What is Happening to the N?

Surface applied nitrogen in the urea form is susceptible to losses from volatilization. These forms include granular urea (46-0-0) and 28% UAN solution, which consists of 1/3 urea. Past experience tells us that when producers apply N in anticipation of forecasted rainfall and precipitation is not received N losses may be considerable.  Factors favouring high volatilization potential are:

Factor Reason
High soil temperatures Speeds urea hydrolysis, diffusion of NH3
Moist conditions followed by rapid drying Granular pellet dissolves, then NH3 is lost with evaporating water
Windy conditions Reduces boundary layer between soil surface and atmosphere which increases losses of water and NH3
High soil pH (>7.5) Increases ratio of NH3:NH4+. NH3 is subject to movement and loss, whereas NH4+ may be held by exchange sites on clay and organic matter
High lime content in surface soil Increases ratio of NH3:NH4+
Coarse soil texture (sandy) Less CEC (cation exchange capacity) to hold NH 4+
Low organic matter content Less CEC to hold NH4+
High amounts of surface residue More urease enzyme present may decrease volatilization if soil temperature and wind speed is reduced and soil moisture is preserved.
Nitrogen source Urea > UAN solution > ammonium nitrate

Several of these conditions have been experienced. Following is an estimate of losses based on Manitoba research:

University of Manitoba: 38-46% of urea N lost in 5 days at 25°C vs less than 7% loss at 15°C.

AAFC Brandon: 40% and 88% loss of urea N after 7 days in May (20-25°C temperatures) and July (30°C temperatures), respectively. Losses for 28% UAN were 7 % and 50% under the same circumstances.  

A 1/4 inch (6mm) rainfall is required to adequately incorporate surface applied urea forms.

Options for topdressing in a dry spring:

  1. Spoke wheel apply 28% UAN
  2. Strip application of 28% UAN solution will improve its efficiency, but losses still occur.
  3. Harrowing after application may not be very effective. Shallow incorporated urea will hydrolyse and the NH3 will move with water that is evaporating from the soil surface.

Research has been done to document benefits of the urease inhibitor NBPT (Agrotain) at AAFC, Brandon.

Farmers that have applied urea form nitrogen may want to estimate losses and make supplemental applications. At present no guideline is available to assist in making this decision, but farmers should inspect fields.

For further information, contact your MAFRD GO Representative.