Mother Earth? Volcanite Premium Pumice is a great soil amendment or can also be used as a stand-alone growing medium. It is derived from a volcanic origin and has a neutral pH. Its porous structure enhances drainage and encourages gas exchange to increase air porosity, yet still holds water and nutrients. Mother Earth? Volcanite Premium Pumice has a density that prevents it from floating to the top of soil and media unlike other aggregates. This ensures complete distribution throughout the root zone for excellent and predictable drainage and aeration characteristics throughout the entire growing cycle. Particle size distribution is 3/8 to 1/32 in.