Volcanic Pumice Rock -15 Dry Quarts - FREE SHIPPING! Top Choice Amendment (100% Pumice) among Gardeners. Simulaneously aerates the soil, enhances soil drainage, and retains soil moisture. Doesn't wash away, blow away, or break down over time. Pumice is light weight rock that enhances all types of soil. 3/8" Screened Volcanic Pumice - largest size particle is 3/8", smaller size particles may be present.