· Lava rock adds space. What does pumice do that the other two don't do? Please Log in or Create an account to join the conversation. Pumice 2 years 8 months ago #45536. Clicio; Away; Platinum Member ; Posts: 3456 ; Thanks received: 1483 ; From this very website, Bonsai Empire: "Pumice is a soft volcanic product, which can absorb water and nutrients quite well. When used in a .