Anhydrous ammonia (82% N) is applied as a gas from high pressure tanks through tubes and anhydrous knifes that inject ammonia (NH3+) six to eight inches below the surface to minimize (NH3+) escape into the air. Ammonia reacts quickly with water in the soil and changes to ammonium (NH4+) form. As a positively charged ion it bonds with negatively charged clay and organic matter. Ammonium is not subject to movement with water.

However, as a gas at normal temperatures, some anhydrous ammonia can be lost during and after application. Soil physical conditions affect the amount of loss. If a soil is hard or full of clods or too wet the slot behind the applicator blade will not seal allowing some ammonia loss to the atmosphere. This loss is often observed as white puffs as the applicator is being pulled by a tractor across the field. In no till conditions, sealing wings or closing disks are often added to the anhydrous applicator knives to help seal the anhydrous slot created by the knife to prevent the loss of N.

When soil temperature rise above 50 degree F to support biological activity, Ammonium ions are converted to the nitrate (NO3-) form by soil bacteria in the process called nitrification.

Nitrification increases at higher temperatures by the action of Nitrosomonas bacteria and then to nitrate by Nitrobacter and Nitrosolobus bacteria.

After nitrification has converted ammonia, the negatively charged nitrate ion is no longer held by negatively charged clay and organic matter. Nitrate-N can be lost from soils by leaching or denitrification. Leaching is defined as moving nitrate-N below the root zone of plants. Sandy soils are more prone to this loss but it can happen with heavy rain in other soils. Denitrification is where nitrate nitrogen is converted back into gaseous nitrogen and lost to the air as N2 or N2O gas in very wet or saturated soil. Plants can absorb N as both ammonium (NH4+) or nitrate (NO3-).Therefore, the goal of products that enhance nitrogen use efficiency is to hold N as ammonium until uptake by plants so there is little risk of loss (except on sandy soil that cannot bind much ammonia).

Urea (46% N) is applied as a solid broadcast or surface banded in limited amounts as dry starter fertilizer. Urea is very hydroscopic and hydrolyzes to ammonium carbonate very quickly when added to the soil. Ammonia carbonate is an unstable compound that decomposes to ammonia and carbon dioxide. The ammonium ion is adsorbed by soil and ultimately nitrified. Hydrolysis of urea speeds up in the presence of the urease enzyme produced by many bacteria and some plants. Once converted to ammonia urea behaves like anhydrous ammonia or any other commercial N.

Urea applied to bare soil and not incorporated by water, rainfall or tillage is subject to volatilization losses of ammonium. The first loss of nitrogen when urea or liquid nitrogen (UAN) is applied to the surface of the soil is into the air. Volatilization begins when urea comes in contact with urease and moisture. This break down of urea results in ammonia being lost into the air. As much as 30% can be lost within days of application. If plant residue is heavy on a moist soil surface, this increases bacterial populations, concentration of urease and volatilization loss of urea.

UAN solutions (28-32%) are mixtures of urea, ammonium nitrate and water in various proportions. 28% is the most common UAN solution in Michigan and is applied: banded, broadcast before or after planting or as side dress applications after crop emergence. There are higher concentrations of UAN solutions (30%) available but don’t store at cold temperatures as well. However, 28% (as farmer’s call it), can be stored over winter in non- pressure tanks and equipment. 28% contains no free ammonia. 28% is most commonly stored over winter because it will not ``salt out” until the temperature drops below – 50 F. All common UAN solutions are formulated to contain 50% of actual N from amide (from urea), 25% as ammonium and 25% as nitrate both from ammonium nitrate.

28% applied to the soil and not incorporated by water, rainfall or tillage is subject to volatilization losses of ammonium. However, in some areas 28% is commonly used as a weed and feed program by custom applicators where both nitrogen and herbicides are applied by broadcast application. The 25% nitrate (NO3-) in 28% is subject to uptake by roots or losses by leaching and denitrification. The 75% ammonium in 28% adheres to clay and SOM until converted by bacteria to nitrate N by nitrification then possibly lost by leaching or denitrification during wet saturated soil conditions.

