Dirt? Are you serious? Sorry to say, but that hardly qualifies as murderabillia.... I thought first that you were expecxting to find some bone fragments or something else of interest but plain dirt is just dirt like from any other place. It's always interesting to visit places with some history, and if you want to bring a piece of it home, why not? But dirt, man.....
How the hell would you expect to find bone fragments? The State/FBI comes in with ground-penetrating radar and tear's the yard apart. The likely hood of that happening is nil. As far as the dirt not being interesting, I have to disagree. It has an actual connection to the crime. And when, in this case, the sk has nothing out there aside from envelopes/letters, this is as good as it gets...
I don't collect dirt. But since I live near Seattle, when I have the time, I plan to visit each Ridgway victim that is buried in Seattle along with Bundy victims and collect dirt from each of their graves.