On commercial soil mixes:

Read the whole discussion at: Sproutoff Soil Discussion

As posted by Ultra Denny on May 27th in response to a question by D4gw00d:

The soil mix placed in growing containers must be superior to native soil in certain respects. Because there is an interruption to fluid flow of the soil mass (in other words, the pot has a bottom), there is not the natural draw of fluids through the system. (This is a process we call capilary action*.) So, we must employ ingredients and container designs that allow for much greater oxygenation of the soil. This encourages root growth.

In order to grow plants quickly enough to make a living at it, growers must maintain optimum nutrient levels and near perfect moisture levels, conditions rarely found in native soils.

Some growers use ingredients that are too far removed from the texture of native soils. Therefore, there is a poor interface. Usually, the problem is an overabundance of large particles that knit poorly with the typical small particle native soils in which they are installed. Therefore, a small amount of native soil or sand particles will improve the interface. Some growers use this: some don't.

Keep in mind that the bigger a plant is when you go to install it, the bigger its needs. Sometimes we tend to underestimate a plant's needs during the adaptation process.

This is a humongous question to answer. That's my best in short order. Feel free to ask for points of understanding.

*Capillary action demonstrated - Take a flat sponge and soak it then, holding it flat, let the immediately available water drip out (only take a couple seconds). Next, take that sponge and flip it 90 degrees to where it's standing on its edge. Watch a lot more water poor out. This is known as capillary action.

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