Nitrogen is an element of fundamental importance for all living organism; like carbon, hydrogen, oxygen and sulfur is important for composition of proteins and constitution of nucleic acids. Most common forms are Nitric nitrogen NO3, Ammoniacal nitrogen NH4, urea nitrogen NH3 (natural or synthetic) and other forms amminoacid related. In soil, the more absorbed form is Nitric nitrogen, which results water-soluble in the soil solution. The ammoniacal form is absorbed to the exchange complex and enter into solution by following the mechanisms of cation exchange capacity. Nitrogen is absorbed by plants both as nitric NO3 and as ammonium ion NH4. The amino acids nitrogen and urea nitrogen can be assimilated by the plants also directly, as well as after the mineralization of the elements. The organic nitrogen is the form most represented in the ground. The organic fraction is represented by organic compounds of mainly protein nature, derived from the decomposition of the remains of animals and plants. The ammonia nitrogen is derived from the decomposition of organic matter that consists of the demolition of the proteins into polypeptides, then into peptones and then into amino acids, so at the liberation of the radical -NH2 of the amino acids with formation of NH3, which is rapidly transformed into NH4 and adsorbed by soil colloids and consequently removed from the leaching. An important fraction of ammonia nitrogen present in the soil can also be derived from the contribution of nitrogen fertilizers and the atmosphere by nitrogen-fixing bacteria. The nitrate nitrogen, aside from the addition of nitrogen fertilizer, the nitrification process of ammonium nitrogen, carried out in two stages by certain species of bacteria. In the fertilization practice it is generally preferred the distribution of ammonia nitrogen, urea or organic in the preliminary stages of fertilization, while the nitric oxide is most preferred in coverage. The urea and ammoniacal forms are particularly useful for the foliar absorption.