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 A chemical element is a chemical substance whose atoms all have the same number of protons. The number of protons is called the atomic number of that element. For example, oxygen has an atomic number of 8: each oxygen atom has 8 protons in i ...

nitrogen Nitrogen is a chemical element; it has Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a Nonmetal (chemistry), nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the Pnictogen, pnictogens. ...

phosphorus Phosphorus is a chemical element; it has Chemical symbol, symbol P and atomic number 15. All elemental forms of phosphorus are highly Reactivity (chemistry), reactive and are therefore never found in nature. They can nevertheless be prepared ar ...

, and

potassium Potassium is a chemical element; it has Symbol (chemistry), symbol K (from Neo-Latin ) and atomic number19. It is a silvery white metal that is soft enough to easily cut with a knife. Potassium metal reacts rapidly with atmospheric oxygen to ...

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 primary 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 . The third number ("K value") is the equivalent content of potassium oxide . For example, a 15-13-20 fertilizer would contain 15% by weight of nitrogen, 13% by weight of , 20% by weight of , and 52% 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. Fertilizers with additional macronutrients (S, Ca, Mg) may add more numbers to the N-P-K ratio to indicate the amount. The additional numbers are similarly reported in the oxide mass fraction form. For example, a Polish fertilizer labeled "NPK (Ca,S) 4-12-12 (14-29)" has an equivalent of 14% soluble calcium oxide and 29% total sulfur trioxide.

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: * 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. * 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 and has the same potassium contents as pure KCl would have to be 63% . 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

NPK values for various synthetic fertilizers

Source: * 15-0-0 Calcium nitrate * 21-0-0 Ammonium sulphate * 30-0-0 to 40-0-0 Sulfur-coated urea (slow release) * 31-0-0 Isobutylidenediurea (~90% slow release) * 33-0-0 to 34-0-0

Ammonium nitrate Ammonium nitrate is a chemical compound with the formula . It is a white crystalline salt consisting of ions of ammonium and nitrate. It is highly soluble in water and hygroscopic as a solid, but does not form hydrates. It is predominantly us ...

* 35-0-0 Ureaform (~85% slow release, sparingly soluble ureaformaldehyde) * 40-0-0 Methylene ureas (~70% slow release) * 46-0-0

Urea Urea, also called carbamide (because it is a diamide of carbonic acid), is an organic compound with chemical formula . This amide has two Amine, amino groups (–) joined by a carbonyl functional group (–C(=O)–). It is thus the simplest am ...

(U-46) * 82-0-0 Anhydrous ammonia * 10-34-0 to 11-37-0 Ammonium polyphosphate * 11-48-0 to 11-55-0 Monoammonium phosphate * 18-46-0 to 21-54-0

Diammonium phosphate Diammonium phosphate (DAP; IUPAC name diammonium hydrogen phosphate; chemical formula (NH4)2(HPO4)) is one of a series of water- soluble ammonium phosphate salts that can be produced when ammonia reacts with phosphoric acid. Solid diammonium ph ...

* 7-7-7 Growmore * 13-0-44 Potassium nitrate * 0-17-0 to 0-22-0 Superphosphate (Monocalcium phosphate monohydrate with gypsum) * 0-44-0 to 0-52-0 Triple superphosphate (Monocalcium phosphate monohydrate) * 0-0-22 Potassium magnesium sulfate (K-mag)

NPK values for mined fertilizer minerals

* 11-8-2 to 16-12-3 bird guano * 0-3-0 to 0-8-0 Raw Phosphate Rock (would be 0-34-0 if it were soluble) * 0-0-60 Potassium chloride * 0-0-19 Kainite * 0-0-17

Carnallite Carnallite (also carnalite) is an evaporite mineral, a hydrated potassium magnesium chloride with formula KCl.MgCl2·6(H2O). It is variably colored yellow to white, reddish, and sometimes colorless or blue. It is usually massive to fibrous with r ...

NPK values for biosolids fertilizers and others

* 9-0-0 dairy manure * 1-0-1 horse manure * 3-2-2 poultry manurefrom "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 * 4-12-0 Bone meal * 5-5-6 Fish blood and boneOrganic Fish, Blood and Bone Feed – Harrod Horticultural (UK)
''www.harrodhorticultural.com'' accessed 15 December 2022 * 6-2-0 Milorganite

References

{{reflist Agricultural chemicals Fertilizers Packaging