Now that ammonium nitrate (34-0-0) prices have increased and supplies are more difficult to obtain, anyone who needs to apply nitrogen (N) during hot weather should evaluate the alternatives, according to an agricultural consultant at The Samuel Roberts Noble Foundation.
“If you are in an area where ammonium nitrate is still available, it is still an excellent source of nitrogen,” said James Locke, Noble Foundation soils and crops consultant. “Although a 34-0-0 product may be available, make sure that it is actually ammonium nitrate. There have been many reports of urea and ammonium sulfate or other products being blended to make 34-0-0. If urea is used in these blends, it has the same volatility risk as using straight urea. Even if it is not available, summer fertilization is still necessary to maintain full productivity.”
While several alternatives to ammonium nitrate are available, the most common choices are urea (46-0-0), UAN liquid (32-0-0 or 28-0-0) and ammonium sulfate (21-0-0-24S).
Urea is a dry nitrogen source that has long been used for fall, winter and spring application, but is quickly becoming the primary choice for summer use. Summer applications of surface-applied urea are typically avoided due to the risk of loss to the atmosphere; however, incorporation of urea by at least 0.25 inches of rainfall or sprinkler irrigation, or tillage within three to four days of application will keep volatilization losses to a minimum, Locke said.
“If there are no rain, irrigation or tillage opportunities, you can have up to 40 percent loss,” Locke said. “The ideal choice is to apply the urea when rainfall is imminent, although we all know that can be very difficult. One can also apply a nitrogen additive containing NBPT to keep the urea from converting to ammonia.”
UAN, or liquid urea-ammonium nitrate, is a nitrogen source produced by combining urea and ammonium nitrate. The ammonium nitrate portion retains all the advantages of its granular form; however, the urea portion has an equal, if not greater, risk of volatilization than its granular form, Locke explained. All of the procedures to limit volatilization losses from the granular form apply to the liquid form in UAN. Other disadvantages of liquid UAN include the potential for leaf burning and difficulty in blending with phosphorus and potassium.
Ammonium sulfate is a dry nitrogen source that has excellent agronomic properties, much like ammonium nitrate. It is non-volatile, the nitrogen is readily plant-available, and it is a good source of sulfur. The primary drawback of ammonium sulfate is the high cost per pound of actual nitrogen. Due to its high cost, ammonium sulfate is used primarily in high value horticultural crops or ornamental settings. Ammonium sulfate has a higher capacity to acidify soils, so Locke recommended paying close attention to soil pH and liming as needed.
Locke also warned consumers to compare prices against other sources and read labels to fully understand what fertilizer is being purchased. “Some products on the market today claim to be excellent sources of nitrogen,” Locke said. “But I advise everyone to make sure they look at the chemical makeup before purchasing.”