Rockwool is made from melted basalt, chalk and sand, basalt is a fine grained hard rock from ancient volcanic activity. The basalt is melted and spun , almost like cotton candy, into rockwool. Horticultural rockwool came into being in the 1960s as an offshoot of the fiberglass industry which manufactures their product in a similar fashion. In fact it was initially called "mineral Insulation" as it was a home insulation. Rock wool is basically fiberglass with some basic changes in composition.
There are various grades of Horticultural Rockwool. The highest quality is made from basaltic rock, which leads to a highly non-reactive rockwool. Nonreactive in the sense that the metals in the rockwool do not react with the minerals in the nutrient solution and produce undesirable results.
Another sign of quality Horticultural Rockwool is that it should not remain saturated. Good drainage is essential in a hydroponic system. Most manufacturers will incorporate mineral wetting agents, Wetting agents lower the surface tension on a liquid, enabling it cover larger surface areas and promoting easier uptake by the roots.
Rockwool, for the longest time was the most popular hydroponic growing medium known. Although it still the most popular for drip systems, it has been waning in popularity in recent years.
Rockwool has many advantages.
1. It has a large water retention capacity, and also holds sufficient air. It holds a lot of water which gives you an advantage against power or equipment failures. And it also holds 18 - 25% air which gives the root system ample oxygen so long as the media is not completely submersed.
2. It is available in multiple sizes and shapes for various hydroponic applications. Everything from 1" cubes to huge slabs.
3. Rockwool is also durable, it doesn't readily fall apart or break down.
Rockwool also has some serious disadvantages
1. That same quality that makes it durable also makes it an environmental nuisance. Thrown away it does not biodegrade - It's rock. A million years from now it will still be sitting where ever it was disposed of.
2. Rockwool is also a health hazard. Rockwool, like asbestos is made from rocks and minerals the fibers and dust thereof have proven to be extremely hazardous. Wearing a dusk mask when using it is advisable. Rockwool is not as serious a threat as asbestos, but it is still unhealthy.
3. Rockwool has a high pH and nutrient solutions must be adjusted to accommodate for that factor. It is also susceptible to pH shifts which creates the need to continuously monitor the ph Levels of your system.
4. Although rockwool has a high water retention capacity it has a restricted root environment and a low buffering capacity for water and nutrients. The water flow to plant roots may be hindered, even when the water content is apparently high.
Other Grow Mediums
1. HYDRAULIC PROPERTIES OF ROCKWOOL SLABS USED AS SUBSTRATES IN HORTICULTURE