Fertilizer NPK Ratios – What Do They Really Mean

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Robert Pavlis

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 - What Do They Really Mean
Fertilizer NPK Numbers – What Do They 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.

Fertilizer label from the UK
Fertilizer label from the UK

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.

Plant Science for Gardeners by Robert Pavlis

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:

  1. Photo Source: SuSanA Secretariat

 

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

23 thoughts on “Fertilizer NPK Ratios – What Do They Really Mean”

  1. Robert: I have a question, what is NPK 18:18:5 ? is that something that is especially blended for a certain use?. Can you please send me your response directly to my email. Thank you very
    much

    Reply
    • Do you mean the accuracy of fertilizer labels? The company making the product would add the ingredients in the right proportion to make the product. They probably do some testing. I don’t think outside agencies do much testing.

      Reply
  2. Hello Robert, please i just want to know if really (as some botanists claimed) that chemical fertilizers can damage the soil in the long run?… like the soil ph and all, and if true how often should one use them for this damage not to occur?

    Reply
  3. Hi Robert

    I emailed Scott’s customer service the following:

    Hi there
    On the Canadian Tire . CA website for your product, Evergreen & Acid Loving Plant Food, the description says:

    Balanced formula with 28% Nitrogen, 10% Phosphorus and 10% Potassium

    However from my research, the NPK values refer not to the % of the nutrient, but to the % of the ingredient. Ie, P = phosphoric acid, which is around half phosphorous, and K= potash, which is around 80% potassium.

    Therefore it is misleading to state that the product is 28% Nitrogen, 10% Phosphorus and 10% Potassium…. It would be more accurate to say 10% phosphoric acid and 10% potash, or if you want to use the elemental name, ~4% phosphorous and ~8% potassium.

    They wrote back, “The numbers represent the percentages of each of the nutrients per container. For example, a fertilizer that has a 24-8-16 analysis contains 24% of the bag’s total weight in nitrogen, 8% of phosphorus and 16% of potassium.”

    What do you think – is their response incorrect?

    Reply
  4. To take it a step further there is the factor of how much of each element in the chemical breakdown is actually available.

    Reply
    • Absolutely correct. what you add to the soil may not reflect what is available to plants. But the availability to plants is a very complex topic with many variables – good for a separate post. some of the variables would include:
      – chemical composition of each nutrient in the fertilizer
      – soil types
      – pH of soil
      – rain fall
      – current levels of nutrients in soil
      – type of plants
      – temperature
      – time of year

      Reply
  5. Just to clarify, in Australia where they report elemental P and K does that mean a fertiliser of 10-10-10 would contain 10% of each?

    Reply
    • That is my understanding, based on Wikipedia, and this reference, the “National (Australia) Code for Labeling Fertilizer” which says:

      “For a fertilizer that contains any of the nutrients N, P, K or S above the minimum level for inclusion on the label, and for which the level of inclusion is stated, then the concentration of N, P, K and S, including zero values where no claim is made for inclusion, are to be printed directly below the product name, enclosed in parenthesis and separated by hyphens in the order N-P-K-S. …… In the elemental form (e.g. P, K) not the oxide form as is the case in some countries (e.g. P2O K2O).”

      Reply
      • If you work in the derived units all the time and just forget the actual elemental amounts you can accurately compare one fertiliser with another.
        Eg as a silly simple example one gram of a 14 14 14 fertiliser equals two grams of Growmore which is 7 7 7
        In general recommendations in the UK are not given in the elemental amount and gardeners just carry on as if they did not exist

        Reply

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