When using more than 100 pounds per acre actual nitrogen, nitrogen use and yields improve by splitting it into spring and summer applications (Ball, NF Ag News and Views, June 2004). Deciding what source to use for summer nitrogen can be difficult, because each has its pros and cons. The deciding factor for most producers is their tolerance for risk versus the cost for each type of nitrogen fertilizer. Although there are several available nitrogen sources, this article will cover three of the most common in Noble's service area: ammonium nitrate (34-0-0), urea (46-0-0) and liquid urea-ammonium nitrate (UAN=28-0-0 or 32-0-0).
Ammonium nitrate has been the standard for summer applications for many years due to its excellent agronomic properties. The nitrate half is mobile in the soil and can move to roots for rapid uptake. The ammonium half attaches to clay particles and releases nitrogen over time. One of ammonium nitrate's advantages is that it is not usually subject to volatilization. One of the disadvantages is the high cost per pound of nitrogen. Due to production costs and increased regulations, ammonium nitrate currently costs 20 plus percent more per pound of nitrogen than other common sources.
Urea has long been used for early spring and late fall applications, however, summer usage is increasing. Urea has several advantages, including cost per pound of nitrogen, higher nutrient density and good handling and storage properties. The biggest disadvantage is the potential for volatilization. This occurs when urea is surface-applied and converted to ammonium carbonate by urease. Urease is a naturally occurring enzyme in soils and plant residues. The ammonium carbonate is rapidly converted to ammonia that can be lost to the atmosphere. The amount of volatilization depends on soil pH, soil moisture, air temperature, humidity and wind speed. The worst-case scenario is urea applied to a moist, alkaline soil (pH > 7.5) with heavy surface plant residues followed by days with morning dews and warm, windy afternoons. Although estimates vary, up to 50 percent of the nitrogen may be lost. Incorporating the urea by tillage, irrigation or rainfall (at least a quarter of an inch) within 24 to 48 hours results in essentially zero loss.
UAN or urea-ammonium nitrate is a combination of the above materials. It is a liquid formulation containing 28 percent to 32 percent nitrogen. The advantages and disadvantages for the ammonium nitrate and urea portions are the same as previously described. The primary advantage of a liquid formulation is the potential to blend the fertilizer with some herbicides. This can save an application trip, and some herbicides work better when applied with fertilizer. Weigh the benefit of saving that trip against the potential for reduced fertilizer response or weed control if the fertilizer or herbicides are not timed properly. Disadvantages of UAN are the potential for foliar burn with higher nitrogen rates and difficulty or expense of applying with other nutrients.
With the current price differential, one strategy is to spend the same amount on urea as you would have on ammonium nitrate. Example: ammonium nitrate costs $310/ton, urea costs $315/ton and 50 pounds/acre of nitrogen is needed to reach the yield goal. As ammonium nitrate, 50 pounds of nitrogen cost $22.79, which would purchase 67 pounds of nitrogen as urea. If the volatilization loss is equal to the price differential, then the choice does not matter. If urea is incorporated within 24 to 48 hours, yield potential increases by about 30 percent without additional costs. If losses were greater than the price differential, then ammonium nitrate would have been a better choice.
A Noble Research Institute soil and crops specialist will be glad to discuss options with you, but, in the end, the producer must decide his or her own tolerance for risk when choosing a summer nitrogen source.