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Understanding Nitrogen

October 27, 2010
By: Darrell Smith, Farm Journal Conservation and Machinery Editor
Fall NH3 MFischer (1)
Nitrification inhibitors can help protect fall-applied anhydrous ammonia (NH3) from loss, saving you money and protecting water.  

When Farm Journal Field Agronomist Ken Ferrie gives a presentation about nitrogen, one question is more common than any other: “Should I use a nitrogen stabilizer?”

Simple question, but the answer is complex. It requires a basic understanding of how nitrogen is lost, which depends on which product you apply.

“Starting from the bottom—understanding various types of nitrogen fertilizers, how each one behaves in the soil and how they can be lost through denitrification, leaching or volatilization—lays a foundation for your decision about whether to use a stabilizer,” Ferrrie says.

Crash course on nitrogen. With nitrogen, the key words are “ammonium” and “urea.” They determine how nitrogen acts in the soil and what type of stabilizer you need to prevent loss.
When a fertilizer company manufactures nitrogen fertilizer, it is taking nitrogen from the air and putting it into a form dealers and farmers can handle and plants can use. The first step in making any form of nitrogen fertilizer is to make ammonia (NH3), which is 82% nitrogen (82-0-0).

Ammonia is derived from natural gas. In the presence of nickel catalysts, under high pressure and temperature, the natural gas reacts with steam, producing hydrogen (H) and carbon monoxide. Nitrogen from the air is added to the hydrogen. Then the mixture is passed over another catalyst, at high pressure, producing liquid ammonia (NH3). Stored under pressure, the ammonia gas (NH3) remains a liquid, which can be transported.

The carbon monoxide produced while breaking down natural gas can be used to make urea—CO(NH2)2 (45% nitrogen). The carbon monoxide is converted to carbon dioxide and reacts with ammonia (NH3), forming urea and water. When the water is evaporated, solid urea remains.
In order to make ammonium nitrate (NH4/NO3), some ammonia (NH3) is burned to produce nitric acid. The nitric acid reacts with more ammonia (NH3) to produce an ammonium nitrate (NH4/NO3) solution. When the liquid is evaporated, it leaves granular ammonium nitrate.

Some fertilizer products are a combination of products. The popular 28% and 32% urea-ammonium nitrate (UAN) solutions—CO(NH2)2 and NH4/NO3—are mixtures of urea and ammonium nitrate. Another common fertilizer, ammonium sulfate, contains ammonium as well as sulfur.

What is pH? Understanding nitrogen fertilizer requires you to know one more set of basics: soil pH, the measurement of soil acidity.

“pH literally expresses the balance of hydrogen (H) ions and hydroxyl (OH) ions,” Ferrie says. “Water (H2O) contains an oxygen molecule hooked to two hydrogen molecules. A certain percentage of water molecules are disassociated, which means an oxygen molecule is hooked to only one hydrogen ion instead of two—that molecule is called a hydroxyl (OH). Some hydrogen (H) ions are left over.


FJ 032 F10410 chart

Drastic changes in soil pH can give you inaccurate soil test results and a greater risk of fertilizer loss.

“If the soil is acidic (below pH 7), it contains more hydrogen (H) ions than hydroxyls (OH). If it’s alkaline (above pH 7), it contains more hydroxyls (OH). If there are equal hydrogen (H) ions and hydroxyls (OH), the pH is 7, or neutral,” Ferrie explains.


Ammoniacal nitrogen in the soil. In certain cases ammonium (NH4) can be converted back to ammonia (NH3). The extent of this conversion is driven by the soil’s pH level.

“In alkaline situations, high amounts of hydroxyls (OH) in soil can strip hydrogen (H) away from ammonium (NH4) to make water (H2O) and ammonia (NH3),” Ferrie says. “In slightly acidic conditions, ammonia (NH3) picks up hydrogen (H) from the soil to make ammonium (NH4). Soil pH above 8.5 drives ammonium (NH4) back to the ammonia (NH3) form.”

Here’s a breakdown of ammonia (NH3) and ammonium (NH4) ions in relation to soil pH:
“At the pH level where most farmers operate, 99% of the ammoniacal nitrogen (NH3 and NH4) in their soil is in the ammonium (NH4) form,” Ferrie explains. “If you crank the soil pH up to 11, the ammonia (NH3) smell would be so strong you would think you were standing in a chicken coop.”
In the soil, ammonia gas (NH3) is not stable; it can convert to ammonium (NH4), a solid, or be lost as a gas. That’s good, because ammonia gas (NH3) is toxic to plant roots.

