Not to be cute, but dig it up...literally.
Put some aluminum foil down on a cookie sheet and spread your "dirt" out a little- breaking up any clumps. Put it in a 200 degree oven for about 45 minutes. This will ensure that all the water is removed and that all bacteria is killed. Put it in a plastic baggie and you have all the dirt you need.
To put it on a model, mix this dirt with a 50/50 mix of white glue and water, tint with water-based paint if desired, and apply...Simple.