Rock Wool Substrate Culture is a horticultural growing media made from the natural ingredients Basalt rock and Chalk. Horticultural Rock wool growing media is primarily available in two general formats. First, as rigid slabs, blocks, and cubes known as "bonded" products because the fibers are held together with a "gluing" or binding agent which renders them stiff and brittle. This is the primary format for the vegetable and cut flower industries. Secondly, rock wool is available as a highly refined and consistent hydrophilic or hydrophobic granulate which is basically water absorbent or water repellent. This format can be used as a component in various peat moss based soilless media or for ground bed incorporation to improve the tilth of heavy clay or light sandy soils.