I never use yard dirt, and neither should you unless you sterilize it. That means baking it to remove any organisms. You should always use something to build up your ground work and give it a slightly uneven surface unless you want to depict a golf green or a parking lot.
I use Celluclay mixed with model railroad ballast, testured woody turf material (which may no longer be produced), brown acrylic craft paint, white (Elmer's) glue and water combined to a thick oatmeal consitency. After this has dried overnight under a desk lamp, I'll add shrubs, static grass, grass tufts, bushes, rocks and stumps to match the figure's environment. For more ground texture, which in 1/35-1/32 scale would be minimal, I'll add more turf or ballast to the mixture, then paint and dry brush before the vegetation.
To attach a figure to a base, I drill a 1/64 hole in at least one supporting leg and run a piece of brass rod up it. This will have a corresponding hole in the base. For single figures, I usually use a 2 inch cube of craft wood I've stained. Before applying my celluclay mixture, I drill a number of 1/32 or 1/8 inch holes to give the mixture more bite. If I use the enclosed base, I'll still usually drill a hole in a leg and through the base. I fare the base onto the block with the Celluclay slurry. To protect the base, I cover it in masking tape until the figure is attached.
Here's a link to my gallery ar Armorama for examples of my figures and vignettes.