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 Phosphorus pentoxide is a chemical compound with molecular formula Phosphorus, P4Oxygen, 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 desic ...

. The third number ("K value") is 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 fertil ...

. 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 Calcium oxide (formula: Ca O), commonly known as quicklime or burnt lime, is a widely used chemical compound. It is a white, caustic, alkaline, crystalline solid at room temperature. The broadly used term '' lime'' connotes calcium-containing ...

and 29% total

sulfur trioxide Sulfur trioxide (alternative spelling sulphur trioxide) is the chemical compound with the formula SO3. It has been described as "unquestionably the most conomicallyimportant sulfur oxide". It is prepared on an industrial scale as a precursor to ...

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 ...

'' 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 sa ...

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

atomic mass Atomic mass ( or ) is the mass of a single atom. The atomic mass mostly comes from the combined mass of the protons and neutrons in the nucleus, with minor contributions from the electrons and nuclear binding energy. The atomic mass of atoms, ...

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 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) * 33-0-0 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 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 dry chemical fire extinguishers. It also has ...

* 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 Potassium nitrate is a chemical compound with a sharp, salty, bitter taste and the chemical formula . It is a potassium salt of nitric acid. This salt consists of potassium cations and nitrate anions , and is therefore an alkali metal nit ...

* 0-17-0 to 0-22-0

Superphosphate Superphosphate is a chemical fertiliser first synthesised in the 1840s by reacting bones with sulfuric acid. The process was subsequently improved by reacting phosphate coprolites with sulfuric acid. Subsequently, other phosphate-rich deposits suc ...

(Monocalcium phosphate monohydrate with gypsum) * 0-44-0 to 0-52-0

Triple superphosphate Superphosphate is a chemical fertiliser first synthesised in the 1840s by reacting bones with sulfuric acid. The process was subsequently improved by reacting phosphate coprolites with sulfuric acid. Subsequently, other phosphate-rich deposits suc ...

(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 Guano (Spanish from ) is the accumulated excrement of seabirds or bats. Guano is a highly effective fertiliser due to the high content of nitrogen, phosphate, and potassium, all key nutrients essential for plant growth. Guano was also, to a le ...

* 0-3-0 to 0-8-0 Raw Phosphate Rock (would be 0-34-0 if it were soluble) * 0-0-60

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 sa ...

* 0-0-19

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, natu ...

* 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 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 us ...
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