The air around us is approximately 80% nitrogen, though it is not available to the plant. Nitrogen is a essential component of the amino acids and proteins in the plant.
Nitrogen does not remain in the plant media in a soluble form for very long and the nitrogen not taken up by plants or micro organisms is easily washed away.

Microorganisms oxidize urea or ammonia into nitrite and then to nitrate. The nitrite form is an intermediate oxidation state that usually quickly converts into the nitrate. The process is not carried out by one form of microbe alone.
karmaxul Reviewed by karmaxul on . Nitrogen Nitrogen Education Organic Nitrogen Compounds:) Urea: Urea has three distinct advantages, namely, it is readily soluble in water and can be applied either as a soil application or as foliar spray. It increases soil acidity only slightly. In urea, the nitrogen is present in the amide form ( diamide of carbonic acid) and is rapidly converted to ammoniacal nitrogen and nitrate nitrogen. The major draw backs of urea are that it is susceptible to leaching, fixation and volatilization. Rating: 5