Elemental Proportion in NPK Fertilizers

Any natural or synthetic material added to soil to provide plant nutrients is called Fertilizer. Fertilizers are available in the market in liquid, solid as well as gaseous form (e.g. anhydrous ammonia).

All chemical fertilizers available in the market are labelled with three numbers, which represent the weight of total nitrogen (N), citrate-soluble phosphorus (expressed as P2O5) and water-soluble potassium (expressed as K2O), respectively. These numbers represent nitrogen, phosphorus and potassium or NPK respectively.

We note here that these numbers are not N-P-K but are actually N-P2O5-K2O. To get the actual elemental proportion of these three elements, we need to multiply fraction of P2O5 with  0.44 and fraction of K2O by 0.83. Nitrogen number represents actual Nitrogen present. Thus, if a fertilizer bag is labelled as 18−51−20, this implies that it has:

  • 18% Elemental Nitrogen
  • 44= 22% elemental Phosphorous
  • 83= 17% of elemental Potassium

Apart from the above, fertilizer bags also come with filler material such as pelleted bio solids, vermiculite, fuller’s earth, diatomaceous earth etc. These filler items help to spread the fertilizer avoid the so called “Fertilizer burn” in plants.

The below table represents the labels found on common fertilizers available in the markets:

Kindly note that Urea has the highest nitrogen content of all solid nitrogenous fertilizers in common use in India.