Fall fertilizer is best applied as close to freeze up as possible to balance two objectives: (1) To allow soil to seal over the band (frozen soil may not seal). (2) To reduce losses due to high microbial activity in warm soils.
The key strategy of fall fertilization is to store nitrogen over the winter in the ammonium form – which is held on clay and organic matter – and is referred to as stabilized N. Urea and anhydrous ammonia are both ammonium-based fertilizers. When the ammonium form is converted to nitrate by soil microbes, then it is vulnerable to losses.
To keep nitrogen in this stabilized ammonium form:
- Apply when soils are below 10°C. Cool soils delay microbial conversion. Read more.
- Place N fertilizer in tight bands in the soil. This will further delay microbial activity in areas of high nitrogen concentration. Band urea at least 2” deep. Band NH3 deep enough that you can’t see gas escape or smell it.
- Consider chemical inhibitors, which can reduce this microbial conversion. Read more.
- Also consider soil moisture. Banding in wet soil conditions will increase ruts, and wet soils can also increase losses if the soil does not close well over the band. Banding in dry soils can also increase gassing-off losses because soil does not provide a proper seal on the band.
Finally, if fields are short of phosphorus, consider a phosphate and urea blend in the fall. Work from Don Flaten at the University of Manitoba shows that phosphate and urea in a dual band applied in the fall can make phosphorus more readily available in the spring. Even with a fall application, some P placed in the canola seed-row next spring is recommended.
Fall soil tests
For results to be as close as possible to the situation next spring, the ideal time to take fall samples is when soil temperatures drop below 10°C and as close to freeze-up as possible. Microbial processes in the soil slow down as temperatures cool, so mineralization should be minimal below that threshold. Read more.
Other considerations for fall fertilizer work:
Dealing with ruts
2016 article on fertilizer application in November
Plan fertilizer over the winter, don’t apply it (includes reference to Manitoba winter restrictions)
Podcast on fall soil sampling