Labeling of fertilizer
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 elements nitrogen, 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 (N), phosphorus (P), and potassium (K), respectively.
More precisely, the first number ("N value") is the percentage of elemental nitrogen by weight in the fertilizer; that is, the mass fraction of nitrogen times 100. The second number ("P value") is the percentage by weight of phosphorus pentoxide P
5 in a fertilizer with the same amount of phosphorus that gets all of its phosphorus from P2O5. The third number ("K value") is analogous, based on the equivalent content of potassium oxide K
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 P
5, 20% K
2O, 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 (15-13-13)".
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 (S) contained in the fertilizer. 
Converting nutrient analysis to composition
The values in an NPK fertilizer label are related to the concentrations (by weight) of phosphorus and potassium elements as follows:
5 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.
2O 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 (whose atomic mass is 39.09 g/mol) for every chlorine atom (whose atomic mass is 35.45 g/mol). Therefore, pure KCl is 39.09/(39.09 + 35.45) = 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 K
2O and has the same potassium contents as pure KCl would have to be 63% K
2O. 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.
NPK values for commercial 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)
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)
- 00-00-60 Potassium chloride
- 00-00-19 Kainite
- 00-00-17 Carnallite
- 09-00-00 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". 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. Archived from the original (PDF) on 28 February 2015. 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; 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 "Archived copy". Archived from the original on 12 March 2012. Retrieved 28 October 2014.CS1 maint: archived copy as title (link)
- Organic Fish, Blood and Bone Feed - Harrod Horticultural (UK)