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.
In the U.K., fertilizer labeling regulations allow for reporting the elemental mass fractions of phosphorus and potassium.
[5] Fertilizers with additional macronutrients (S, Ca, Mg) may add more numbers to the N-P-K ratio to indicate the amount.
For example, a Polish fertilizer labeled "NPK (Ca,S) 4-12-12 (14-29)" has an equivalent of 14% soluble calcium oxide and 29% total sulfur trioxide.
[6] The values in an NPK fertilizer label are related to the concentrations (by weight) of phosphorus and potassium elements as follows: The N value in NPK labels represents actual percentage of nitrogen element by weight, so it does not need to be converted.