With the recent rains in Ontario, there have been questions about the potential losses of N from applications to winter wheat. Rate of nitrogen loss depends on several factors, including:
- Form of N fertilizer
- Rate applied
- Application timing
- Precipitation received
- Soil texture
- Temperature
In general, volatilization losses are expected to be quite low – there has been enough precipitation to move N into the soil profile where it is protected. Denitrification may be the larger concern due to wet conditions, especially on heavier clay soils, but cool soil temperatures have mitigated the loss potential. Table 2 shows the effect that soil temperature has on denitrification losses in saturated conditions.
| Soil Temperature (°C) | Potential denitrification rate (percentage of N03-N in soil) |
|---|---|
| < 12 | 1% – 2% per day |
| 12 to 18 | 2% – 3% per day |
| > 18 | 4% – 5% per day |
On lighter sandy soils, nitrate leaching is the more dominant form of loss, but is likely not a major concern for most of the province. In areas with extreme rainfall events, there is potential for movement of nitrate out of the rooting zone through the soil profile, but loss is dependent on the rate of soil infiltration. A week of steady rain equating to 125mm (5”) will move nitrate further down in the soil profile than a single rainfall event of the same amount. A deep, healthy root system may still pick up nitrate that has moved downward assuming it hasn’t left via tile drains.
The timing of application is important as well. Urea fertilizer may take 7-14 days to be converted to nitrate, which is the form of N that can be lost via denitrification or leaching. Split application is a way to reduce the risk of N loss from early season applications and gives an opportunity to adjust rates later in season depending on crop conditions.
