Nitrification inhibitors are the other inhibitors marketed, but they have a completely different mode of action. Any N supplied as a commercial fertilizer is ultimately transformed to a nitrate form of N (or at least a significant fraction of that supplied). In the presence of adequate oxygen, warm temperatures (>50° F) and some moisture, ammonium-N is converted to nitrate-N through a biochemical process (known as nitrification) that requires two forms of soil bacteria. The first bacterium, Nitrosomonas, converts ammonium-N to nitrite-N. The second bacterium, Nitrobacter, converts nitrite-N to nitrate-N. And as you know, nitrate-N is the form we are most concerned about being lost (whether by leaching or denitrification).

Nitrification inhibitors have one primary way of delaying the nitrification process, and that is eliminating the bacteria Nitrosomonas in the area where ammonium is to be present. There are three common nitrification inhibitors commercially available:  2-chloro-6-(trichloromethyl)-pyridine (nitrapyrin), dicyandiamide (DCD), and ammonium thiosulfate (ATS).

Nitrapyrin is the active ingredient found in N-Serve and Instinct. The biochemical activity of nitrapyrin and its ability to suppress growth of Nitrosomonas has been known since the 1970s and it was initially registered in 1974. It is quite effective even at relatively low rates. Dicyandiamide (DCD) is the active ingredient in nitrification inhibitors such as Agrotain Plus, SuperU and Guardian. Dicyandiamide is required at a significantly larger concentration to be effective.

Since each of the products discussed above is highly sensitive to concentration, it is imperative that if they are used they are applied at labeled rates. Cutting rates is not in your best interest as an end user because a lower concentration may not allow the product to perform its job in the soil.

Applying anhydrous with nitrapyrin in the fall (which is not the recommendation for summer crops in Ohio) may realize a benefit of the nitrapyrin (as it is out in the field for a long time), but for most who apply anhydrous ammonia in the spring soon before planting and as a sidedress treatment, the utility of a nitrification inhibitor is difficult to justify since the risk of N loss is low. No-till situations are more likely to show positive yield results than conventional till systems for spring-applied anhydrous. 

Nitrification inhibitors are less likely to show an economic benefit when high N rates are used in the field. Nitrogen losses at high N-application rates are not likely to affect yield as much if lower N rates are applied (fewer bushels per acre are gained with each additional level of N at the high levels).

Summary

Application timing, N source, application method, soil texture and tillage are all factors that should be evaluated to determine where urease and nitrification inhibitors should be used. Before buying an inhibitor make sure scientific evidence backs up its claim. A producer and/or consultant should be wary of any product that does not have solid scientific data demonstrating that the inhibitor activity matches the advertised benefit.