Most references say that the NPK values indicate the % nitrogen, % phosphorus and % potassium. They say this, not because it’s true, but because it makes it simple to explain fertilizer concepts to the general public.
What does NPK really mean?
Fertilizer NPK Numbers
Each country has their own requirements for labeling fertilizer, so this post may not apply to you. In Australia, they report elemental P and K as well as sulfur (S). This post does apply to North America and most of Europe. Please let me know if your country has different rules.
Update: In Ireland and the UK, product labels can show both values, as in the image below.
The letters N, P and K are the elemental symbols used by chemists as a short hand to describe the chemical. N is used for nitrogen and P for phosphorus. The letter K is used for potassium and stands for kalium, the original Latin name for potassium. If you have trouble remembering whether P stands for phosphorus or potassium, remember that the three nutrients are listed in alphabetical order. Phosphorus comes before potassium, alphabetically, and so the last letter in the list, K, is short for potassium.
The N value is the % nitrogen.
The P and K values are the % P2O5 and % K2O and NOT the %P and % K as so many references claim.
Why is this important? It is critical to understand this if you are trying to figure out how much fertilizer to add to your garden. For example, if your soil test suggests you add 5 Kg P per 1,000 sq meters, you need to be able to convert this requirement to the weight of fertilizer needed.
The following will help you convert to % P and % K:
- P2O5 consists of 56.4% oxygen and 43.6% phosphorus by weight. To get the % P value, multiply the reported NPK value by 0.436, or approximately half of the reported value.
- K2O consists of 17% oxygen and 83% potassium by weight. To get the % K value, multiply the reported NPK value by 0.83.
Using this information you can see that a fertilizer NPK number of 10-10-10 contains 10% nitrogen, 4.36% phosphorus, and 8.3% potassium. These conversion numbers will help you determine the correct amount of fertilizer to add to your garden so that you meet the soil test requirements.
Calculating The Amount of Fertilizer Needed
Let’s say have a 25Kg bag of rock phosphate with an NPK value of 0-32-0.
To figure out how much phosphorus you have in the bag, multiply 32 x 0.436, to get the % P = 14%.
Since you have a 25 Kg bag it contains 3.5 Kg P (25 x 14%).
Why Is This So Complicated?
Why report values for P2O5 and K2O instead of the elemental P and K? It is a historical thing. The tests chemists used to use measured P2O5 and K2O and not P and K. So they reported fertilizer NPK numbers as %P2O5 and %K2O. In most countries we still do that today.
For more information on soil testing for fertilizer NPK numbers see Soil Testing for NPK.
Other Countries
Australia, New Zealand, S. Africa and some other African countries report the NPK of fertilizer differently. In Australia and New Zealand the NPK values are the percent weight of nitrogen, phosphorus and potassium – not the oxides as described above for North America and Europe.
In S. Africa a bag of 10-10-10 (USA) would be labeled as 5-2-4(22) because instead of reporting the oxides of P and K they report the actual N, P, K, so a 10-10-10 is actually 10-4-8, and when that is reduced to a ratio it is 5-2-4 (22). The 22 is the total weight of the NPK nutrients.
References:
- Photo Source: SuSanA Secretariat
Thank you, Robert !
I would like to comment on a few points:
1- The percentages correspond to mass percentages when the fertilizer is in solid form, and may be expressed (not always) as mass per liter when the fertilizer is in solution.
2- P2O5 (phosphorus pentoxide, not phosphoric acid as is often incorrectly stated) is not actually present in fertilizers. The same applies to K2O (potassium oxide, often incorrectly referred to as potash). These compounds are used mainly for historical reasons as chemical equivalents to express the overall percentage of phosphorus- and potassium-containing in the different materials composing the fertilizer.
Both P2O5 and K2O are highly hygroscopic (with P2O5 commonly used in chemistry as a desiccant) and would be impractical to handle, sell, or apply as fertilizers. P2O5 energetically reacts with water to form phosphoric acid (a strong acid), while K2O reacts with water to form potassium hydroxide (a strong base).
In practice, fertilizers consist of salts or small molecules, such as K2HPO4 (dipotassium hydrogen phosphate), CaSO4 (calcium sulfate), or simple organic molecules like urea (H2NC(O)NH2). These compounds are composed of a positively charged entity (cation), such as potassium (K+), calcium (Ca(2+)) or ammonium (NH4+), and a complementary negatively charged entity (anion), such as phosphate species like dihydrogenophosphate (H2PO4-) or sulfate (SO4(2-)). These salts are generally readily soluble in water. The “-ium” suffix typically refers to a positively charged species, while “-ate” refers to a negatively charged species. The combination of cation(s) and anion(s) forms neutral salts.
3- As an example of percentage calculations for P and K, consider a fertilizer that contains K2HPO4 (potassium hydrogen phosphate, composed of two potassium cation K+ and one hydrogenophosphate anion HPO4-) at a level of 2 kg in a 10 kg bag. Assuming this salt is the only source of elemental P and K, the fertilizer would contain 3.6% P and 9.0% K on an elemental basis.
This arises from the fact that the mass percentages of P and K in K2HPO4 are 17.8% and 44.9%, respectively. In other words, 2 kg of K2HPO4 contains approximately 356 g of elemental P and 898 g of elemental K (in addition to about 734 g of elemental oxygen and 12 g of elemental hydrogen).
In many countries, however, P and K contents are expressed as mass equivalents of P2O5 and K2O. Thus, the 10 kg bag containing 356 g of elemental phosphorus corresponds to 816 g of P2O5, since phosphorus accounts for 43.6% of P2O5 (i.e., 43.6% of 816 g = 356 g). As a result, the fertilizer would be labeled as 8.2% P2O5.
Similarly, 898 g of elemental potassium corresponds to 1082 g of K2O, since potassium accounts for 83% of K2O (i.e., 83% of 1082 g = 898 g). Therefore, the fertilizer would be labeled as 10.8% K2O.
In summary:
• Real percentages: P 3.6% (K2HPO4), K 9.0% (K2HPO4)
• Commercial equivalent percentages: P 8.2% (P2O5), K 10.8% (K2O)
The difference tends to be larger for the P value than for K value due to the large %age of K in K2O (83.0%) as compared to the percentage of P in P2O5 (43.6%).