I use real "dirt" dug from the prototype locations I'm modeling. I don't use "soil" which , to me, is dirt with a lot of organic matter in it...like rotting grass bits, manure, bugs, leaf particles, jack rabbit poop, etc.
Since I'm digging it up from select locations, I try and minimize the organic matter in it, and since I'm modeling a desert region with sandy/dusty/rocky soil, finding a spot that looks like it doesn't have much organic material in it is pretty easy to do.
After I dig at least a couple of buckets from each spot...I take it home and bake it in my kitchen oven at 350 deg. for an hour, which I hope is going to kill any bugs and cook any fresh organic stuff.
Then, I run it through several sieves and colanders, grading it into four different "finenesses" from small rocks to a very powdery dust.
For regular old "dirt", which is put over most of my painted extruded Styrofoam scenery base, I sift it on with a tea strainer, the sprinkle larger sand particles on it....then, and this is important, I pat it down, or squash it with my fingers, so that it doesn't appear to be loose (unless I want it to look loose). Mashing it with my fingers compacts it and I sprinkle on larger bits of graded dirt and rocks, mashing them down into the first application if they're not too large.
Then, I wet a small section with "wet water"...no, I don't use alcohol, but I don't see why you couldn't...and then I mist on my adhesive...which is my ballast glue mix of Modge Podge, water and a couple of drops of Dawn dish-washing detergent...soaking the location thoroughly.
This hardens up very nicely...and if I don't like some spots, I can easily add more, and glue it down over the first layer.
I don't see why using different grouts, with maybe some sifted sand or dirt material thrown in for texture wouldn't work too...but without having to "glue" it down...just wetting it (I assume, since I've never worked with grout).
Photo (1) - Here's a section of Wilhemina Pass on my old Ntrak module done using my described method...but with a teased tan felt layer glued to my extruded Styrene scenery base first...colored with areas of green permanent inks from my airbrush, then trimmed before adding my graded dust, sand, and dirt....with ballast glue applied with a big dose-dropper rather than being misted on so the felt fibers don't lay down from being wet (I want them standing up).
Photo (2) - Here's a small section of my present layout with a caboose on the Park City Branch trackage with rocky soil applied over teased felt. A different area, different "dirt":
Have FUN!
Cheerio!
Bob Gilmore
