This technique was quite common in the 1950's for creating texture. The plaster would be "stippled" to make it look like loose dirt. A stiff brush is held vertically and bounced in and out of the almost dry plaster. This is my variation on this technique. I am using latex paint and sand.
I use a base flat latex for my scenery. I have found that mixing fine paver sand into it makes a texture paint. I originally felt that would create a realistic texture. Stippling the latex/sand mixture adds a whole new texture to the surface. The strained paver sand has an element of dust in the mix. When the mixture is stippled it becomes dead flat.
I use paver sand from a home improvement store, it is used to "set" patio blocks. It comes in a variety of colors. This is my red clay color. I have matched a latex paint to that color. A fine strainer produces the "stipple" sand. I mix it with the latex paint in a fairly liquid mix.
I paint the areas that aren't to be covered by foliage with a coat of latex paint/sand mixture. Allow it to dry until it is tacky and then stipple the surface.
The stippled area will look like fine sandpaper while wet.
When dry the powder in the sand forms on the ridges and the stippling makes it dead flat. It looks like dirt. The coarser sand forms in the gullies and looks like sediment. (Click Image to Enlarge)
The stippled ridge with a HO man & woman on the right and an OO scale man on the left.