NPK rating

NPK rating
"NPK" redirects here. For the company that uses the NPK stock symbol, see National Presto Industries.

NPK rating (or N-P-K) is used to label fertilizer based on the relative content of the chemicals nitrogen(N), phosphorus(P), and potassium(K) that are commonly used in fertilizers.

Unlike the N number, the numbers for P and K do not reflect the amount of elemental phosphorus and potassium in the fertilizer. Rather they represent the amount of oxide in the form of P2O5 and K2O that would be present in the fertilizer if all the elemental phosphorus and potassium were oxidized into these forms.[1]

Method for converting an N-P-K value to an actual composition

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

Using these conversion factors we can determine that an 18−51−20 fertilizer contains by weight:

  • 18% elemental (N)
  • 22% elemental (P), and
  • 16% elemental (K)

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.[2]

NPK values for commercial fertilizers

NPK values for various synthetic fertilizers include[3]:

  • 82-00-00 Anhydrous Ammonia
  • 21-00-00 Ammonium Sulfate
  • 46-00-00 Urea
  • 35-00-00 Ureaform (~85% slow release, sparingly soluble ureaformaldehyde)[4]
  • 40-00-00 Methylene Ureas (~70% slow release)[4]
  • 31-00-00 Isobutylidene Diurea (~90% slow release)[4]
  • 30-00-00 to 40-00-00 Sulfur-coated Urea (slow release)[4]
  • 33-00-00 Ammonium Nitrate
  • 15-00-00 Calcium Nitrate
  • 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)
  • 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

NPK Values for mined fertilizer minerals

  • 00-3-00 to 00-8-00 Raw Phosphate Rock (would be 00-34-00 if it was soluble)[5]
  • 00-00-60 Potassium Chloride
  • 11-08-02 to 16-12-03 bird guano[6]

NPK values for biosolids fertilizers and others


  1. ^ Libbert, Dr. Robert "Why is phosphorus expressed as P2O5 and potassium expressed as K20 in the soil test report recommendations and by the fertilizer companies?" 21-12-2008
  2. ^ UK Fertilizers Regulations 1990, Schedule 2 Part 1, Para. 7.
  3. ^
  4. ^ a b c d J. B. Sartain, University of Florida ((c)2011), "Food for turf: Slow-release nitrogen", Grounds Maintenance, 
  5. ^ George Rehm, Michael Schmitt, John Lamb, Gyes Randall, and Lowell Busman (Reviewed 2010). "Understanding Phosphorus Fertilizers". University of Minnesota Extension Service. 
  6. ^ Bat and seal guano are lower in fertilizer value than bird guano. see Guano
  7. ^ a b c 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 [1]

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