What do the numbers after NPK mean?

Assume you walk into a gardening store looking for the right kind of balanced nutrient mix for your plants. You come across a couple of balanced solutions. The label on the first one reads NPK 05-05-05 while on the other one it says NPK 10-10-10. You are wondering which one to go for. Answering following questions may help you pick the right one.

  1. Which one is better, NPK 05-05-05 or NPK 10-10-10?
  2. Which one of these is balanced?

To answer these questions, it is imperative to first understand what does the numbers in front of NPK denote.

Decoding NPK Formula:
Most of the literature and references available in public domain will tell you that the numbers in front of NPK denote the amount of Nitrogen, Phosphorus, and Potassium contained in the nutrient mix. Thus, NPK 10-10-10 will mean that the Nutrient mix consists of 10% Nitrogen, 10% Phosphorus, 10% Potassium, and remaining 70% being filler or inert matter.

This widely propagated and accepted explanation is, in fact, incorrect.

This is because the terms NPK do not denote the amount of Nitrogen, Phosphorus, and Potassium but the amount of Nitrogen, Phosphorus pentoxide (P2O5), and Potassium oxide (K2O) in the mix.  Thus, NPK 10-10-10 actually consists of 10% Nitrogen, 10% Phosphorus pentoxide, and 10% Potassium oxide, remaining 70% being filler or inert matter.

Let us see how that translates into the amount of major macronutrients, N, P, and K.

Converting NPK Formula to individual macronutrients:
For sake of understanding, we will consider NPK 10-10-10. As explained above, NPK 10-10-10 consists of :

  1. Nitrogen – 10%
  2. P2O5 – 10%
  3. K2O – 10%

Consider P2O5: each molecule of P2O5 consists of 2 atoms of P and 5 atoms of O. In elementary chemistry classes, we learned that the atomic weight of P is 31 and of O is 16. Thus a molecule of P2O5 will weigh 142 (=31×2 + 16×5) of which 62 is contributed by P (=31×2). Thus the amount of P in P2O5 is 43.6% (=62/142).

Consider K2O: similar calculations for K2O will show that the amount of K (atomic weight 30) in K2O is 83%.

So, in terms of macronutrients, NPK 10-10-10 consists of

  1. Nitrogen – 10%
  2. Phosphorus – 4.36%
  3. Potassium – 8.3%

Having understood this we can now answer the queries that were posed above

  • Which one is better NPK 05-05-05 or NPK 10-10-10?
    • None. They both are same. it’s just that the former has half the strength of latter.
  • Are both balanced?
    • Depends, if you are looking for a balance in N, P2O5, and K2O, then they both are balanced. However, if you are looking for a balance in N, P, and K, then, none of them are balanced.

How does that compare with CityGreens Proprietary Nutrient blend NPK?
CityGreens NPK differs on couple of counts

  1. Along, with NPK, it has 6 other vital micronutrients
  2. When used as instructed, it results in a ratio that provides almost equal amount of N, P, and K to the plants (instead of equal amount of N, P2O5, and K2O)

Having said that, just to reinforce, as we had pointed out in our last week’s post, there is no perfect nutrient formula that can work wonders with all the plants and in all the situations. What your plant needs may be entirely different from what your neighbors’ may need. You will need to experiment to arrive at the best solution for your specific case. We hope you do that.

Happy Farming!

Gaurav is an entrepreneur whose first Start-up was focused on providing services to patients suffering from Chronic Diseases. While researching about the causes of lifestyle diseases and the ways to reduce their incidence, he chanced upon the idea of growing healthy and nutritious food using advanced farming techniques.

He founded CityGreens with a mission to enable City Dwellers to access Safe, Healthy and Fresh food.

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