Down To Earth™ Diatomite Rock is a naturally mined mineral composed entirely of the skeletal remains of trillions of microscopic, unicellular plankton called diatoms. It permanently improves heavy soils and horticultural growing media because it is indefinitely stable and non-compactable. The porous aggregate particles wet rapidly and will absorb greater than 100% of their own weight in water, acting as a reservoir for plant available water and nutrients. Fine to medium particle size (2-7 mm).