Nitrogen Stabilizers / Additives

Product List:

1. Nitrification and/or urease inhibitors (Ex: N-Serve 24, Instinct, DCD, Guardian-DF, Guardian –DL 31-0-0, Guardian LP 15-0-0, Super U, , Super N, Agrotain, Agrotain Plus, Agrotain Ultra)
2. Polymer- and sulfur-coated urea (Ex: ESN, Polyon, Tricote, CoteN, Duration)
3. Urea reaction products (Ex: Nitroform, Nutralene, Nitamin, CoRon)

Following are descriptions:

NITRIFICATION INHIBITORS

Nitrification inhibitors are compounds that slow the conversion of NH4-N to NO3-N prolonging the period of time that nitrogen is in the ``protected” form and reducing its loss from the soil. By inhibiting bacteria that convert NH4-N to NO3-N, the ammonia N is held longer to the clay and organic matter in the soil and kept from leaching or denitrification. Most inhibitors can slow this process by a few weeks if added to anhydrous ammonia, urea, ammonia nitrate, liquid nitrogen solutions (UAN or 28%), urea, or manure. N Serve 24 (nitrapyrin) was originally a DOW Ag Department product for sale since 1976. N Serve 24 slows the conversion of ammonium nitrogen NH4-N to Nitrate Nitrogen in the soil. Nitrapyrin 2- chloro-6-(trichloromethyl) pyridine is very specific to Nitrosomonas bacteria, actually killing part of the Nitrosomonas population in the soil. DOW Nitrapyrin products for delaying nitrification of ammonia and urea fertilizers include: N Serve 24 and Instinct was labeled in 2009. Instinct is an encapsulated nitrapyrin formulation that can be applied to fertilizer left on the soil surface for up to 10 days delay for ammonium fertilizer nitrification. However, ongoing research trials in the Midwest have had inconsistency in yield increase due to the use of Instinct related to rainfall pattern in the experiments.

DCD (dicyandiamide) is another compound that can slow nitrification but is not as volatile as N Serve 24. Products containing only DCD (Guardian & Agrotain Plus) are generally used with nitrogen solutions and liquid manure. DCD keeps the Nitrogen in the ammonium form longer. In the USA products that contain DCD include: Guardian-DF, Guardian- DL 31-0-0, Guardian –LP 15-0-0 Agrotain Plus and Super U.

When to consider Nitrification Inhibitors: The highest value for Nitrification Inhibitors is when nitrate N losses are expected to be high under these conditions:

1. Tile drained soils when leaching potential is high
2. Wet or poorly drained soils 
3. Fields with N applied in the fall.

N Serve 24 is very effective on poorly drained or saturated soils that are water logged for several weeks in spring where denitrification occurs. Any soils in MSU Soil Management Groups with drainage class b-somewhat poorly or c-poorly drained characteristics could benefit from nitrapyrin.

When not to consider Nitrification Inhibitors: when Nitrate N losses are least likely, for example, when N is applied side-dress, as crop demand is high at this time.

DCD and nytrapyrin have current widespread use in North American Agriculture.

UREASE INHIBITORS

Urease Inhibitors are compounds that inhibit the action of the urease enzyme on urea and thus, delay urea hydrolysis. This allows time for urea to be incorporated into the soil (e.g., by rainfall) where volatilization losses are unlikely when hydrolysis occurs. NBPT (N-butyl thiophosphoric triamide) sold under the trade name Agrotain reduces the activity of the urease enzyme found in crop residue for up to 14 days. NBPT is the one product widely used in Ag as a urease inhibitor.

NBPT is a structural analog of urea and inhibits urease by blocking the active site of the enzyme. NBPT is the active ingredient in the Agrotain urease-inhibiting products.

Agrotain, with the active ingredient NBPT, is an additive for use primarily with urea (applied by the urea retailer) and secondarily with liquid N solutions. Agrotain Ultra is a more concentrated formulation of Agrotain. Agrotain Ultra can be mixed with urea or UAN. Eventually, these product degrade, allowing urea hydrolysis to naturally occur. Once in the NH4, N from urea is subject to denitrification of NO3-an N form that may be lost to the soil. Agrotain and Agrotain Ultra provide no activity against nitrifying bacteria. Agrotain Plus is a dry concentrate formulation for application of liquid nitrogen solutions (UAN) or 28%... Agrotain Plus contains NBPT plus an additional nitrogen stabilizing agent that keeps nitrogen in the ammonium form for a longer period of time. Agrotain Plus can be mixed with UAN or liquid livestock manure. Super U is a Granular Stabilized Urea fertilizer containing two N inhibitors: a urease and a nitrification inhibitor keeping N in the plant available ammonium form longer by reducing potential N loss by volatilization and slowing nitrification. Guardian DL (Conklin) contains DCD and Ammonium thiosulfate (ATS) having nitrification and soil urease inhibiting properties.