To convert from ammoni a (NH3) to ammonium (NH4), the ammonia (NH3) ion can pick up a
hydrogen (H) ion from the soil solution or rip a hydrogen (H) ion away from a water molecule.
“That’s why, if you spill anhydrous ammonia (NH3) on your flesh, you apply water,” Ferrie explains. “The ammonia (NH3) rips hydrogen (H) ions away from the water instead of from your flesh.

“By ripping hydrogen (H) ions away from water (H2O) molecules when the ammonia (NH3) converts to ammonium (NH4), the ammonia (NH3) creates more hydroxyls (OH), shifting the pH upward in the ammonia core (the area where you placed ammonia) and making the soil more alkaline,” Ferrie says.

Once in the ammonium (NH4) form, microbes work to convert the ammonium (NH4) to nitrate (NO3). In this conversion, hydrogen (H) molecules are released into the soil solution, creating an acid ammonia core. These two reactions can cause the pH of the ammonia core to temporarily swing as high as 13 or as low as 4 or 4.5.

The swings in pH can lead to inaccurate soil test results. “These swings make it difficult to accurately measure soil pH after nitrogen has been applied,” Ferrie says. “This is especially true if you don’t know where the ammonia core is located.

“Probing the core early in the reaction will result in false alkaline readings. After nitrate (NO3) levels start to rise due to ammonium (NH4) conversion to nitrate, false acidic readings will result,” Ferrie explains.

“Soil tests should be pulled early in the spring before nitrogen applications are made, or in the fall after the corn crop, when pH swings have had a chance to settle down,” he says.

Urea in the soil. Like ammonia (NH3), urea fertilizer goes through changes in the soil. The first breakdown process is driven by the urease enzyme in the soil. In a series of reactions, ammonium (NH4) is created.

As the urea hydrolyzes, it collects hydrogen (H) ions from the soil, raising the pH around the urea molecules. “Urea hydrolysis will drive soil pH around the nitrogen molecule to near 9,” Ferrie says. “This can happen within two to four days of application. In moist, warm, high-pH conditions, it can happen a lot faster.”

A high-pH environment, rich in hydroxyls (OH), around the ammonium (NH4) molecule has a tendency to strip hydrogen (H) away from the ammonium (NH4) to create water (H2O) and ammonia (NH3). The ammonia, a gas, will be released into the atmosphere—a form of nitrogen loss called volatilization.

“With surface fertilizer applications, the extent of nitrogen loss is determined by the rate you apply,” Ferrie says. “The more nitrogen or urea you apply, the greater the shift in surface pH and the higher the risk of loss.” (This does not happen with ammonium [NH4] forms of nitrogen, except when soil pH is above 8.5.)

Also, if you are applying urea to a high-pH soil, the risk of loss is greater. “This is something no-till and strip-till farmers must be aware of because they may have higher surface pH readings,” Ferrie says. “If they apply lime in the fall, they will have a high pH on the surface the following spring. Also, remember that the urease enzyme, which starts the process that leads to volatilization of nitrogen, may be 10 times higher on the surface.”

How nitrate is lost. Nitrate is lost after ammonium (NH4) is converted to nitrate (NO3). Denitrification occurs when soil is saturated with water, causing oxygen to be driven out. The
denitrifying bacteria multiply. They strip oxygen away from nitrate (NO3) molecules, releasing nitrogen as a gas. Denitrifying bacteria are temperature driven, so you get more nitrogen loss when there is standing water combined with warm temperatures.

Leaching occurs because the negatively charged nitrate (NO3) molecules are easily flushed out. The water carries the nitrate (NO3) downward through the soil or out through tile lines, creating both yield and environmental issues.

Slowing the volatilization of urea. To prevent volatilization, you can till in urea or hope for a timely rain to wash the urea into the soil, where there is more hydrogen (H) to offset the pH swing. You also can apply a urease inhibitor to delay the breakdown of the urea until it rains or you till it into the soil. “If you till urea in immediately after application, there is very little need to apply a urease inhibitor,” Ferrie says.

“No-till and strip-till farmers do not incorporate, so if they apply urea in the spring, especially following a fall lime application, they need to consider using a urease inhibitor,” he adds.
Remember that 28% and 32% UAN solutions are 50% urea, 25% ammonium (NH4) and 25% nitrate (NO3).

