Answer (provided by John Heard, MAFRI Crop Nutrition Specialist)
The amount of nitrogen remaining for your canola crop will be affected by the timing of fall N application, amount of rainfall you have received in May and soil type. Early fall applied nitrogen (regardless of form) will likely have converted to the nitrate-N form by May of this year. It is this nitrate-N form that is vulnerable to loss by leaching or denitrification. But fall and overwinter losses will have been negligible since the fall was so dry. But the heavy rainfall experienced in some parts of the province the past couple weeks may have moved that nitrate-N deeper in the soil profile – especially on sandy soils. On clay soils that become waterlogged the nitrate-N is lost at a rate of 2-4 lb N/ac/day due to denitrification.
So some of your nitrogen will still be present for canola. The standard soil nitrate-N test will pick up this form of nitrogen and could be used now. Or one could seed now but apply a high rate of N in a strip across the field. If colour differences are apparent by bolting – then make a supplementary N application.
More details on such a tough decision are covered in the attached article from the July 2011 edition of the Crops e-News.