To get the texture this is what I'd do - frost the cake like normal, without worrying about getting it perfectly smooth & let it set up a bit; then take a piece of plastic wrap; wad it up; then open it back up a little (don't get it or let it be smooth - you want the wrinkles); then lay it on the frosting and gently press all over....you want all of the plastic to come in contact with the frosting or you'll end up with un-textured spots...you can sort of smooth it with the back of your hand - not to get the wrinkles out but just to ensure full contact everywhere....when you pull it off you will have the texture of the plastic imprinted on it.
I've done this on a wall with red paint and a brown glaze (let the paint dry, then apply the glaze and then the plastic wrap...smooth all over for good contact, then peel off) When you remove the plastic it leaves the texture behind and when it's all dry it looks like leather...at least on a wall it does. I think if you did this with your frosting it would look alot like dirt and the texture of the ground or a rock wall or the like.
Just an idea...HTH!!