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 species of atoms that have a given number of protons in their nuclei, including the pure substance consisting only of that species. Unlike chemical compounds, chemical elements cannot be broken down into simpler sub ...

nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...

phosphorus Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ear ...

, and

potassium Potassium is the chemical element with the symbol K (from Neo-Latin ''kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmosphe ...

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 Phosphorus pentoxide is a chemical compound with molecular formula P4 O10 (with its common name derived from its empirical formula, P2O5). This white crystalline solid is the anhydride of phosphoric acid. It is a powerful desiccant and dehydra ...

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 Potassium oxide ( K O) is an ionic compound of potassium and oxygen. It is a base. This pale yellow solid is the simplest oxide of potassium. It is a highly reactive compound that is rarely encountered. Some industrial materials, such as fertiliz ...

. 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 , 20% , and 52% of some inert ingredient.

Other labeling conventions

In the

U.K. The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain, is a country in Europe, off the north-western coast of the continental mainland. It comprises England, Scotland, Wales and North ...

, 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 Calcium oxide (CaO), commonly known as quicklime or burnt lime, is a widely used chemical compound. It is a white, Caustic (substance), caustic, alkaline, crystalline solid at room temperature. The broadly used term "''lime (material), lime''" co ...

and 29% total

sulfur trioxide Sulfur trioxide (alternative spelling sulphur trioxide, also known as ''nisso sulfan'') is the chemical compound with the formula SO3. It has been described as "unquestionably the most important economically" sulfur oxide. It is prepared on an ind ...

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 Sylvite, or sylvine, is potassium chloride (KCl) in natural mineral form. It forms crystals in the isometric system very similar to normal rock salt, halite ( NaCl). The two are, in fact, isomorphous. Sylvite is colorless to white with shades of ...

'' is a naturally occurring mineral consisting mostly of

potassium chloride Potassium chloride (KCl, or potassium salt) is a metal halide salt composed of potassium and chlorine. It is odorless and has a white or colorless vitreous crystal appearance. The solid dissolves readily in water, and its solutions have a salt ...

, KCl. Pure potassium chloride contains one potassium atom (whose

atomic mass The atomic mass (''m''a or ''m'') is the mass of an atom. Although the SI unit of mass is the kilogram (symbol: kg), atomic mass is often expressed in the non-SI unit dalton (symbol: Da) – equivalently, unified atomic mass unit (u). 1&nbs ...

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

* 15-00-00

Calcium nitrate Calcium nitrate, also called ''Norgessalpeter'' (Norwegian salpeter), is an inorganic compound with the formula Ca(NO3)2(H2O)x. The anhydrous compound, which is rarely encountered, absorbs moisture from the air to give the tetrahydrate. Both anhyd ...

* 21-00-00

Ammonium sulphate Ammonium sulfate (American English and international scientific usage; ammonium sulphate in British English); (NH4)2SO4, is an inorganic salt with a number of commercial uses. The most common use is as a soil fertilizer. It contains 21% nitrogen a ...

* 30-00-00 to 40-00-00 Sulfur-coated urea (slow release) * 31-00-00

Isobutylidenediurea Isobutylidenediurea (abbreviated IBDU) is an organic compound with the formula (CH3)2CHCH2. It is a derivative of urea (OC(NH2)2), which itself is highly soluble in water, but IBDU is not. It functions as a controlled-release fertiliser owing to ...

(~90% slow release) * 33-00-00 to 34-00-00

Ammonium nitrate Ammonium nitrate is a chemical compound with the chemical 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, although it does not form hydrates. It is ...

* 35-00-00 Ureaform (~85% slow release, sparingly soluble ureaformaldehyde) * 40-00-00 Methylene ureas (~70% slow release) * 46-00-00

Urea Urea, also known as carbamide, is an organic compound with chemical formula . This amide has two amino groups (–) joined by a carbonyl functional group (–C(=O)–). It is thus the simplest amide of carbamic acid. Urea serves an important r ...

