How to calculate NPK formulae

          


Growers sometimes ask me how come that potassium nitrate 13-0-46 = 59% and not 100%.
Let’s start with a basic explanation about the NPK nutrients and their percentages (%) in fertilizer formulae.
Fertilizer's formula usually refers to the content of nutrients in their oxide forms (N-P₂O5-K₂O).
In Scandinavia, Australia and South Africa it is more common to refer to elemental forms (N-P-K).

For example:
The nutritional composition of Multi-K™ Classic is 13-0-46.
This means that the fertilizer contains

• 13% nitrogen (N)
• 0 % phosphorus oxide (P₂O₅) = 0% phosphorus (P)
• 46% potassium oxide (K₂O) = 38.3% potassium (K) 

 
Now, 13% N as NO₃ means that the product contains 13x4.43= 57.6% NO₃  
and 46% K₂O are 46/1.2= 38.3% K.
These sum up to 57.6+38.3=95.9%
Not 100% yet….
The difference is attributed to

• Actual analysis which is somewhat higher than 13-0-46
• Presence of moisture and trace impurities in the product

Note that Multi-K™ GG, Multi-K™ pHast and Multi-K™ Reci contain higher rates of nutrients, and less impurities.