Rockwool is a horticultural growing medium made from natural ingredients - basalt rock and chalk. Rockwool for hydroponics is formed when heated at 1600 degrees Celsius, into lava. The rockwool lava is next blown through a large spinning chamber, where its pulled into fibers, which resemble cotton candy or the same lava fibers that fly around in a live volcano. After the rockwool fibers are spun, they are compressed into mats that can be cut into slabs or cubes for hydro growing.

Rockwool has some disposal problems associated with its use because it's made from spun molten rock and does not break down. Even though it's possible to sterilize rockwool between hydroponics crops and use it for two or more seasons, it tends to separate and needs to be handled with extreme care.

Hydroponic rockwool as a growing medium is available in the following two forms:

Rockwool rigid slabs, blocks or cubes - are referred to as "bonded" rockwool because the fibers are held together with a binding agent, which makes the blocks and cubes stiff and brittle. This stiff, rigid rockwool is often seen in displays of freshly cut flowers, the flower stems are pushed into the rockwool for support and water is poured over the rockwool and stored within.

Hydrophilic rockwool or hydrophobic granulate rockwool - is often incorporated into peat moss soil in order to improve the tithe of the soil. Hydrophilic and Hydrophobic granulate rockwool are water absorbent or water repellent.

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