Dyna-Gro DYNA-ROK II Advanced Hi-Silica Growing Medium is a natural, non-toxic and environmentally safe growing medium. It is suitable for all indoor and outdoor plants, including orchids, roses, annuals, perennials, shrubs, vegetables and fruit trees. This lightweight, porous mineral is super absorbent, taking up as much as 150% of its weight in water and nutrients, which are then slowly released back to the plant's roots. This material is sedimentary rock consisting of the fossilized remains of unicellular fresh water plants known as diatoms. It consists of approximately 90% amorphous silicate with the balance made up of other mineral elements needed for plant growth.