Urea - Significant Losses Converting to Plant Available Ammonium
In a conventional till system where the urea can be worked in shortly after application, it is a very efficient and effective source. Unfortunately when it is applied to the soil surface and rain is the method of incorporation, we can experience between 5 - 60% N losses. The losses come from how urea is converted to plant available ammonium (,NH4). For urea (NH2)2CO2, to be converted to plant available NH4, it needs the enzyme urease. Urease is present everywhere but in the highest concentrations on plant residue. The figure below shows the reaction, urease converts urea into NH3 as soon as the prill dissolves. In the presence of moisture, the NH3 (gas) is turned immediately to NH4 (solid) and is absorbed onto the soil particle.
The problems come when there is no soil particle for the NH4 to bind with. It usually takes 0.50 inches of rain or irrigation to fully dissolve and incorporate urea into the soil. So if we only get a few tenths or hundredths, even heavy dews, some of the urea will dissolve, be converted to NH3 then NH4 and be left on the plant/residue. When the moisture dries, some or all of the NH4 goes back to NH3 and will gas off into the atmosphere. I have even seen this happen when urea is applied on a wet/damp soil, not incorporated and it doesn’t rain for significant period of time. If the temps are cooler, the urease is slower so less of the urea is converted to NH4, but if the temps are warm 60+ degrees these little enzymes can act very quickly.
While the recent rains are a blessing and will surely help germination, it is not aiding our N use efficiency especially in no-till. That is why in some parts of the state you may see some grain drills running right now. Some of those producers are not planting wheat they are actually applying there pre-plant urea. I have even been told in the SW part of the start some producers are using air-seeders to apply their urea. While this seems like a costly venture I have worked with the Ag Economist to create a calculator to figure up the break even for when it would pay to use an air-seeder over the traditional spinner spreader in no-till.
Source: OSU_NPK . DOWN AND DIRTY WITH NPK