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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.”

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

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