First Part Here:
http://mattrize.blogspot.com/2010/10/what-does-organic-soil-less-mean-and.html
Part 2: Why use soil-less?
The short answer is simple preference. The long answer is much more controversial. Here we go.
IMO soil-less organic media with soil-less inorganic natural media are ideal for advanced indoor gardens.
Here are my reasons:
a. Water Retention: Soil-less literally/practically means lower water retention in comparison to true soil. By having lower water retention we can feed more often, which is important when relying primarily on liquid/powdered plant food. Faster wet/dry cycles may speed up the overall life-cycle, a common claim of hydro growers.
b. Perlite: This goes back to water retention. The soil-less community loves perlite, although personally I am attempting to give it up completely for reason's related to my grow ideals. My mom and grandma explained to me the importance of perlite for container plants. Some plants need it, some don't, ganja LOVES it. In containers perlite helps fight soil compaction... but watering gently and slowly does as well.
c. Coco/Peat/Bark: These are the popular soil-less organic choices, although I'm not sure as to the proper label for the amendments. ie rice hulls, small sticks, EWC, and compost. These natural and organic products are great for growing Cannabis. pH seems to be the major issue in these products, as their buffering capacity is generally less than true soil.
Coco/Peat/Bark are extremely common for growing Cannabis. In fact, around here the majority of indoor gardens are soil-less organics. The common Cannabis designed potting mixes that most indoor folks use: Ocean Forest, Happy Frog, Promix, Sunshine, Roots Organics, Light Warrior, Bio Terra Plus, Humboldt Nutrients mix... these are all technically a combination of soil-less organics (peat or coco plus other stuff) and soil-less inorganic natural media (perlite). Of exception is the Bio Terra Plus which is actually just soil-less organic as it contains no perlite.
d. Control: By not having food in slow release form you are in control over what the plant gets. It is then your job to "read and feed" your plants. This really gives you the opportunity to learn what your plants uses (food) and when they use it (stage of life cycle). This also lets you push your plants to the max while keeping it organic.
e. Chlorosis: Advanced chlorosis at harvest may lead to smoother smoking flowers. Soil-less media has no slow release food, so Nitrogen must be provided or the plants instantly yellow. This is true until two weeks from harvest for all soil-less organics. Once the N is cut the green is literally sucked from the plant, by design for soil-less organics. This means the freshly dried herb will have a jump start on the curing process. The chlorophyll is consumed by the plant. This reveals the secondary pigments already present in the leaves and flowers but buried in a sea of cholorphyll. Secondary pigments (not green) include: purple, blue, lime green, red, orange, gold, and other light green colors.
Here goes the numbers for Bio Terra Plus. This is a super heavy (high water retention) soil-less media because it has no perlite.
Organic matter min 75% of gross product
Electrical Conductivity: 1.1-1.3mS/cm
pH (H2O): 5.5-6.5
Water Retention: 7.1 (gms per gm of organic matter)