WHEN TO CONSIDER UREASE INHIBITORS 

Urease inhibitors may be considered whenever urea-containing fertilizers are broadcast and not incorporated with tillage or irrigation. Research shows that N loss from broadcast urea can be significant; loss is greatest with warm, windy weather and a moist soil surface or sod. Urease activity increases as temperature increases; therefore, hydrolysis is normally completed in 10 days at a temperature of 40 deg. F and within 2 days at a temperature of 85 deg. F. Urea’s rapid hydrolysis in soils may be responsible for ammonia injury to seedlings when large quantities are placed too close to the seed. Hydrolysis is also highly correlated with the % organic matter, total N and cation exchange capacity (CEC) of the soil and increases as any of these factors increase. Urease inhibitors help prevent valorization, potentially for 2 weeks or more, thus increasing the chances that rainfall will incorporate urea before losses occur. As long as it rains within that 14 day period (1/4 inch), urea is moved into the soil where it is converted to NH4-N without risk of volatilization. Nitrogen volatilization loss is most likely to occur when soil temperature is warming (greater than 60 F).

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N volatilization is potentially higher under the following environmental conditions at the time of urea application: high pH (recently limed), dry soil moisture conditions, high surface residue cover, (broadcast spread urea on no-till corn after wheat residue, corn residue) or pasture conditions during warm wet weather followed by a breezy dry period. This is because when urea fertilizers are broadcast applied to a soil covered with residue there is more urease enzyme available to convert it to ammonia gas, hence more potential for ammonia volatilization, and the ammonia is directly exposed to the atmosphere.

UREASE INHIBITOR AND NITRIFICATION INHIBITORS

SuperU (marketed by Agrotain International) is urea with both NBPT and a nitrification inhibitor (dicyandiamide DCD). SuperU was designed to prevent losses associated with surface application of urea as well as leaching and denitrification losses. Super N is a liquid concentrate containing the same inhibitor combination for use with UAN.

Further research and field trial studies are on-going to refine the rate and timing of these various products to maximize their use with minimizing the loss of Nitrogen to the environment. The goal is to competitively price these products and maintain crop yield and profitability to the grower.

CONTROLLED- RELEASE NITROGEN FERTILIZERS

Controlled released nitrogen products were available 30 years ago. Sulfur coated urea is probably the best known product. A sulfur coating is applied to urea granules. The elemental sulfur coating oxidizes slowly by bacterial activity. The urea then dissolves/diffuses through imperfections in the coating. The thicker the sulfur coating, the slower N is released. However, it is fairly expensive to coat sulfur on urea. Therefore, it was never widely accepted or adopted in U.S. agriculture as a common fertilizer.

Polymer-coated fertilizers were manufactured as early as the 1960s in Japan. A variety of polymers are used to form semi-permeable coatings on soluble N sources, usually urea. The N release mechanism is by diffusion through the coating rather than by disruption of the coating. Release is regulated by polymer chemistry, coating thickness, soil moisture, and soil temperature. Recent advancements have decreased production costs to an economical level for commodity grain crops. Examples: ESN, Polyon.

Polymer coated urea (PCU)

Polymer-coated urea fertilizers use a hydrophobic (water- insoluble) coating that temporarily isolates the urea prill from the soil environment. These polymer coatings may be resins or mineral-based products that act as a semi-permeable membranes or impermeable membranes with tiny pores. Nutrient release through these membranes is controlled by the properties of the coating material; in. e. its permeability characteristics as affected by temperature and moisture.

HOW PCU Works:

1. Water moves through the coating
2. N dissolves into solution inside the prill granule
3. N moves out through the polymer coating into the soil solution

PCUs are not significantly affected by soil properties such as pH, salinity, soil texture, microbial activity, redox potential, or CEC. Therefore it is possible to predict and control the nutrient release rate from these products more accurately than for sulfur coated ureas. (Trinket, 1010)

Sulfur coated Urea are used in the turf and horticulture industry for years, but their high cost excluded their wide spread use on Midwest farms. AGRIUM has patented newer plastic coating technology (PCU) called ESN that is competitively priced in the agricultural market. Modern polymers allow chemists to create release curves that closely match the uptake characteristics of target crops. The amount of the release is controlled by the thickness and other characteristics of the polymer.” Three years of Nitrogen form, rate, placement and timing research on corn by the University of Nebraska with ESN, urea, 28% and Anhydrous Ammonia concluded that: ESN consistently provided higher yields and less Nitrous Oxide emissions than the conventional N fertilizers used on corn. (Eghball, 2016, ASA Meeting, Phoenix, AR).

ICL Everris, sells two controlled release urea products: Agrocote MAX with E-Max Release Technology and Agrocote S Poly S Technology Controlled Release Fertilizer. Agrocote Max, “is a reacted polymer coating for use on macronutrients and micronutrients to improve nutrient efficiency and plant utilization. How the Technology Works: 1/ Water moves in through the coating, 2/ Nutrients dissolve into a solution in the granule, 3/As soil temperatures rise, N releases through the Poly-S coating, 4/ The polymer dissolves.”

