Why does my puppy eat so much grass?

My 4 month husky eats TONS of grass. He doesn't just nibble it, he rolls around rapidly taking in mouthfuls at a time, trying to get it all down, hacking, then eating more. He's totally obsessed with it. He eats lots of dirt as well. When I try to get him to stop (I'm worried he's going to choke) he sometimes growls and/or bites me, but he is by no means an aggressive dog, and he never gets aggressive over anything else. He doesn't throw up the grass either, and we feed him enough. He eats Nature's Domain dry food, Kirkland grain free wet food, white rice, and green beans, plus a doggie multivitamin. We've taken him to the vet twice and apparently nothing's wrong with him, but I'm not so sure about that.