What Happens to Surface Applied Nitrogen?
Prepared by John Heard, MB Ag Soil Fertility Specialist
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 to 4/10" (6-10 mm) of rainfall is required to adequately incorporate surface applied urea forms.
Options for topdressing in a dry spring:
- Apply UAN (20-0-0) below the soil surface. Options include spoke wheel applicators and RTK guided,
- Dribble application of UAN (28-0-0) solution will improve its efficiency over broadcast sprayed UAN,
- 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.
- Use of a urease inhibitor such as Agrotain (active ingredient NBPT) will delay losses for up to 7-14 days.
Farmers that have applied urea form nitrogen may want to estimate losses and make supplemental applications. Losses due to volatilization can be estimated by using Nitrogen Rich Strips (NRS) or measuring with clositubes. NRS are an applicator width of 150 percent the recommended N rate applied as a strip or in cells across the field. Simply compare crop growth (colour, height, biomass) between the field and he NRS. If differences are apparent, supplemental Nitrogen may be required. Use of NH3 clositubes for estimating volatilization loss is described here:
For further information, contact your MB Ag Office.