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Pioneering The Future
How to calculate NPK formulae

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Haifa Blog

06Dec2018
How to calculate NPK formulae
By Ronald Valke

This blog is part of an ongoing knowledge sharing activity of the Haifa Group Modern Greenhouses  special task force.

A frequent question I get from growers is how come potassium nitrate 13-0-46 = 59% and not 100%

Nutrients content
I’d like to start with a basic explanation about the NPK nutrients and their percentages (%) in fertilizer formulae.
Fertilizer's formula usually refers to the content of nutrient 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)


So how we sum to 100% ?

Here we have to consider the actual form of the nutrients in the fertilizer:
In mineral fertilizers, nitrogen may be present as ammonium, nitrate or amide (urea), phosphorus is present mainly as phosphate, and potassium is present as K+ ion.  The following table shows the ratios between elements, oxides, and the forms mentioned above.

Ammonium

NH4 =

N x 1.28

 

Nitrate

NO3 =

N x 4.43

 

Amide

(NH2)2CO=

N x 2.14

 

Phosphate

H2PO4=

P x 2.68

P2O5 x1.15

Potassium

K =

K x1

K2O x 0.83

 

Now, 13% N as NO3 means that 13x4.43= 57.6% of the product is NO3  
and 46% K2O are 46/1.2= 38.3% K.
Together these sum to 57.6+38.3=95.9%

 Not 100% yet….
The difference is attributed to
-    The fact that the actual analysis is somewhat higher than 13-0-46
-    Presence of trace impurities in the product
-    Moisture 

 

Premium Multi-K™ products such as Multi-K™ GG and Multi-K™ Reci contain higher analysis, and less impurities. For actual analyses see product data sheets.
 

 

Pure Element

Multiply by

Oxide form

Multiply by

Pure Element

N

4.426

NO3

0.226

N

N

1.288

NH4

0.776

N

P

2.292

P2O5

0.436

P

K

1.205

K2O

0.83

K

Ca

1.399

CaO

0.715

Ca

Mg

1.658

MgO

0.603

Mg

S

2.996

SO4

0.334

S

S

2.497

SO3

0.4

S

 

This blog is part of an ongoing knowledge sharing activity of the Haifa Group Modern Greenhouses task force.