Higher overall transportation costs for ammonium nitrate (34 percent nitrogen) compared to urea (46 percent nitrogen), combined with hazardous material issues associ-ated with ammonium nitrate, have driven the dry fertilizer market toward urea.

Advantages associated with urea compared to ammonium nitrate include: higher analysis, lower cost, less potential losses via leaching (movement through the root zone with soil water) or denitrification (a process in which nitrate converts to gas under wet soil conditions), and less foliar burn when applied to late season corn and grain sor ghum.

The major disadvantage to urea compared to ammonium nitrate is that when it is surface broadcast, urea-nitrogen can be lost from the system via volatilization. Since we are moving toward urea becoming the dominant dry nitrogen fertilizer source, volatilization issues need to be addressed.

Urea must be converted to ammonium or nitrate before becoming plant available. The enzyme urease is required for this conversion to occur . Mother Nature has provided all of our cropping systems with enough urease to drive equation 1.

By analyzing components of equations 1 and 2, we can identify when urea-N volatilization potential is greatest. Urease activity is much greater on plant surfaces and in crop residues compared to bare soil. Therefore, urea-N volatilization is greater in pasture/hay production and reduced tillage compared to conventional tillage systems.

When urea is incorporated through tillage, rainfall or irrigation, the ammonia produced is quickly converted to ammonium in the soil similar to what happens when anhydrous ammonia fertilizer is used (equation 2). After incorporation, little urea-N is lost to the atmosphere. Once incorporated, urea will perform equal to or better than ammonium nitrate.

Another requirement for volatilization to occur is that water must be present. Very little N loss occurs when urea is applied to dry soils. Conversely, when urea is applied to wet soils, volatility is greatest.

Urease activity occurs predominately from 50 degrees to 110 degrees F. At temperatures outside this range, very little N loss occurs. As temperatures rise above 50 degrees F, it is estimated that urease activity doubles for every 10 degrees F temperature change. Therefore, if you can potentially lose 5 percent of the applied urea-N at 55 degrees F, 10 percent could be lost at 65 degrees F and 20 percent could be lost at 75 degrees F, etc. These are potential loss numbers. Actual losses are usually smaller based upon other conditions present. In our region, volatility is minimal between Oct. 15 and April 15. During these times, urea is generally equal or superior to ammonium nitrate.

Soil pH and CEC also af fect the loss of urea-N via volatilization. Volatility potential is greatest as pH increases and CEC decreases. Therefore, high pH, sandy soils have highest potential urea-N volatility. From equation 2, as ammonia converts to ammonium, the resulting OH- ions produce an alkaline microenvironment around the urea granule, increasing the likelihood of volatility.

Many people falsely believe that ammonium nitrate is superior to urea when applied to winter cereals or cool-season grasses in late winter through mid-spring.

Most research shows no advantage to ammonium nitrate compared to urea in wheat until mid April, unless very high surface residue exists. Spring applied N response on cool-season grasses has shown similar responses. Over a 10-year time period, Merle V igil and Dave Kissel at Kansas State University showed no dif ference in average cool-season grass yields between ammonium nitrate and urea.

The most common situations in which ammonium nitrate has performed superior to urea has been in no-till corn, mid-season (no later than 7-leaf stage) sidedress corn and warm-season forages. Under these situations urea should be applied as close to an incorporating rainfall or irrigation event as possible.

Another option to improve urea-based nitrogen ef ficiency in high volatility environments is the product AGROTAIN. Chemically, AGROTAIN is N-(n-Butyl)-thiophosphoric triamide (NBPT) combined with the solvent, N-methyl pyrrolidone. It is environmentally safe and possesses little handling risk. AGROTAIN inhibits urease activity, which helps prevent urea-nitrogen (N) volatilization for up to 14 days after application. Most research has shown that it works reasonably well.

AGROTAIN has been around since the 1980s. It is one of the most extensively researched fertilizer enhancement products on the market. Research at several universities has shown that when urea or UAN are broadcast in no-till or conservation tillage corn, the addition of AGROTAIN consistently produces 5 to 15 bushel per acre yield increases compared to urea or UAN materials alone. However, the research has also shown that anhydrous ammonia, UAN knife applied or ammonium nitrate are superior nitrogen management strategies in no-till corn compared to urea + AGROTAIN. When economics or product availability dictates converting from ammonium nitrate to urea/UAN + AGROTAIN in no-till corn, N rates may need to be increased 5 to 10 percent.