Rock wool cube is the most widely used medium in hydroponics.
Rock wool is an inert substrate suitable for both run to waste and recirculating systems.
Rock wool is made from molten rock, basalt or 'slag' that is spun into bundles of single filament fibres, and bonded into a medium capable of capillary action, and is, in effect, protected from most common microbiological degradation.
Rock wool is a substrate which is already used routinely, along with coconut and arlita in marijuana cultivation, especially in hydroponics, with large results and benefits.
Rock wool has many advantages and some disadvantages. The latter being the possible skin irritancy (mechanical) whilst handling (1:1000). Flushing with cold water usually brings relief. Advantages include its proven efficiency and effectiveness as a commercial hydroponic substrate. Most of the rock wool sold to date is a non-hazardous, non-carcinogenic material, falling under Note Q of the European Union Classification Packaging and Labeling Regulation (CLP).
Of course, it is a material that has been modified to employ industrially in our crops. A modification made as follows, as described from Hydroenv “is first extracted basalt rock and gets into furnaces at temperatures of 1500 ° C. Once it is in a liquid state (lava) it is passed hoops that rotate at high speed, allowing lava to be spun. Then the rock wool threads harden when cooled with hot air at 230 ° C; and subsequently bind the fibers are compressed. Finally cut and packed into the desired presentation. ”
As is the case with other natural substrates, it is necessary to stabilize the pH thereof for 24-48 hours, time in which we change the water in which we introduce our rockwool. The water should have a pH of 5.5 and changing water every day that passes is explained precisely by the fact of having to maintain the pH, so that the quality of our substrate is not damaged, its pH stabilized and thus not contaminate our plant with any substance that does not belong to this mineral.
Note: some information transfered from Noelia Jiménez, thanks