Nitrogen Stabilizers / Additives
Nitrification inhibitors are compounds that slow the conversion of ammonium to nitrate, thus prolonging the period of time that N is in the "protected" form, and reducing its loss from the soil. Several compounds have proven effective for this purpose, including nitrapyrin, DCD (dicyandiamide), and ammonium thiosulfate. Of these, only nitrapyrin and DCD have current widespread use in North American agriculture.
Figure 4. Nitrapyrin added to anhydrous ammonia may reduce N losses, especially if N is applied in the fall.
Photo courtesy of Case-IH.
Nitrapyrin, or 2-chloro-6-(trichloromethyl) pyridine, works by inhibiting and depressing the activity of Nitrosomonas bacteria. In contrast to some other products, nitrapyrin has a bactericidal effect, actually killing part of the Nitrosomonas population in the soil. Thus it is effective until the bacterial population recovers in the zone of application and diffusion. This bacteriocidal activity is very specific to Nitrosomonas.
In warm soils, nitrapyrin can degrade in about 30 to 40 days. However, it is very persistent in cool soils, which contributes to its effectiveness for fall and winter applications. Measurable activity against Nitrosomonas often occurs for about 6 to 8 weeks in warm soils conducive to crop growth, and 30 weeks or more in cool soils typical of late fall and winter in the midwestern U.S. (Trenkel, 2010).
Nitrapyrin products for delaying nitrification of ammoniacal and urea fertilizers include N-Serve® 24 (launched in 1976) and Instinct® (launched in 2009). According to the product label, N-Serve 24 N stabilizer is an oil-soluble product that may be used with anhydrous ammonia, dry ammonium and urea fertilizers. When combined with a compatibility agent, N-Serve 24 may be used in the application of aqua ammonia and other liquid ammoniacal or urea fertilizer compositions. N-Serve 24 must be injected or incorporated in a zone or band in the soil with the fertilizer at a minimum depth of 2 to 4 inches during or immediately after application.
The product label for Instinct N stabilizer indicates that it is a water-based microencapsulated formulation of nitrapyrin that may be used in the application of aqua ammonia and other liquid ammoniacal or urea N fertilizer compositions such as 28%, 30% or 32% UAN. Instinct may be mixed with liquid fertilizer, insecticides, herbicides and/or water and applied as a preplant incorporated, preemergence, or postplant application. Incorporation may occur at any time up to 10 days after application and may be either by mechanical means or moisture (a minimum of 0.5 inches of rainfall or overhead irrigation).
DCD (dicyandiamide) - Following extensive use in western Europe and Japan, DCD was introduced into the U.S. in 1984, and officially approved by the EPA as a nitrification inhibitor in the late 1990s. Products containing only DCD are generally used with N solutions and liquid manure. The rate of DCD used is relative to the amount of fertilizer N applied, rather than the area of application. This may limit its practicality at very high broadcast rates of UAN solutions (e.g., > 30 gallons/acre of 28% N solution.)
In the soil, DCD has a bacteriostatic effect on Nitrosomonas, i.e., the bacteria population is not entirely killed, even with repeated applications, but its activity is suppressed or inhibited for a certain period of time (Trenkel, 2010). Depending on the amount of mineral N applied and the moisture and temperature of the soil, DCD may stabilize ammonium-N for about 4 to 10 weeks (Trenkel, 2010).
University studies have demonstrated that DCD can be effective in maintaining N in the ammonium form and increasing corn yield with both fall and spring applications. However, like other nitrification inhibitors, DCD was not always cost-effective in these studies, or significantly different than the control (no treatment).
In the U.S., products that contain DCD include Guardian® DF, Guardian-DL 31-0-0, Guardian-LP 15-0-0, Agrotain® Plus, and Super U®. Guardian-DF and Guardian-DL are intended for application with N solutions or liquid manure. In addition, Guardian DF can be impregnated on dry N fertilizers. Agrotain Plus contains a urease inhibitor as well as the DCD nitrification inhibitor, and will be discussed in a later section. Super U is a urea fertilizer with a urease inhibitor and DCD already applied; this product will be discussed in a future publication.
When to Consider Nitrification Inhibitors - The highest value of nitrification inhibitors should be realized when NO3- losses are expected to be high from leaching or denitrification, including the following conditions (Ruark, 2012):
- Tile-drained soils when leaching potential is high
- Wet or poorly drained soils
- Fields with N applied in the fall
On the other hand, nitrification inhibitors are usually least valuable when NO3- losses are unlikely, including these situations (Ruark, 2012):
- Sidedress applications, as crop demand is high at this time
- Applying on very coarse-textured soils with low CEC; if exchange sites are limited, any NH4+ ions not held on the exchange can leach out of the zone containing the inhibitor
For the N in urea to be available to plants, it must undergo a chemical reaction that transforms the amide groups of the urea molecule to ammonia (NH3). The urease enzyme, ubiquitous in soils, catalyzes this hydrolysis reaction. If this process occurs at the soil surface, ammonia can be lost to the air. However, if this reaction is delayed until surface-applied urea is incorporated into the soil by tillage, rainfall, or irrigation, the risk of ammonia loss is greatly reduced.
Certain compounds are known to inhibit the hydrolytic action of the urease enzyme on urea and thus delay urea hydrolysis. Though many have been tested, only 1 product has been widely used in agriculture as a urease inhibitor. That product, N-butyl-thiophosphoric triamide, or NBPT, is a structural analog of urea and as such inhibits urease by blocking the active site of the enzyme. NBPT is the active ingredient in the Agrotain family of urease-inhibiting products.
Urease activity increases as temperature increases, thus hydrolysis is normally completed within 10 days at a temperature of 40 F and within 2 days at a temperature of 85 F. Hydrolysis is also highly correlated with the organic matter, total N and cation exchange capacity (CEC) of the soil; increasing as any of these factors increase.
Agrotain, with the active ingredient NBPT, is an additive for use primarily with urea (applied to urea by the retailer) and secondarily with urea-ammonium nitrate solutions. Agrotain Ultra is a more concentrated formulation of Agrotain. Agrotain or Agrotain Ultra use may be considered when urea is broadcast and not incorporated with tillage or irrigation. Research shows that N loss from surface-applied urea can be significant. The amount of loss depends on weather conditions; loss is greatest with warm, windy weather and a moist soil surface. Agrotain and Agrotain Ultra help prevent volatilization, often for 2 weeks or more, increasing the chances that rainfall will incorporate urea before losses occur.
Eventually, Agrotain and Agrotain Ultra degrade, allowing urea hydrolysis to naturally occur. This is necessary so that plants can take up and use the N from urea. However, once in the NH4+ form, this N is subject to denitrification to NO3-, a form that may be lost from the soil. Agrotain and Agrotain Ultra provide no activity against nitrifying bacteria.
Agrotain Plus is an additive specifically for UAN solutions, according to the product label. Agrotain Plus contains both the urease inhibitor NBPT and the nitrification inhibitor DCD. Thus, it acts against both the volatilization and nitrification processes that lead to N losses from UAN solutions. However, it does not protect the portion of the solution originally in the nitrate form, (i.e., the 25% of the N content of the solution derived from nitrate in ammonium nitrate).
Super U® is a urea fertilizer with the same ingredients as in Agrotain® Plus already applied. This product will be discussed in a future publication.