NPK rating
The labeling of fertilizers varies by country in terms of analysis methodology, nutrient labeling, and minimum nutrient requirements. The most common labeling convention shows the amounts of nitrogen, phosphorus, and potassium in the fertilizer.
Macronutrient fertilizers are generally labeled with an NPK analysis, based on the relative content of the chemical elements nitrogen (N), phosphorus (P), and potassium (K) that are commonly used in fertilizers. However, numbers used in this labeling scheme do not directly represent the source composition or absolute nutrient content of the fertilizer. The N value is the percentage of elemental nitrogen by weight in the fertilizer. The value for P is the fraction by weight of P2O5 in a fertilizer with the same amount of phosphorus that gets all of its phosphorus from P2O5. The value for K is analogous, based on a fertilizer with K2O.
For example, the fertilizer sylvite is a naturally occurring mineral consisting mostly of potassium chloride (KCl). As such, it contains one potassium atom for every chlorine atom, and is 52% potassium and 48% chlorine by weight (because of the difference in atomic weights of the elements). K2O is similarly 83% potassium. Therefore, a fertilizer that gets all its potassium from KCI would have to be 63% K2O (.52/.83 is .63). Pure KCl fertilizer would thus be labeled 0-0-63; because sylvite is less than pure (it contains other compounds that contain no potassium), it is labeled 0-0-60.
The factors for converting from P2O5 and K2O values on a fertilizer label to the concentrations (by weight) of P and K elements are as follows:
Using these conversion factors, an 18−51−20 fertilizer contains by weight:
In the U.K., fertilizer labeling regulations allow for reporting the elemental mass fractions of phosphorus and potassium. The regulations stipulate that this should be done in parentheses after the standard N-P-K values. In Australia, macronutrient fertilizers are labeled with an "N-P-K-S" system, which uses elemental mass fractions rather than standard N-P-K values and includes the amount of sulfur (S) contained in the fertilizer.
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