They're much better for agriculture than they used to be (though still pretty poor in Siberia). The change in productivity is wholly due to climate change. Warmer weather leads to a longer growing season, rain instead of snow, etc.

A lot of this land is nutrient packed as well. Siberia could be prime growing land if it gets warm enough. Similar to Ukraine today.

Saying that, there are a number of concerns about soil resilience in Siberia. If soil resilience doesn't improve, Siberia will remain very poor for agriculture.