FluidsensorOnline.com

The information on the back of a bottle or packet of fertiliser can be difficult to understand. It may list the ingredients and  appear to give the % of each element, however things are not as simple as they may appear.

The three main elements in most fertilisers are nitrogen (N), Phosphorous (P) and Potassium (K). You may see three numbers prominently displayed on some fertilisers – 4 1 3 for example. This is the ‘NPK’ ratio and means (in theory) that it contains 4% nitrogen, 1% phosphorous, and 3% potassium.
However on the list of ingredients although you may find nitrogen (or N) you may not find ‘P’ or ‘K’. Instead you might see P2O5 and K2O (phosphorous pentoxide and potassium oxide) even though the fertiliser does not actually contain either of them!

It is done this way in order to comply with UK fertiliser laws.
The reasons are historical. In the 19th century and without the aid of the analytical instrumentation available today, the phosphorous and potassium content could not be measured directly. Instead the fertiliser was burnt and the residual ash, phosphorous pentoxide or potassium oxide, was measured instead.

There is another source of confusion to those of us who are not chemists! Many articles written about fertilisers refer to nitrate (NO3) and phosphate (PO4) rather than to the elements themselves. This is understandable because it is usually a nitrate that is providing the nitrogen and a phosphate that is providing the phosphorous. The good news is that it is easy to calculate the actual quantity of the individual elements and vice versa:-

If something contains 12% P2O5 it will contain 12 x 0.436 = 5.23% phosphorous

Other symbols and abbreviations often used in connection with fertilisers are %, ppm, W/V and V/V

% (per cent) is the same as parts per hundred. Potassium sulphate contains 44.9% potassium. This means that 100 grams of potassium sulphate contains 44.9 grams of potassium.
ppm is an abbreviation for parts per million. Some of the nutrients required by plants are needed in such small quantities that using % as a measure would have too many zero’s. 1 ppm = 0.0001%

Sometimes, after a % or ppm value ‘W/W’, ‘W/V’ or ‘V/V’ might appear. W stands for weight and V for volume. ‘W/W’ Means that the % or ppm has been calculated on a weight for weight basis.
In the example above 44.9% is actually 44.9% W/W i.e. weight per weight.

It is usual to measure solids (powders) by weight e.g. grams (g) or kilograms (kg) and liquids in litres (l) or mille-litres (ml).
When a solid is dissolved in water however it is often easier to calculate the % of an element or the ppm as W/V (Weight per Volume)

One Response to “What is NPK? – Understanding Fertiliser Labels”