Pumice

Pumice is a relatively light weight porous stone material that is used in soils to provide aeration to mixes. Because it is a stone material, it does not break down in the soil. Its porous nature allows it to suck up and hold onto moisture, while permitting excess water to flow through the mix easily. Its alternative in a soil mix is rice hulls. Some prefer the pumice as rice hulls will eventually break down. However, in our own observation this takes at least 4-5 years to occur. And considering that when rice hulls break down, they release their significant silica content to the surrounding soil, then there are obvious advantages to both materials in a soil mix.