An essential element required by all life
Nitrogen is essential for all life on Earth. Although molecular nitrogen makes up most of the atmosphere, virtually all organisms are incapable of using it in this form. Most plants can only absorb nitrogen in the form of ammonium or nitrate ions. Specialist organisms (eg. some cyanobacteria, and legume root nodules) are capable of "fixing" atmospheric nitrogen into a form that can be used by other plants.
Much of the green revolution came about due to the widespread use of artificial nitrogen fertilizers. The nitrogen content of an artificial "NPK" fertilizer is given by the first value.
Use this tag for all questions about the nitrogen content of soils and the relevant behavior of legumes.
Nitrogen input and content are important aspects of compost heap management, but this is usually relative to carbon. Consider using the carbon-nitrogen-ratio tag for these questions, instead.