Muddy stuff

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.

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.

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.

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