Nitrogen Fertilizers

Anhydrous ammonia (82-0-0) is a gaseous material that is compressed and stored as a liquid. At 60oF, a gallon of anhydrous ammonia weighs 5.15 pounds. It is manufactured by reacting nitrogen from the air with hydrogen (usually from natural gas) in the presence of a catalyst at high temperature and high pressure.

Anhydrous ammonia is the source of nitrogen for other commercial fertilizers. It is also used for direct application. Since it is a compressed gas, it must be injected in the soil to prevent loss from vaporization. When soil conditions are favorable for injection and closure of the injection channel, ammonia is an effective source of nitrogen for crops. Because anhydrous ammonia needs to go through the nitrification process (see Soils - Part 5), it is more resistant to losses from the soil by leaching or denitrification because it is converted by bacterial action to the nitrate form more slowly than other nitrogen sources. N-Serve is an anti-bacterial agent which slows the conversion of NH3 to the nitrate form.

When anhydrous ammonia is injected in the soil, the injection sites will have a high pH and a very high ammonium concentration. This is lethal to most microorganisms in the band. It also provides a rich protein source for microbes on the fringes of the band, which enhances their reproduction. Research studies show that recolonization is complete within a few months after application.

Aqua ammonia (20-0-0) is anhydrous ammonia dissolved in water. It is a low-pressure solution and contains free ammonia. The amount of free ammonia increases as air temperatures increase. It is stored in closed low-pressure tanks and is injected into the soil much like anhydrous ammonia. Since aqua ammonia is a low-pressure solution and contains only a small amount of free ammonia, direct soil application does not need to be as deep as with anhydrous ammonia. Aqua ammonia is not commonly used because of the cost of handling the water in the product.

Ammonium nitrate (34-0-0) is a dry granular material manufactured by reacting nitric acid with anhydrous ammonia. The reacted material is concentrated, prilled and coated to prevent caking. In ammonium nitrate, one-half of the nitrogen is in the nitrate form, and the other half is in the ammonium form.

Ammonium nitrate is the preferred form of nitrogen fertilizer when it is to be applied on the soil surface and not incorporated. It does not volatilize as free ammonia, except on high pH soils. Ammonium nitrate use has declined in recent years and is being replaced by urea, which is less expensive and easier to store and maintain. Under the right conditions, ammonium nitrate can become explosive.

Ammonium sulfate (21-0-0 + 24S) is a dry crystalline material produced by reacting anhydrous ammonia with sulfuric acid. It contains 21 percent nitrogen and 24 percent sulfur, which makes ammonium sulfate an excellent source when both nutrients are needed. Ammonium sulfate has excellent storage properties. The low nitrogen content, when compared to other sources, the crystalline form, and relative cost limit its use in Nebraska. While all nitrogen fertilizers have an acidifying effect, ammonium sulfate is more acidifying than other nitrogen sources, which limits its use in areas where soils require liming. Its most common use would be in sandy soil areas where sulfur is also needed.

Ammonium nitrate-sulfate (30-0-0 + 6.5S) is a dry nitrogen fertilizer relatively new to the United States. It is manufactured by reacting anhydrous ammonia with a mixture of nitric acid and sulfuric acid. The nitrogen content of the product will vary depending on the proportion of nitric to sulfuric acid. A common grade is 30-0-0 that contains 6.5 percent sulfur. Ammonium nitrate-sulfate has good storage and handling properties. It is very satisfactory for direct application, use in blended fertilizers, and is a good replacement for ammonium nitrate.

Urea (46-0-0) is a dry nitrogen material produced by reacting ammonia with carbon dioxide. Of the commonly used dry fertilizers, urea contains the highest percentage of nitrogen and is rapidly replacing ammonium nitrate. When surface applied, urea is the most readily volatilized of the dry nitrogen materials. After application to the soil, urea will hydrolyze (combine with water) to form ammonium carbonate. This reaction is driven by the enzyme urease, which is present in the soil and on crop residues. Ammonium carbonate is very unstable and decomposes into water, ammonia, and carbon dioxide. Thus, nitrogen is lost as ammonia.

This risk of loss can be minimized by incorporating urea into the soil with tillage, injection, or with one-half inch or more of water from rainfall or pivot irrigation. A new urease inhibitor called Agrotain® retards the hydrolysis of urea for about two weeks under most field conditions. Agrotain® also can be used with UAN solutions. Careful consideration should be given to the potential loss of urea applied to the soil surface without this protection.

Urea-ammonium nitrate (UAN) is a non-pressure solution of ammonium nitrate, urea, and water. Two grades are most common: 28-0-0 (10.7 pounds/gal) and 32-0-0 (11.1 pounds/gal). The lower analysis material contains more water and can be stored at lower temperatures. Salt crystals will form at about 0oF for 28 percent solution and at about 32oF for the 32 percent solution.

Individual solutions can be made from urea or ammonium nitrate; however, higher analysis solutions are possible when urea and ammonium nitrate are combined. Commonly available nitrogen solutions will contain about one-half of the nitrogen from urea and the other half from ammonium nitrate.

UAN solutions have an advantage in terms of handling. They can be pumped, mixed with chemicals and sprayed. They are corrosive and will quickly destroy brass, bronze and zinc fittings. Carbon steel and cast iron are also seriously corroded. UAN doesn’t corrode aluminum alloys, stainless steel, rubber, neoprene, polyethylene, vinyl resins, and glass.

The performance characteristics of urea-ammonium nitrate solutions are the same as for the two main ingredients, urea and ammonium nitrate. The ammonium nitrate portion will perform like ammonium nitrate, and the urea portion like urea. The urease enzyme is present on crop residues and on the soil surface, so the urea portion of this fertilizer is subject to loss unless incorporated by tillage or rain.