Fall Nitrogen Applications

Fall nitrogen applications will be commencing shortly.  There is an important rule and several guidelines for legal and efficient use of nitrogen.

RULE: November 10.  Nitrogen and phosphorus applications must be made before this date to comply with regulations under The Nutrient Management Regulation of the Water Protection Act (C.C.S.M. c. W65).


A)     Cool soil temperatures – when nitrogen is applied to cool soils, the biological conversion to the nitrate form is reduced.   The following table (adapted from Manitoba research by Tiessen et al) illustrates the dramatic impact of cool soils on slowing nitrification rates.  Conversely  on warm soils this conversion is rapid, and we wish this to happen during the growing season.  But once nitrogen is in the nitrate form it is vulnerable to leaching losses and denitrification under wet soil conditions.

Table 1.  The estimated rate of conversion of ammonia-N  from banded urea to nitrate-N. (Heard from Tiessen et al, 2003)


Growers and agronomists can measure soil temperatures at the depth of injection.  Soil temperature at 2” depth under sod is posted for a number of Manitoba sites at MAFRD’s Ag-Weather Program website: http://tgs.gov.mb.ca/climate/SoilTemp.aspx

B)     Banding N in the ammonia form also slows conversion to nitrate.

C)      Nitrification inhibitors can slow this conversion. These include the DCD component in SuperU, nitrapyrin in eNtrench for urea or N-Serve for anhydrous ammonia.

D)      Controlled release products, like ESN,  slow the physical release of urea, which in turn slows this conversion to nitrate.

Following the 4R Approach – especially source, time and placement – will maximize performance of fall applied nitrogen fertilizer.

Submitted by:  John Heard, Crop Nutrition Specialist, MAFRD

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