Ain't nothin' like the real thing, baby!
Yupper, sifted dirt do nicely... Get as fine a dirt as you can, then sift it through a screen. Then sift what passes through that in a finer screen...
Make a "mud" mix with a 50-50 Elmer's glue and water mix, and a drop or two of dishwashing liquid, apply here and there, and you're done! If you want the mud to look really wet, paint it with thinned clear gloss...
One thing to note about using real dirt... It's best to bake your outside dirt (if you take it from a lawn or anywhere around your house) in a 350-degree oven for about 10-15 minutes to kill any critters living in it.. You want the dirt to be "dead".
Dirt taken from the street is already dead... Don't over-look that suff in the street either, the little rocks and stuff work great on your landscaped bases and dioramas as either the rocks they are, or if they aren't "smooth", it makes for great "rubble" around a bombed-out building.. Be sure to sift them into different "grades" and bag them separately...
Use "dust" sparingly... Unless your tank calls for an even, heavy coating of dust, an airbrush is too much.. Instead, use artitist's pastels... Dust accumulates in corners and recesses, floors, aound hatches, etc..
They aren't expensive either... One box of 12 will run about 12.00-15.00 dollars, and will supply enough for a good part of your lifetime... If you don't have an arts & crafts store near you, like Hobby Lobby (if you DO have a Hobby Lobby, use the 40% off coupon to buy them), you can get them on-line at Hobby Lobby...
http://www.hobbylobby.com/