Surface N Applications & Dry Soils

Planting is proceeding at full tilt and some questions are arising about surface N applications.  When urea forms of N are surface applied, they are vulnerable to hydrolysis (the cleaving of the urea molecule with water) and subsequent losses of ammonia gas (NH3) called “volatilization.  This is a well established risk  and risk factors are well known, and more information can be in the article Volatilization Losses From Surface Applied Nitrogen on the MAFRD website.

Conditions favouring high volatilization potential are:

  • high soil temperatures
  • moist conditions, followed by rapid drying
  • windy conditions
  • high soil pH (>pH 7.5)
  • high lime content in surface soil
  • coarse soil texture (sandy)
  • low organic matter content
  • high amounts of surface residue (e.g. Zero tillage)
  • nitrogen source: urea>UAN solution
  • UAN application: broadcast UAN> dribble or strip UAN

Our observations in Manitoba have been that urea or UAN solution (28-0-0) applied to bone dry soil actually suffers little volatilization loss  since there is little moisture to drive hydrolysis. At this point with dry conditions forecasted, the riskiest situation may be by exposing your applied N to enough soil moisture to start the volatilization process – such as applying immediately after some soil disturbance like seeding or secondary tillage.  Likewise, very shallow incorporation through seeding or harrowing may start this process.  Light rains of 1/10″ may also be enough to start this volatilization process, yet inadequate to incorporate the urea molecule.  Generally rainfall of ¼ to 0.4” is considered sufficient to incorporate surface urea or UAN.

If you must apply urea or UAN to moist soils or in other high risk situations, a NBPT urease inhibitor like Agrotain is recommended.  Although volatilization losses on already dry soil surfaces are usually low neither is this surface N doing any good until rainfall is received.  A strategy of waiting until imminent rainfall may still be a preferred option.

Submitted by: John Heard, Crop Nutrition Specialist, Manitoba Agriculture

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