how much mud to make soil good?

[Red Shoe Paul]

I'm dumping water on stone to try and make cavern soil, does any amount of mud make the soil good or is it a certain amount?

[Fel]

If you mean for farming, even a dusting of mud (the lowest amount) is fine and anything above that makes no difference.
All that matters is not being in the soil layers, any amount of mud and being underground (technically the tiles being "dark", never having been exposed to the sky).
For above-ground crops it's similar except they don't have "poor soil" in the soil layers and the tiles need to have been exposed to the sky directly at some point (turning them into "light" attribute).