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Nitrogen fertilization is an economic necessity in modern agriculture. Nitrogen fertilizers are sold in gaseous, liquid, and solid forms. When properly applied, all can be equally effective sources of nitrogen. The typical physical properties of common nitrogen fertilizers are given in Table 1.
Most U.S. anhydrous ammonia is produced by reacting natural gas (CH4) with steam in the presence of a catalyst to obtain hydrogen (H2). The hydrogen is then reacted with atmospheric nitrogen to produce ammonia. This process uses some 33,000 cubic feet of natural gas to produce a ton of anhydrous ammonia:
CH4 + H2O → CO + 3H2
CO + H2O → CO2 + H2
3H2 + N2 → catalyst, heat, pressure → 2NH3
Anhydrous ammonia (NH3) is hygroscopic, meaning it readily absorbs water. It should be applied 4 to 8 inches below a soil surface which is friable enough to seal the slot behind the applicator to minimize potential vapor loss.
Ammonia will react almost immediately with soil moisture to form the ammonium ion (NH4+) which readily attaches to the exchange surfaces of the soil clay and organic matter. Nitrogen in the ammonium form is not mobile.
The ammonium form will convert to the nitrate (NO3) form of nitrogen within several weeks if soil temperatures are favorable, above 50°F and below 90°F. The nitrate ion (NO3-) is not attracted to the exchange surfaces, so is highly mobile in the soil. This allows good root availability under favorable conditions and predisposes this form of nitrogen to leaching under periods of excess moisture.
There are some who frequently comment about anhydrous ammonia "killing off" soil organisms or causing the soil to become “hard”. Research does not show this to be true. Although there is an initial reduction of soil microorganism numbers in the soil injection zone, this is a short lived effect. Microbial populations will recover quickly and, in some cases, will increase.
Kansas State research has shown no detrimental effects on the soil bulk density in comparisons between anhydrous ammonia and other nitrogen sources in long-term studies (twenty years in duration).
Anhydrous ammonia is the primary building block of other commonly used nitrogen fertilizers. Figure 1 outlines the basic manufacturing processes used to manufacture various nitrogen fertilizers.
Urea is a widely used nitrogen fertilizer and has some unique properties. It is non-explosive, less hygroscopic, and less corrosive than ammonium nitrate. Urea is manufactured by combining carbon dioxide and ammonia under heat and pressure using a catalyst:
2NH3 + CO2 → catalyst, heat, pressure → (NH2)2CO + H2O
When urea is applied to soil, an enzyme (called “urease”) causes chemical hydrolysis and produces ammonium carbonate ((NH4)2CO3). Ammonium carbonate is a very unstable compound and is quckly decomposed, which produces ammonia and carbon dioxide. At this point, urea reacts like any other ammoniacal fertilizer and eventually is converted to nitrate:
(NH2)2CO + H2O → urease → (NH4)2CO3
(NH4)2CO3 → NH3 + CO2
Ammonia loss from surface applied urea has received a lot of attention. Some research shows significant ammonia loss from urea when applied to a sod cover or to highly calcareous soils. Other research shows urea to be equal to other nitrogen fertilizers as a nitrogen source. Any surface-applied, ammonia-containing or ammonium-containing fertilizers can lose nitrogen under adverse conditions. Ammonia loss is negligible when the urea is quickly incorporated by tillage, by rainfall, or by irrigation following application.
Urea-ammonium nitrate solutions are manufactured by dissolving ammonium nitrate and urea in water. One-half of the nitrogen content is in the urea form, one-fourth is in the ammoniacal form, and one--fourth is in the nitrate form.
UAN solutions are widely used due to ease of handling and application. UAN solution is commonly used in starter fertilizers, for topdressing, in layby or side-dress applications, and in combination with herbicides. Solutions with 28% nitrogen are used for storage in colder months since crystallization or "salt out" occurs at 1°F. Crystallization of 32% UAN solution occurs below 32°F, so they must be stored during warmer months.
Leaf burn can occur when UAN solutions are applied directly to the foliage of actively growing plants unless diluted in irrigation water. No more than 60 pounds of actual nitrogen should be broadcast applied to four-leaf corn according to University of Minnesota research.