(U-46) * 82-00-00

Anhydrous ammonia Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous was ...

* 10-34-00 to 11-37-00

Ammonium polyphosphate Ammonium polyphosphate is an inorganic salt of polyphosphoric acid and ammonia containing both chains and possibly branching. Its chemical formula is H4 PO3sub>n(OH)2 showing that each monomer consists of an orthophosphate radical of a phosphorus ...

* 11-48-00 to 11-55-00

Monoammonium phosphate Ammonium dihydrogen phosphate (ADP), also known as monoammonium phosphate (MAP) is a chemical compound with the chemical formula (NH4)(H2PO4). ADP is a major ingredient of agricultural fertilizers and some fire extinguishers. It also has sign ...

* 18-46-00 to 21-54-00

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 phosp ...

* 7-7-7 Growmore * 13-00-44

Potassium nitrate Potassium nitrate is a chemical compound with the chemical formula . This alkali metal nitrate salt is also known as Indian saltpetre (large deposits of which were historically mined in India). It is an ionic salt of potassium ions K+ and nitrat ...

* 00-17-00 to 00-22-00

Superphosphate Triple superphosphate is a component of fertilizer that primarily consists of monocalcium phosphate, Ca(H2PO4)2. Triple superphosphate is obtained by treating phosphate rock with phosphoric acid. Traditional routes for extraction of phosphate rock ...

(Monocalcium phosphate monohydrate with gypsum) * 00-44-00 to 00-52-00

Triple superphosphate Triple superphosphate is a component of fertilizer that primarily consists of monocalcium phosphate, Ca(H2PO4)2. Triple superphosphate is obtained by treating phosphate rock with phosphoric acid. Traditional routes for extraction of phosphate rock ...

(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 Guano (Spanish from qu, wanu) is the accumulated excrement of seabirds or bats. As a manure, guano is a highly effective fertilizer due to the high content of nitrogen, phosphate, and potassium, all key nutrients essential for plant growth. G ...

* 00-3-00 to 00-8-00 Raw Phosphate Rock (would be 00-34-00 if it were soluble) * 00-00-60

Potassium chloride Potassium chloride (KCl, or potassium salt) is a metal halide salt composed of potassium and chlorine. It is odorless and has a white or colorless vitreous crystal appearance. The solid dissolves readily in water, and its solutions have a salt ...

* 00-00-19

Kainite Kainite ( or ) (KMg(SO4)Cl·3H2O) is an evaporite mineral in the class of "Sulfates (selenates, etc.) with additional anions, with H2O" according to the Nickel–Strunz classification. It is a hydrated potassium-magnesium sulfate-chloride, natura ...

* 00-00-17

Carnallite Carnallite (also carnalite) is an evaporite mineral, a hydrated potassium magnesium chloride with formula KMgCl3·6(H2O). It is variably colored yellow to white, reddish, and sometimes colorless or blue. It is usually massive to fibrous with rare ...

NPK values for
biosolids Biosolids are solid organic matter recovered from a sewage treatment process and used as fertilizer. In the past, it was common for farmers to use animal manure to improve their soil fertility. In the 1920s, the farming community began also to use ...
fertilizers and others

* 09-00-00 dairy manure * 01-00-01 horse manure * 03-02-02 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 * 04-12-00

Bone meal Bone meal is a mixture of finely and coarsely ground animal bones and slaughter-house waste products. It is used as a dietary supplement to supply calcium (Ca) and phosphorus (P) to monogastric livestock in the form of hydroxiapathite. As a slow-r ...

* 05-05-06 Fish blood and boneOrganic Fish, Blood and Bone Feed – Harrod Horticultural (UK)
''www.harrodhorticultural.com'' accessed 15 December 2022 * 06-02-00 Milorganite

References

{{reflist Agricultural chemicals Fertilizers Packaging