Growers should be monitoring soil temperatures to guide fall nitrogen management.  Provincial weather station soil temperatures are posted at: http://www.gov.mb.ca/agriculture/weather/soil-temperature.html

The principle of fall fertilization is generally to delay applications until soils have cooled so microbial activity is curtailed.  That way less of the stable ammonium form-N (NH4+,) that is held on clay and OM, is converted to nitrate (NO3-) which can leach or denitrify.

The rate of nitrification of banded N to nitrate is illustrated in the following table from http://www.gov.mb.ca/agriculture/crops/soil-fertility/a-fall-fertilization-refresher.html

Table 1. Nitrification rates of ammonia to nitrate form-N from banded urea (calculated from Tiessen et al, 2003¹).

So as soils cool and eventually freeze, the microbial activity is reduced such that ammonium-N is retained in its stable form.  If one chooses to apply nitrogen before Mother Nature provides cool soil – they may consider using one of several enhanced efficiency fertilizers – N-Serve, eNtrench, ESN or SuperU. 

Submitted by: John Heard, Soil Fertility Specialist