Rockwool as a Substrate

     Rockwool is a commonly used substrate in hydroponics and aquaponics.  If you have never worked with it the best description of rockwool would be to say it looks and feels somewhat like insulation.  It isn't hard on your skin to work with, though.  Rockwool manufactured for growing purposes is made from basaltic rock.  The rock is heated to its melting point and spun into fibers.  A binder is then added to the fibers and they are compressed into slabs.  These slabs are then cut to a variety of sizes which serve different growing purposes.
     Rockwool is great for aquaponic purposes because of its water retention and air holding capabilities.  It's also very clean and easy to work with.  I use rockwool in my raft systems because it won't break down when constantly moist and is very inexpensive.  Seeds can be germinated directly in the rockwool or seedlings can be put in rockwool once they've sprouted.  It's a very versatile medium.  There are disadvantages, though.  The earth must be mined to get the basaltic rock required for the production of rockwool.  Many would argue that this makes it an unsustainable product.  It naturally has a higher pH, and it's recommended that you soak rockwool in a solution of lower pH to neutralize it.  I've never adjusted the pH and have never run into any problems, though.  Because it's manufactured from rock it won't break down.  So, once used, disposing of rockwool is tough for someone striving to be very green.  Putting it in a landfill is not acceptable for some people.
     There are a couple of viable alternatives to rockwool.  The best alternative, in my opinion, is to use net pots and hydroton or growstones.  This way when plants are harvested the substrate can be cleaned off and reused.  Seeds can be started in coco coir or a similar substrate and planted in the net pots when they've grown an inch or two.  There is also a product called Oasis foam that is mostly used in the floral industry.  It works, but I'm not fond of it.  The foam breaks apart easily and the cubes are too small, in my opinion.  I'd rather use rockwool than Oasis foam.
     If I had to pick my top two choices for raft and NFT system substrates I would choose rockwool or net pots.  Rockwool is a great product as far as using it in a system goes.  But, long term it may not be the best choice.  And it is another financial input since you have to buy new slabs occasionally.  Net pots will cost less over time and are the better choice for someone who wants to be as green as possible.  More information on rockwool can be found at the Grodan website.