Thanks for that answer! I've been busily baking some dirt here as well--a pleasant change from all the Christmas puddings and mince pies! 
Do you mix the dirt with any or just glue it down? I ask because you've nailed the texture so well.
Because the forest floor has so many bumps I just left all the dirt with its clumps and stones and such. I’ve got a 5 gallon pail of it. I painted white glue full strength all over the module, sprinkled the dirt, and then did several applications of sprayed on alcohol followed by diluted matte medium. Until it was all saturated. Some falls off now and again but it’s all pretty much locked in place. An interesting note. Some modules are different colours. I guess the dirt is different depending one which part of the yard I dig it from? Or maybe how saturated it got? Anyway I opted not to paint again it as it’s going to be almost completely covered in understory. Or maybe I’ll shoot some brown with my airbrush just to blend the module joints?
Oh and I prepainted my plaster covered foam, or exposed foam, in brown latex paint before I did the dirt layer. It was just a left over brown I had from when I painted a room brown in my house.
