When and why did that change?
Soil doesn't change much. Climate and farming techniques do. You could put much of the change to the medieval warm period, new plows and methods to deal with stuff.
When and why did that change?
If it's just knowledge of techniques, that can change with earlier Roman penetration after Teutoburg Forest causing cultural exchange. Roll forward 200 years and it suddenly looks much more attractive.
That only works if either side has the tools or technique or if it came with from said intermixing.
Soil doesn't change much. Climate and farming techniques do. You could put much of the change to the medieval warm period, new plows and methods to deal with stuff.