Labeling of fertilizer

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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.

Video Labeling of fertilizer

Labeling of macronutrient fertilizers

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.

Converting nutrient analysis to composition

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:

P₂O₅ consists of 56.4% oxygen and 43.6% elemental phosphorus. The percentage (mass fraction) of elemental phosphorus is 43.6% so elemental P = 0.436 x P₂O₅
K₂O consists of 17% oxygen and 83% elemental potassium. The percentage (mass fraction) of elemental potassium is 83% so elemental K = 0.83 x K₂O
Nitrogen values represent actual nitrogen content so these numbers do not need to be converted.

Using these conversion factors, an 18-51-20 fertilizer contains by weight:

18% elemental (N)
22% elemental (P), and
17% elemental (K)

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. 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.

Maps Labeling of fertilizer

NPK values for commercial fertilizers

NPK values for various synthetic fertilizers

15-00-00 Calcium nitrate
21-00-00 Ammonium sulphate
30-00-00 to 40-00-00 Sulfur-coated urea (slow release)
31-00-00 Isobutylidene diurea (~90% slow release)
33-00-00 to 34-00-00 Ammonium nitrate
35-00-00 Ureaform (~85% slow release, sparingly soluble ureaformaldehyde)
40-00-00 Methylene ureas (~70% slow release)
46-00-00 Urea (U-46)
82-00-00 Anhydrous ammonia
10-34-00 to 11-37-00 Ammonium polyphosphate
11-48-00 to 11-55-00 Monoammonium phosphate
18-46-00 to 21-54-00 Diammonium phosphate
13-00-44 Potassium nitrate
00-17-00 to 00-22-00 Superphosphate (Monocalcium phosphate monohydrate with gypsum)
00-44-00 to 00-52-00 Triple superphosphate (Monocalcium phosphate monohydrate)
00-00-22 Potassium magnesium sulfate (K-mag)