It has to be dry earth and I think you’ll see this form of feline behavior occur almost exclusively in warm weather. Perhaps it is because the earth has to be dry and also the warm wether is pleasant to a cat and it excites the cat to seek more pleasure.
I know my cat likes this, but the downside is that her ears get dirty, so I’m constantly having to clean them. She’s very good about letting me do it, since it’s a regular routine.
I use baby swabs with a little coconut oil on them, which works pretty well.
In the past, I had a huge vet bill, and serious side effects for her, due to a deep cleaning for “debris” in her ears. I don’t want to go through that again.