Vermiculite is used in Bonsai soil to keep it aerated and loose, which helps with drainage, but it also absorbs substantial amounts of and later releases it as it's needed. This Vermiculite is sterile, free of disease and has a neutral pH. It's also makes a great potting medium to use when planting seeds or root cuttings.
Horticultural Vermiculite is an excellent soil enhancement. It's clean, permanent, odorless, lightweight and inexpensive.
* What is this stuff? Horticultural Vermiculite is the geological name given to a group of hydrated laminar minerals which are aluminum-iron magnesium silicates. It has the appearance of mica, and is found in various parts of the world. When processed for horticultural use, the mineral is subjected to intense heat, expanding it into accordion-shaped granules with countless layers of thin plates.