There are no problems when topdressing dormant wheat with UAN solutions. Some minor leaf burn can be expected if UAN solution is applied when wheat breaks dormancy. Experiments by Kansas State showed more leaf burn with higher nitrogen rates and finer droplet size when growing wheat and fescue were treated with UAN solution. However, neither grain nor forage yields were affected by the amount of leaf burn.
UAN solutions are quite corrosive to brass, bronze, and galvanized metal. Avoid mixing UAN solutions with certain herbicides (like 2,4-D amine formulations) because they are incompatible.
Ammonium sulfate is manufactured by reacting anhydrous ammonia with sulfuric acid:
2NH3 + H2SO4 → NH4SO4
Ammonium sulfate is a byproduct of the coke industry. Itis one of the oldest forms of solid nitrogen fertilizers. The nitrogen content (21%) is fairly low by today's standard, but it can be a useful nitrogen source when sulfur is needed.
Soil applied ammonium sulfate breaks down into the ammonium ion (NH4+) that attaches to soil colloids and the sulfate ion (SO4-2), which is soil-mobile.
Ammonium nitrate was once a popular nitrogen fertilizer, but is not readily available in the U.S. because of it can be used as an explosive material. It is manufactured by combining anhydrous ammonia with nitric acid (HNO3):
HNO3 + NH3 → NH4NO3
Half of the nitrogen is in the ammoniacal form; half is in the nitrate form. When applied to soil, the ammonium portion attaches to soil exchange surfaces while the nitrate portion is readily mobile in the soil solution.
Ammonium nitrate is hygroscopic and can draw substantial moisture from the air. The fertilizer material commonly “caked” during storage when exposed to humid conditions. Fertilizer-grade ammonium nitrate was manufactured to produce a high-density material to help assure the prills remained intact during transport and application. Low density grades of ammonium nitrate are used as a blasting agent when coated with a combustible oil. Low density, blasting grade ammonium nitrate is processed to make a porous prill that absorbs oil.
¶ Table 1a. Physical Properties of Commonly Used Nitrogen Fertilizers |
||
| Urea | Ammonium Nitrate | |
| Color | white | white/reddish |
| Molecular weight | 60 | 132 |
| Density, lb/cu ft | 46 – 48 | 66 – 68 |
| Critical relative humidity @ 60ºF | 80.7 | 81.0 |
| Nitrogen content | 45% – 46% | 21% |
| Sulfur content | --- | 24% |
| Angle of repose | 40º | 28º |
¶ Table 1b. Physical Properties of Commonly Used Nitrogen Fertilizers |
|||
| Anhydrous Ammonia | 28% UAN Solution | 32% UAN Solution | |
| Color | colorless | greenish | greenish |
| Odor | pugnant | --- | --- |
| Molecular weight | 17 | --- | --- |
| Density, lb/gal | |||
| @ 60ºF | 5.15 | 10.67 | 11.08 |
| @ 32ºF | 5.33 | --- | --- |
| Nitrogen content | |||
| lb. N/gal. @ 60°F | 4.22 | 2.98 | 3.54 |
| Total-N | 82% | 28% | 32% |
| Ammoniacal N | 82% | 7.0% | 8.0% |
| Nitrate-N | --- | 7.0% | 8.0% |
| Urea N | --- | 13.9% | 15.9% |
| Component content by weight | |||
| Ammonium-nitrate | --- | 36% – 42% | 41% – 48% |
| Urea | --- | 29% – 33% | 33% – 38% |
| Water | 0.2% – 0.5% | --- | --- |
| Free ammonia | --- | 0.07% | 0.08% |
| Boiling point @ 1 atm. | -28% | --- | --- |
| Salting-temperature | --- | 1% | 32% |
| Specific gravity @ 60°F | --- | 1.28 | 1.32 |
CF Industries. 2020. Product Specification Sheet: Urea Ammonium Nitrate Solution (UAN). 1 page. accessed 22Jun2022 230https://www.cfindustries.com/globalassets/cf-industries/media/documents/product-specification-sheets/uan---north-america/urea-ammonium-nitrate-solution-28-30-32.pdf
Agrico. 2017. Safety Data Sheet: Urea Ammonium Nitrate Solution (UAN). 9 pg. accessed 22Jun2022. http://www.agricocanada.com/wp-content/uploads/2016/09/agric-101-SDS_Urea-Ammonium-Nitrate-UAN-Solution.pdf