Dirt smell is the odor released by atinomycetes organisms that live in the soil. If you dig a scoop of dirt from some place with reasonably healthy, productive soil and take a big whiff, that's the smell that you smell.
Some hunters want to concoct a dirt smell with the idea that this odor will mask their human scent and make them smell more "natural." I'm not at all sure that works, because any blend of EOs or an FO that smells like dirt to our limited human noses might smell entirely different to animals' noses.
I suggest using ingredients with strong scents that aren't formulated by humans. One example is pine tar soap. I'm still not sure something like pine tar soap really masks human scent, but at least it's a step in the right direction of using smells that are direct from nature (burnt wood).