I have been using dirt for ages, not exclusively though. I find it gives a unique texture.
I model in my garage and usually get dirt from around the garage door and sift it with my fingers to break it up a bit then sprinkle over the pva glue. I also use cat litter crushed and a bit of fine building sand to get the required texture. It all must depend on the type of weather conditions you want to set the scene in!

Sam

i mix a little sand from the beach with dirt and very fine sand from the crafts store, the fine sand is the dirt, the sand from the beachrepresents the small stones and thick dirt, and the real dirt represents the bigger stones and gravel.

I believe that mother nature can provide us with many matterials to build a coutry side scene. Real dirt, moss, little stones, branches roots and many other useful things may help us build a dio without spending any money. I used real dirt in my first dio with good results. I sifted the dirt, then mixed it with white glue and water. The result was a pasty mixture which was applied on the base. Then, when it was still wet I placed the moss for grass, some little stones and it was done. It is uncrackable ( have in mind that the base fall from 2 m high and it is undamaged). I didn't paint the dirt as there is nothing more natural than nature.