The ground material is real dirt I found locally. It has a high clay content, and thus has little “sparkle”.
I make several grades by sifting it through various sizes of brass screen.
I apply it over diluted white glue, applied with a brush. I start with a fine grade.
Later add applications of coarser grades, leaving the fine material on areas like pathways, etc.
On the final applications I drop in larger sized rocks and bits of sawdust for leaves (most of it sticks).
The oil mess is done with applications of thinned Floquil Engine Black and Roof Brown.
I don’t have a specific ratio; I just mix it in the lid of the thinner bottle until it looks “oily”.
I use paint thinner (mineral spirits) as it gives me more working time.
This application is very subjective; I bring it up (add more paint) and take it back (more thinner) several times until I like it.
You can’t tell the final result until the thinner totally evaporates.
I find it hard to get a nice “blend” between the oil and the dirt, so sometimes it takes a while before I’m happy.
My railroad runs Shays (drive line facing the edge), so that side of the track was treated with the same oily mixture.
Photos from the West Side and Diamond and Caldor Lumber companies were used for reference.
I also take a sharp tipped brush and add the occasional drop of thicker paint to represent grease spots.