First you need a cow.
Make cows drop cowdung. Little shits that need to be collected and can be spread across fields to up the quality of the crops a little bit.
This will not affect the soil q at all, it will only, either make the crop go up a point in q, or make the +2/-5 go to perhaps +2/-3 or something (or both), to even out the general q of the harvest.
Might be a tradable commodity for villages to smaller local farmers.
Grow local!