I'm not quite sure thatsoil quality itself should affect farming. At least not at the rate of soil nodes and qualities we see in distribution now. No one would ever see decent crops.
If soil is allowed to be cultivated, however, then we see the effect of trees going up a few hundred points of potential quality which is going to drastically increase the margin of almost every other product's potential development.
However, if we would like to see farming be softcapped by a natural resource like soil the we can simply make it a quintessential ingredient to compost. If this is done then there should be a scale that means the ideal peak is determined and Q10 should be ABLE to achieve that but it should be extremely difficult. Q90ish soil should be able to reach 2x-3x that potential value. If you develop your compost to beyond the potential development of Q10, then Q10 should nerf it. So that high quality soil is used sparingly to raise the quality to a level of resource that reflects what a given community has more sustainable access to.
The tiles/plots/etc. themselves would have a cultivated value. Fertilizing would average out the land quality and growing crops should cost quality. Thus cultivated land becomes valuable because it takes a lot of investment to achieve. The value of high quality baked goods goes up because of their hunger:FEP ratio current demand and increased cost of production while remaining something that everyone can develop for themselves with or without access to good soil. It also takes out stupid margins of farming development.
It would put farming on par with where sausages are now.