The most common issue I see when it comes to someone choosing the right medium is just plain education. Most people tend to go with the he said-she said choice. It is important that you find what works best for you and your situation.
Soil less Mixes will include some if not all of the following - Coco fibers, Rice hulls, Peat Moss, Perlite, and barks. A lot of companies will also throw in some humic acids, compost, endomycorhizae, castings, and other natural fertilizers or stabilizers in the mix. I prefer to have the least amount in my medium so I can control everything my plant gets. It makes it easier to break down problems when they arise.
Lets go over some of your choices
ROCKWOOL:
Rockwool comes from the basalt rock. It is mined, heated down, made into long fibers, spun like cotton candy, and chopped down to size. Rockwool can put off some itchy dust but once it is soaked in water it is a lot easier to handle. Rockwool must be pre-soaked, the Acidity of the product must be neutralized prior to use.
Clay Rock or Hydtroton/Hydrocorn/hydrokorrels All of these are going to be very closely related. The difference is where they are mined. You will see different shapes, sizes, color, and even dust content will be different. Clay rock doesn't need to be soaked but it does need to have the dust washed off as well as possible. For those companies that claim you don't have to wash their rock, just wash it. I can not imagine how precise they can possibly be in such large productions, I've hand washed cup fulls and still have residue but its your call. You can use this rock in many different ways. I have seen flood tables filled up with them but once they get rooted in there you can no move them! They work well in any aeroponic or DWC systems in net pots.
COCO COIR:
Coco Coir comes from the husk of a coconut. It has many other applications other than gardening but thats the most important right? It also comes in many forms; bricks you can expand, loose bags, chips, or chunks.
Volcanic glass that is heated and expanded to create a foam like appearance. It comes in various sizes depending on what you need. You can use perlite on its own or in other mixes. Perlite provides room for both air and water to pass.
PEAT MOSS:
Exactly what it sound like, Moss. It can hold large amounts of water and retain nutrients. You will mostly see this as an additive to other mixes and not necessarily being used on its own.
Each choice will give you a different level of Moisture retention and vary in size and shape. Ideally you dont want a heavy soil that will compact and suffocate your root system. You want to make sure you have sufficient drainage. Those of you who would like to find a happy medium (no pun intended), can mix your soils with any of the aerating products above to increase oxygen levels and space for your roots. Every plant is different and requires a different environment. Customizing your mixes is the best way to go to ensure the plant reaches its maximum potential.
Different methods of growing will require a different options for a mediums.
