Plants are surrounded by the nitrogen (N) in our atmosphere. Every acre of the earth’s surface is covered by thousands of pounds of this essential nutrient, but because atmospheric gaseous nitrogen is present as almost inert nitrogen (N₂) molecules, this nitrogen is not directly available to the plants that need it to grow, develop and reproduce.

Healthy plants often contain 3-4% nitrogen in their above-ground tissues. These are much higher concentrations than those of any other nutrient except carbon, hydrogen and oxygen, nutrients not of soil fertility management concern in most situations. Nitrogen is an important component of many important structural, genetic and metabolic compounds in plant cells. It is a major component of chlorophyll, the compound by which plants use sunlight energy to produce sugars from water and carbon dioxide (i.e. photosynthesis). It is also a major  component of amino acids, the building blocks of proteins. Some proteins act as structural units in plant cells while others act as enzymes, making possible many of the biochemical reactions on which life is based. Nitrogen is a component of energy-transfer compounds, such as ATP (adenosine tri phosphate) which allow cells to conserve and use the energy released in metabolism. Finally, nitrogen is a significant component of nucleic acids such as DNA, the genetic material that allows cells (and eventually whole plants) to grow and reproduce.

Soil Nitrogen
Soil nitrogen exists in three general forms - organic nitrogen compounds, ammonium (NH₄⁺) ions, and nitrate (NO₃^-) ions. At any given time, 95-99% of the potentially available nitrogen in the soil is in organic forms, either in plant and animal residues, in the relatively stable soil organic matter or in living soil organisms, mainly microbes such as bacteria. This nitrogen is not directly available to plants, but some can be converted to available forms by microorganisms. A very small amount of organic nitrogen may exist in soluble organic compounds, such as urea, that may be slightly available to plants. The majority of plant-available nitrogen is in the inorganic (sometimes called mineral nitrogen) NH₄⁺and NO₃^-forms. Ammonium ions bind to the soil’s negatively-charged cation exchange complex and behave much like other cations in the soil. Nitrate ions do not bind to the soil solids because they carry negative charges, but exist dissolved in the soil water, or precipitated as soluble salts under dry conditions. Some NH₄⁺and NO₃^- may also exist in the crystal structure of certain soil minerals, and may be quite available; however, such nitrogen is important in only a few soils.

Different Nitrogen Sources