The reason why the Midwestern United States have a very fertile soil, but Canada doesn't, despite both of them being covered by glaciers until relatively recently, and both having the same age of soil, is the climate and vegetation.
Canada is much colder than the Midwest, its winters are longer, and the summers mild, while the Midwest has higher temperatures, prolonged period of drought, and seasonal rainfall.
Because of this, Canada has been covered by coniferous forests. They leave relatively little biomass, and also the climate is making the decomposition of the biomass very slow, thus resulting in relatively poor soil quality.
In the Midwest, the climate is perfect for the development of the grasslands. The grasses grown and die each year. They live a lot of biomass, and the higher temperatures contribute that the biomass decomposes pretty quickly, thus resulting in good quality soil.
