NPK rating

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"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 chemical elements nitrogen (N), phosphorus (P), and potassium (K) that are commonly used in fertilizers. The N value is the percentage of elemental nitrogen by weight in the fertilizer. The values for P and K 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]

Component functions[edit]

The three elements promote plant growth in three different ways. In simple terms, these are:

  • N – nitrogen: promotes the growth of leaves and vegetation[2]
  • P – phosphorus: promotes root and shoot growth
  • K – potassium: regulation of water and nutrient movement in plant cells, and should promote flowering, fruiting and general hardiness[3]

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

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 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 we can determine that an 18−51−20 fertilizer contains by weight:

  • 18% elemental (N)
  • 22% elemental (P), and
  • 17% 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.[4]

NPK values for commercial fertilizers[edit]

NPK values for various synthetic fertilizers include:[5][edit]

NPK Values for mined fertilizer minerals[edit]

  • 11-08-02 to 16-12-03 bird guano[7]
  • 00-3-00 to 00-8-00 Raw Phosphate Rock (would be 00-34-00 if it were soluble)[8]
  • 00-00-22 Potassium magnesium sulfate (K-mag)
  • 00-00-60 Potassium chloride

NPK values for biosolids fertilizers and others[edit]


  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. ^ Fertilisers / Royal Horticultural Society
  3. ^ Fertilisers / Royal Horticultural Society
  4. ^ UK Fertilizers Regulations 1990, Schedule 2 Part 1, Para. 7.
  5. ^ NPK (Fertilizer)
  6. ^ a b c d J. B. Sartain, University of Florida ((c)2011), "Food for turf: Slow-release nitrogen", Grounds Maintenance 
  7. ^ Bat and seal guano are lower in fertilizer value than bird guano. see Guano
  8. ^ George Rehm, Michael Schmitt, John Lamb, Gyes Randall, and Lowell Busman (Reviewed 2010). "Understanding Phosphorus Fertilizers". University of Minnesota Extension Service. 
  9. ^ 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]
  10. ^ Organic Fish, Blood and Bone Feed - Harrod Horticultural (UK)