Roots can only utilize two nitrogen forms. These are nitrate (NO3-) and ammonium (NH4+). Plants cannot directly take up organic forms of nitrogen. Also, a plant cannot distinguish between, nor does it care about the original source of ammonium or nitrate. Therefore, nitrate or ammonium derived from urea is no different from that derived from ammonium nitrate or swine effluent. What differs among nitrogen sources is the rate and timing of their nitrogen availability.

Note that nitrate has a negative charge and ammonium has a positive charge. Soil mineral and organic fractions have a negative charge. Think of a magnet in terms of soil nitrogen retention. Like charges repel, opposite charges attract. Therefore, ammonium tends to "stick" to soil particles and nitrate remains in soil solution.

Another basic principle is that nitrogen is not stable in soils and can move among forms and among environmental locations. To understand nitrogen pathways, get to know the following terms: mineralization, immobilization, fixation and denitrification.

Mineralization is the conversion of organic nitrogen from soil organic matter, crop residue or waste materials into plant available inorganic nitrogen. This process occurs as soil microbes decompose organic materials. Since this is an organism-driven process, it occurs primarily in the microorganism comfort zone between 70 and 90 degrees F.

Immobilization is basically the reverse of mineralization. It is the tie up of plant-available nitrogen through plant or microbial uptake. The immobilization/mineralization process helps regulate nitrogen in the soil environment. This is nature's way of protecting against large-scale swings in soil nitrogen pools. Net mineralization occurs when organic matter or microbial feed sources are rich in nitrogen. Net immobilization occurs when microbial feed sources are poor in nitrogen.

Fixation is the conversion of atmospheric nitrogen into plant-usable mineral forms. Biological fixation occurs as legumes convert atmospheric nitrogen utilizing a symbiotic relationship with rhizobium bacteria.

Nitrification is the conversion of ammonium into nitrate. The process occurs through the activity of soil microorganisms. Under aerobic (oxygen containing) conditions and temperatures above 50 degrees, this process occurs rather quickly. Conversely, cool temperatures and saturated soil conditions delay nitrification. This is why early winter-applied ammonium-based nitrogen does not nitrify until the following spring.

Denitrification is the conversion of nitrate to gaseous nitrogen. This process leads to a net loss of plant-available nitrogen. Denitrification occurs under anaerobic (low oxygen) conditions. Anaerobic bacteria need oxygen to survive. They have evolved to thrive in low oxygen environments by stealing oxygen from the nitrate molecule (NO3-) to meet their metabolic needs. The byproduct of this reaction is gaseous nitrogen. In clay soils, which tend to become waterlogged, denitrification is the major nitrogen loss pathway.