Rockwool is a clean media which has a perfect Air:Water ratio and comes in a ready to use cube format.
Rockwool is a natural media made from a volcanic rock. The production process ensures sterility in the final product.
When hydrated, each cube has a consistent 80% Water 20% Air ratio which makes it an ideal media for hydroponics as it is difficult to over water the cubes. This prevents seedling problems such as damping off.