What happens to the fertiliser I apply?

Fertiliser N is usually supplied as nitrate-N or ammonium-N.  The latter is converted rapidly to nitrate-N in the soil and this is the form of N that the plant takes up and uses to transport N around the plant.  In order to use it to make proteins it must first be converted back to ammonia.  This combines with organic acid-based molecules to form the amino acids, the building blocks of proteins.

At any one time, therefore, N is present in plants in a number of forms.  Shortly after fertiliser application it will mainly be as nitrate-N but over time this will change to protein-N.  As the plant grows the total N concentration will also reduce.