Richard Taylor, Extension Agronomist; rtaylor@udel.edu
There’s a debate going on as to what the best source of nitrogen (N) is for broadcasting over winter wheat or barley at this time of year. The question arises because it is known that urea can volatilize to ammonia (NH3), a gas, and be lost to the atmosphere because the enzyme urease, which helps break down urea, is present in all organic matter. What a lot of people overlook is the speed of this conversion which is affected by the soil pH, soil and air temperature, and moisture conditions. When temperatures are relatively low, below 70°F, and soil temperatures remain well below 50º, the activity of the enzyme is significantly reduced. Another factor involved is soil acidity or pH. When the soil surrounding the urea particle is acidic (pH<7.0), there are available hydrogen ions (H+) that can quickly react with ammonia to form an ammonium ion (NH4+). An ammonium ion is a cation that can occupy a place on the cation exchange sites in clay and soil organic matter and be held for plant absorption. The conversion of urea into ammonium bicarbonate and a hydroxyl (OH–) and then into ammonia (or an ammonium ion if an H+ is available), carbon dioxide, and water raises the localize soil pH and increases the likelihood that some of the N will volatilize off as ammonia.
In general, we found in the Deep South that you more economically apply urea to pastures or wheat fields in the early spring and often into mid-spring with only minor losses of N as ammonia. Since the soil temperature in Delaware soils seldom reaches the 50º F. level until well into April and we often have long periods of cool rainy weather in the spring, the choice of fertilizer to use on small grains is most likely best decided by economics rather than concern over just how much might be lost through volatilization. The most likely choices of fertilizer products are a urea ammonium nitrate solution (UAN) and granular urea. Since UAN does contain half urea and half ammonium nitrate, the small percentage N loss from ammonia volatilization is not likely to impact the economics between the two fertilizers very much. Growers should evaluate available fertilizers and choose the most economic fertilizer based on the cost per pound of N plus the expected application cost and the availability of the fertilizer through their usual dealer rather than arbitrarily sticking with what they’ve used in the past or what their dealer prefers to sell them.