Muddy stuff

That`s an excellent idea Hatewall,I could see that working for all sorts of things to be pressed into the groundwork!

Something similar to what Rod mentioned above but probably much cheaper is
"Miracle Podge". Have a look at my "lost in the Bavarias" dio and check out the road,
I wanted a sloppy muddy roadway as well so just painted this stuff on top of the painted road,wheels etc. I also took an old toothbrush and dipped it into a mix of this stuff and paint and flicked it onto the snow and the vehicle. I was happy with the result.

1/144th?

I would suggest making (or buying) a piece of styrene strip that is the same size as the "foot" of the tread and stamping it into the groundwork.

I made a quick and dirty photo using clay to show what I mean.

I used this stuff on my driveway called "WETLOOK."It makes anything look well,wet.It is sold in "home depots"I think or other home shops. But I dont think they have small tubs its mainly big tubs but you can try.