Not all fertilizers are pure. Some have impurities, such as clay, silt and sand particles, also called “carriers”, that do not provide any nutrients to the plant. After calculating the amount of fertilizers that needs to be added to a nutrient tank, the weight of the impurities should be taken into account and subtracted from the total amount of fertilizers calculated. So if a fertilizer is 90 % pure, it means that it contains 10 % impurities or carriers that do not provide nutrients to the plant, the total amount of fertilizers should be adjusted accordingly.

In the post Formulating a hydroponic nutrient solution – Part I: Macro Nutrients, potassium nitrate /ce{KNO3} was used to provide potassium (K) and nitrogen (N) to the plants. The total amount used was 581.43 g.1000 L-1. This provided 80.59 g.1000 L-1 nitrogen and 224.49 g.1000 L-1 potassium. However, potassium nitrate is not pure. If the fertilizer obtained was 95 % pure, the total amount of fertilizer had to be adjusted. Use the following formula to compensate for impurities.

F_a=\dfrac{100 \times \textit{Calculated amount}}{\textit{Purity \%}}

F_a=\dfrac{100 \times 581.43g.1000L^{-1}\ce{KNO3}}{95\%}


The adjusted amount of fertilizer would now provide the right amount of nutrients to the solution. The percentage impurity should be obtained from your local fertilizer provider or it is printed on the container or bag in which the fertilizers are transported. However, and indication of how pure some fertilizers are, are listed in the table below.

Some fertilizers and their percentage purity as found in many commercial fertilizers.
FertilizerPurity (%)
Ammonium sulphate (NH4)2SO4)±95
Ammonium nitrate (NH4NO3)±98
Calcium nitrate (Ca(NO3)2)±90
Potassium nitrate (KNO3)±96
Potassium sulphate (K2SO4)±90
Magnesium sulphate (MgSO4.7H2O)±98
Mono potassium sulphate (KH2PO4)±98