It is a macronutrient and is required in a large amount.
Nitrogen is very essential for plants becauseit is a main component of chlorophyll, the compound by which plants use sunlight energy to produce sugar from water and carbon dioxide during the process of photosynthesis.
Nitrogen is mostly absorbed by plants as nitrates and some amount as nitrites and ammonium ions.
Nitrogen is also the main constituent of amino acids which are the building blocks of proteins. In the absence of proteins, plants wither and die.
Nitrogen is required by all the metabolically active cells.