Slowing the nitrification process. In the ammonium (NH4) form, some nitrogen is taken up by microbes and plants. But if there is more ammonium (NH4) than the microbes and plants can utilize, it is converted by bacteria called nitrosomonas and nitrobacter to nitrite (NO2). Nitrobacter bacteria break down nitrite to nitrate (NO3).

Nitrification inhibitors delay the bacteria conversion of ammonium (NH4), which is attached to soil particles and unlikely to move, to nitrate (NO3), which is subject to loss.

Stabilizers do not prevent these processes from occurring, but delay them, increasing the chance that plants and microbes will take up the nitrogen.

Sandy soils, which do not have the capacity to hold nitrate (NO3) molecules, are candidates for nitrification inhibitors, as are soils that stay wet. Here, you want to limit denitrification to keep nitrogen around until plant roots can take it up.

At temperatures less than 50°F, the nitrosomonas bacteria are not very active, so there is little risk of nitrogen loss. Otherwise, “the effect of nitrification inhibitors is related to temperature,” Ferrie says. “If you apply a nitrification inhibitor when soil temperatures are above 60°F, its activity won’t last long.

“Using an inhibitor is not an excuse for applying anhydrous ammonia (NH3) in the fall when the soil is too warm. But if you put ammonia (NH3) on when soil temperature is fluctuating between 40°F and 55°F, the inhibitor will keep ammonia (NH3) stabilized until the soil cools down.”

Don’t expect too much from your nitrogen inhibitor, though. “You can’t expect a fall treatment to protect ammonia (NH3) all the way through the end of June, especially if the spring is warm and wet,” Ferrie says.

Assessing risk. When analyzing the risk of nitrogen loss, take tillage practices into account. “In conventional-tillage fields, if you surface-apply nitrogen after planting without incorporating it, you are at just as much risk as a no-till or strip-till farmer,” Ferrie says. “It doesn’t take a lot to stabilize nitrogen—a simple pass with a drag or harrow can do a lot.

“While incorporation can eliminate the risk of nitrogen loss through volatilization in sandy soils, you still run a high risk of leaching, even if you incorporate,” he adds. “In sandy soils, it’s important to split your application through the growing season.”

While many farmers apply anhydrous ammonia in the fall due to time constraints in the spring, avoiding that practice is one of the best ways to minimize nitrogen loss. “If you must fall-apply, wait until soil temperatures drop below 50°F and stay there,” Ferrie says. “And use a nitrification inhibitor.

Splitting applications between pre-plant, starter and sidedress fertilizer is the best way to minimize nitrogen loss, he says. “It keeps plants happy, but you never apply too much at one time.”

Applying nitrogen through irrigation water is another way to spoon-feed plants throughout the season. Finally, eliminate soil compaction in fields, so water infiltrates, rather than stands on compacted soil layers. FJ 032 F10410

FJ 038 F10410


Nitrogen Definitions

Knowing the chemical nomenclature for a few elements and compounds will help you understand the processes described in this story.

Ammonium: NH4
Ammonium nitrate: NH4/NO3
Anhydrous ammonia: NH3
Hydrogen: H
Hydroxyl: OH
Nitrate: NO3
Urea: CO(NH2)2
Urea-ammonium nitrate:
CO(NH2)2 and NH4/NO3
Water: H2O

Know Your Stabilizers for USDA Programs

Besides maximizing yield, avoiding the loss of expensive fertilizer and protecting water from pollution, you need to understand nitrogen stabilizers if you are enrolled in USDA programs that offer cost-share or incentive payments to farmers who use stabilizers.

Specifically, explains Farm Journal Field Agronomist Ken Ferrie, you need to understand whether a nitrification inhibitor protects against denitrification or volatilization.

“If the purpose of a program is to reduce the risk of nitrate (NO3) reaching surface water supplies or groundwater, nitrification inhibitors, which help prevent leaching, would qualify for payment,” Ferrie says. “But urease inhibitors, which prevent the surface-applied urea from volatilizing and being lost to the atmosphere as a gas, may not.”

If you’re enrolled in such a program, read your contract and check with your Natural Resources Conservation Service staff so you know which type of product to use.

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FEATURED IN: Farm Journal - November 2010
RELATED TOPICS: Corn Navigator

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