Most of Canada's north is in fact granite "Canadian shield" country with thin or non-existent top soils. See map here: https://cdn.britannica.com/s:700x450/98/180898-050-810A6879/...
Also the bulk of the population lives next to the border, in areas as far south as northern California / southern Oregon.
In reality Canada will not likely benefit economically from global warming because for its entire history it's been mostly an exporter of resources, not a producer of finished goods. Right now a large part of GDP from exports is from oil and gas, and the dominant economic class here benefits from keeping it that way. Similar to Russia, actually.
Canada already has plenty of fertile soils (rapidly being destroyed by urban sprawl, but that's another story). And it used to be one of the world's biggest food exporters. It is now a net food importer. Because it's cheaper to produce food elsewhere with dirt cheap labour in Mexico and so on. That isn't likely to change.