Along with the arrival of spring, anhydrous ammonia applications will come. Some planned for the season, some making up for missed applications last fall.  Regardless, including a nitrification inhibitor in the tank with anhydrous ammonia or ammonium for nitrogen sources is a good 4R practice of the right source guarding against nitrogen loss.  Fact is, a significant portion of nitrogen loss occurs in the spring of the year as soils warm up and rains come.  Additionally, with spring NH3 costing more per unit N than fall product, loss of spring applied nitrogen is even more costly to replace.  

As a matter of good practice fall applied nitrogen with nitrification inhibitor is typically applied when soil temperatures reach 50⁰F and are trending lower.  At soil temperatures ≤ 50⁰F little if any nitrification occurs leaving much of the protection nitrification inhibitors provide until soil temperatures move above 50⁰F in the spring of the year.  Ammonium-N once converted to nitrate is subject to several fates: leaching, denitrification or the most desired fate plant uptake.  Leaching is the movement of nitrate downward in the soil profile with gravitational water flow.  The amount of downward movement that occurs depends on soil texture (Table 1), soil organic matter, soil structure, soil moisture content with relation to saturation point and infiltration rate of rainfall.  With around 80% of the corn root system in the top foot of soil, nitrate leached below this range is typically lost to the elements.

Table 1: Inches of water held per foot of soil by soil textural class

Source: USDA Plant & Soil Sciences eLibrary

Denitrification is a bacterial process that begins to occur in soils when approximately 60% or more of the pore space typically occupied by air becomes filled with soil water.  At this point anaerobic bacteria, which are capable of scavenging oxygen from other sources, begin robbing nitrate (NO3-) of oxygen until all that is left is nitrogen gas that is subject to escape the soil into the atmosphere.  The goal of corn production and applied nitrogen fate is ultimately plant uptake.    

So, why not forgo the nitrification inhibitor and apply additional nitrogen at a lower cost per acre to offset loss? Nitrification is not only a biological process but also a chemical one.  Those bacteria responsible for conversion from ammonium form (NH4+) to nitrate form (NO3-) are chemotrophic in nature.  Chemotrophic organisms obtain energy through oxidizing (adding oxygen to) molecules through the exchange of electrons.  Unlike plants, these bacteria are unable to utilize sunlight as a source of energy to make their own food.  This leaves these bacteria relying on applied anhydrous ammonia as an energy source.  Increasing the energy source by applying additional ammonia instead of a nitrification inhibitor will most likely only support an increase in population of bacteria with the rate of nitrification increasing under an abundant bacterial energy source.  In a sense, an abundant energy (food) source supports a high bacterial population.  

Fertilizer sources of nitrogen vary in the amount of ammonium versus nitrate form contained within each (Table 2).

Table 2: Percent Ammonium content various fertilizer sources