2O.

For example, a 15-13-20 fertilizer would contain 15% by weight of nitrogen, and the same amounts of phosphorus and potassium as a mixture of 13% by weight of P2O5, 20% K2O, and 52% of some inert ingredient.