The NPK system for rating fertilisers provides the relative quantity of the elements nitrogen, phosphorus and potassium in the fertiliser. The acronym NPK stands for the abbreviations for each element -- N for nitrogen, P for phosphorus and K for potassium. The NPK system expresses the percentage of the fertiliser that is nitrogen (N), phosphorus pentoxide (P2O5) and potassium oxide (K2O). Conversion of the NPK rating to parts per million, or PPM, requires you to multiply each of the three numbers in the NPK rating by a specific value.

Read the fertiliser label and record the NPK rating. Assume the NPK rating of the fertiliser is 10-20-30 for this example. This means the fertiliser contains 10 per cent nitrogen, 20 per cent P2O5 and 30 per cent K20.

Multiply the nitrogen percentage by 10,000 to obtain its PPM value. The nitrogen value in this example is 10 per cent, so the fertiliser contains 10 x 10,000 = 100,000 PPM of nitrogen.

Multiply the P2O5 percentage by 4,400 to obtain its PPM value. The P2O5 value in this example is 20 per cent, so the fertiliser in this example contains 20 x 4,400 = 88,000 PPM of phosphorus.

Multiply the K2O percentage by 8,300 to obtain its PPM value. The K2O value in this example is 30 per cent, so the fertiliser in this example contains 30 x 8,300 = 249,000 PPM of potassium.

Summarise the results of your calculations. A 10-20-30 fertiliser contains 100,000 PPM of nitrogen, 88,000 PPM of phosphorus and 249,000 PPM of potassium.