ICL Everris Agrocote S Poly S Technology Controlled Release Fertilizer, “is a dual-acting coating technology that limits the solubility of urea and minimizes nitrogen exposure to the soil environment at the same time. When activated by water, this dual action begins metering nutrition as needed to optimize crop uptake and utilization during growth. This action also significantly reduces the amount of inefficient nutrition that escapes into the atmosphere, minimizing the environmental impact.”

Chemical Controlled Release Products

The first chemical controlled release products were available 50 years ago as fertilizers… Their high cost relative to other N Fertilizers has limited their use in large scale production of commodity crops such as corn. They are more commonly used in specialty crops as vegetables, orchards, nurseries…etc. Interest in there use is limited but increasing in row crops due to higher crop prices, environmental concerns and regulations or for foliar feeding. Common classes of controlled-release products are described below:

Urea-Formaldehyde/ Methylene Urea

Most synthetic organic N fertilizers, constituting a large portion of the CRN fertilizers available in the U.S., are known as urea-aldehyde reaction products such as urea formaldehyde and methylene urea. Urea formaldehyde, first patented in 1924 in Germany, was first commercially produced in 1955. Commercial fertilizers are mixtures of urea and ‘urea-form’ polymers of varying molecular weight and typically contain about 30-35% N. These slowly soluble organic compounds release N in the soil by chemical and/or biological decomposition processes. Solubility and N release are varied by altering the molecular weight and cross-linking of the urea polymers. Primary soil factors governing N release are moisture, temperature, pH and soil microorganisms and their activity. Today, some liquid products are available by making soluble ‘urea-form’ polymers. These products have been limited to horticultural application because of high cost.

Koch Agronomic Services LLC has released NITAMIN, a mixture of urea-reaction compounds that breakdown with soil microbial activity and release N by their patented Steady State DeliveryTM system. Nitamin fertilizer is a foliar-applied liquid that offers row crops a safe and efficient release of nitrogen and has a lower salt index compared to urea solution or UAN. Nitamin can be applied to plant foliage at higher rates with less potential for leaf burn. NITAMIN delivers nitrogen to crops at a rate that they can effectively utilize throughout the growing cycle. The Steady Delivery System provides row crop with a rapid initial uptake of N followed by steady penetration during the critical growth stage, NITAMIN binds to the soil, even in heavy rain, reducing loss through leaching.

Nitamin Fusion is a soil applied slow release liquid nitrogen fertilizer.

Examples: Nitroform, Nutralene, Nitamin, Nitamin Fusion & CoRon

Urea-Triazones 

These compounds are produced by reacting and aldehyde and ammonia (or primary amine) with excess urea in an aqueous medium under controlled conditions. The liquid fertilizer produced is a stable solution containing N from both triazone and unreacted urea. Products containing triazone-N remain in a liquid phase on plant tissue surfaces longer than some other urea-based products. These products have also proven safer to plant foliage than urea and UAN solutions in research studies. (Clapp, 2011) The Urea Triazones are often used as foliar fertilizers.

References:

• Agrocote, ICL EVERRIS USA, H54606xx13
• Agrocote Max, ICL EVERRIS USA, H54409xx13
• Agrocote S, ICL EVERRIS USA, H54506xx14 
• Blaylock, A.D. 2006. Review of Enhanced-Efficiency Nitrogen Fertilizers, 2006 proceedings of Southern Plant Nutrient Management Conference, Olive Branch, MS, Agrium, Denver, CO.
• Blaylock, A.D. 2010. Enhanced efficiency fertilizers. Colorado State University soil fertility lecture, 10-20-2010. Agrium Advanced Technologies, Loveland, CO.
• Clapp, J. 2001. Urea-triazone N characteristics and uses. Proceedings of the 2nd International Nitrogen Conference on Science and Policy. The Scientific World. (2001) 1(s2), 103-107. ISSN 1532-2246; DOI 10.1100/TSW.2001.356.
• Franzen D. W. 2011. SF-1581. Nitrogen Extenders and Additives for Field Crops, NCERA-103 Committee NDSU
• Georgia Pacific. 2005. Nitamin your steady-delivery nitrogen solution for crops. (Brochure)
• Hergert, G., R Ferguson, C. Wortmann, C. Shapiro and T. shaver. 2011. Enhanced efficiency fertilizers: will they enhance my fertilizer efficiency? Proceedings of the 3rd Annual Crop Production Clinics, Univ. of Nebraska-Lincoln Ext.
• Trenkel, M. 2010. Slow and controlled-release and stabilized fertilizers: an option for enhancing nutrient use efficiency in agriculture. Int’l Fertilizer Industry Association, Paris, France.
• Schwab, G.J. and Murdock L.W. 2005. Nitrogen Transformation Inhibitors and Controlled Release Urea. AGR-185, University of Kentucky-College of Agriculture, Cooperative Extension Service.