Pumice (filtered 3/8³ to 1/4³ in size) is relatively lightweight volcanic rock used as an amendment in all types of soil Its porous composition allows it to retain moisture and nutrients during watering, and release them slowly over time as the root system requires Think of these ...Pumice (filtered 3/8³ to 1/4³ in size) is relatively lightweight volcanic rock used as an amendment in all types of soil Its porous composition allows it to retain moisture and nutrients during watering, and release them slowly over time as the root system requires Think of these components as little batteries for your plants! Sol Soils- Pumice Amendments Pumice (filtered 3/8" to 1/4" in size) is relatively lightweight volcanic rock used as an amendment in all types of soil. Its porous composition allows it to retain moisture and nutrients during watering, and release them slowly over time as the root system requires. Think of these components as little batteries for your plants!