Since the dawn of agriculture farmers and growers have obsessed with what their plants are growing in. While there's something to be said for growing your plant symbiotically with other creatures in a rich black dirt, having your soil achieving that zen like state can take years to achieve and most of us don't have the time or patience to do this.
Besides being used as a source of water and place to anchor itself to, the plant also takes nutrients from the medium. When growing in traditional soil systems the medium can provide much of the nutrition. Additional fertilizer can added to it, either as organic or synthetic.
Since there is typically a nutritional value in naturally occurring and potting soil giving proper dosages of fertilizers can be challenging. The advantage of a soilless medium is that when it comes to nutrients it is a clean slate, and so the grower can add whatever nutrients at whatever concentration they want and not have to worry about overdosing. There are a number of factors to consider when deciding which type of media works the best in a particular situation including cost, drainage, and potential aeration of the roots.
This is a byproduct of the coconut industry and is the husk of the seed which has been ground up and is essentially pure plant fiber. It is capable of holding lots of moisture, and while aeration can be okay one should be careful not to over water as the aeration levels will quickly decrease.
Depending on location it can be relatively cheap and is probably the most environmentally sustainable type of growing medium.
The first generation of coco had a lot of contamination from sea water bringing up the salinity which restricted how fertilizers could be used. It appears that this has been remedied and shouldn't be a concern in the future.
These are spherical in shape around 2 cm (1 inch) in diameter and are used in water culture. They offer excellent aeration due to the large particle size and much of the time small amounts are used, only to keep the plants anchored. Because they have essentially zero water retention nutrient solution has to continually flow over the roots using this media.
While they can be more expensive on a volume basis, the fact that less is needed makes their pricing competitive.
It is also recommended that clay pellets be used on the bottom of pots containing coco or peat based media. This will give a space for drainage to occur as well as give more surface area for roots to take oxygen from.
A mineral that has become expanded due to being heated. It is extremely light and porous and can be a bit inconvenient to use because it floats on water. Seldom used alone it will add aeration to heavier blends.
Similar to perlite but heavier. It is better at holding water than perlite, but will hold less air.
Peat Moss (Sphagnum)
Very commonly used in the indoor gardening industry in Canada but is less common elsewhere. This is simply due to economics as the resource is common in Canada and due the bulky nature shipping it can be expensive.
Peat moss comes from bogs where these primitive plants thrive. When they die they don't readily decompose and can be harvested. It is quite similar to coconut husk being mainly fibrous material, but with a bit more contaminants like small roots from trees.
Once peat moss is dry it's very hard to rehydrate and does have some aeration issues and pH issues. For this reason almost all of the time it is used in blends, usually with perlite and vermiculite along with a pH buffer.
Another mineral that has been modified into a growing medium. Instead of particles being expanded by heat like perlite or vermiculite it changes into long fibers and is related to fiberglass insulation used in homes.
The advantage it has over clay pellets is that rock wool comes in preformed cubes and so it is not necessary even to have a pot to contain it in, simply put the plant into the piece of rock wool and connect it to an irrigation source.
Rock wool is a key component in a new hydroponic system known as drain to waste. In traditional water culture systems, the nutrient solution is constantly being recycled. It goes from the reservoir to the plant and then drains back into the reservoir. A drain to waste system does not recycle, but rather the solution is fed in short bursts and is discarded afterwards. The big advantage to using this system is that a recirculating nutrient solution will want to warm up over time, but because drain to waste solutions are not kept in circulation long enough to heat up, lower root temperatures are easier to achieve. In a recirculating system, if a root born pathogen infects one plant it is transported everywhere, but in drain to waste any zoospores will be likely be flushed out of the system minimizing plant to plant infections.
When growing high end crops it is highly recommended to use some combination of the previously mentioned media. There are others which many modern growers would find unconventional including saw dust, straw, and sand. These are obviously extremely cheap and maybe in some situations they might be considered. Straw and sawdust would hold water and possibly harbour pathogens. Sand has very poor water retention and it will have to be closely monitored so that plants growing in it don't dry out.
Recycling Growing Medium
It is everyone's responsibility to keep as much out of the landfill as possible, that being said using the same growing medium for multiple crops carries a big risk. Fertilizers build up over time and increase the salinity, and any root borne pathogens will be passed from crop to crop. The best option for many of these types of media is to use them one time in an indoor gardening situation and then spread the material on lawns or gardens as soil builders. If you don't have a lawn or a garden yourself there are certainly plenty of people out there who would happily take it off of your hands for free. Just be sure that if you live in an area which has a prohibition to take out the root stock and dispose of it separately.