Labeling of fertilizer
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.
- 1 Labeling of macronutrient fertilizers
- 2 NPK values for commercial fertilizers
- 3 See also
- 4 References
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 potash is a naturally occurring mineral consisting of nearly pure 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. To have 52% potassium, therefore, a fertilizer that gets all its potassium from K2O would have to be 63% K2O (.52/.83 is .63). Pure KCl fertilizer would thus be labeled 0-0-63; because potash is less than pure (it contains other compounds that contain no potassium), potash 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. 
NPK values for commercial fertilizers
- 15-00-00 Calcium nitrate
- 21-00-00 Ammonium sulfate
- 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)
NPK values for mined fertilizer minerals
- 11-08-02 to 16-12-03 bird guano
- 00-3-00 to 00-8-00 Raw Phosphate Rock (would be 00-34-00 if it were soluble)
- 00-00-22 Potassium magnesium sulfate (K-mag)
- 00-00-60 Potassium chloride
- 01-00-00 dairy manure
- 01-00-01 horse manure
- 03-02-02 poultry manure
- 04-12-00 Bone meal
- 05-05-06 Fish blood and bone 
- 06-02-00 Milorganite
- Chambers, Albert; Sally Rutherford (27 November 2007). "Voluntary Standards for the Fertilizer and Supplements Industry". Retrieved 3 February 2010.
- "Draft Code of Practice for Fertilier Description and Labeling" (PDF). Fertilizer Industry Federation Association (FIFA). 15 September 2008. Retrieved 3 February 2010.
-  22 October 2014
- UK Fertilizers Regulations 1990, Schedule 2 Part 1, Para. 7.
- "National Code of Practice for Fertilizer Description & Labelling" (PDF). Australian Government Department of Agriculture, Fisheries and Forestry. Retrieved 27 October 2014.
- NPK (Fertilizer)
- J. B. Sartain, University of Florida, "Food for turf: Slow-release nitrogen", Grounds Maintenance
- Bat and seal guano are lower in fertilizer value than bird guano. see Guano
- George Rehm, Michael Schmitt, John Lamb, Gyes Randall, and Lowell Busman (2002). "Understanding Phosphorus Fertilizers". University of Minnesota Extension Service.
- from "Average total N, ammonium N, phosphate and potash content of manure at the time of land application" in Animal Manure As a Plant Nutrient Resource, Bulletin ID-101 (Reviewed 02/05/01), Cooperative Extension Service, Purdue University. West Lafayette, IN 47907 
- Organic Fish, Blood and Bone Feed - Harrod Horticultural (UK)