Many countries have standardized the labeling of fertilizers to indicate their contents of major nutrients. The most common labeling convention, the NPK or N-P-K label, shows the amounts of the chemical elementsnitrogen, phosphorus, and potassium.
Common labeling conventions
The NPK analysis label
Fertilizers are usually labeled with three numbers, as in 18-20-10, indicating the relative content of the macronutrients nitrogen , phosphorus , and potassium , respectively. More precisely, the first number is the percentage of elemental nitrogen by weight in the fertilizer; that is, the mass fraction of nitrogen times 100. The second number is the percentage by weight of phosphorus pentoxide in a fertilizer with the same amount of phosphorus that gets all of its phosphorus from P2O5. The third number is analogous, based on the equivalent content of potassium oxide. 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, 20%, and 67% of some inert ingredient.
Other labeling conventions
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, as in "15-30-15 ". In Australia, macronutrient fertilizers are labeled with an "N-P-K-S" system, which uses elemental mass fractions rather than the standard N-P-K values and includes the amount of sulfur contained in the fertilizer.
Converting nutrient analysis to composition
The values in an NPK fertilizer label are related to the concentrations of phosphorus and potassium elements as follows:
consists of 56.4% elemental oxygen and 43.6% elemental phosphorus by weight. Therefore, the elemental phosphorus percentage of a fertilizer is 0.436 times its P value.
consists of 17% oxygen and 83% elemental potassium by weight. Therefore, the elemental potassium percentage is 0.83 times the K value.
The N value in NPK labels represents actual percentage of nitrogen element by weight, so it does not need to be converted. So, for example, an 18−51−20 fertilizer contains by weight
18% elemental nitrogen,
0.436 × 51 = 22% elemental phosphorus, and
0.83 × 20 = 17% elemental potassium.
As another example, the fertilizer sylvite is a naturally occurring mineral consisting mostly of potassium chloride, KCl. Pure potassium chloride contains one potassium atom for every chlorine atom. Therefore, pure KCl is 39.09/ = 52% potassium and 48% chlorine by weight. Its K value is therefore 52/0.83 = 63; that is, a fertilizer that gets all its potassium from and has the same potassium contents as pure KCl would have to be 63%. Pure KCl fertilizer would thus be labeled 0-0-63. Since sylvite contains other compounds that contribute no N, P, or K, it is usually labeled 0-0-60.
