Well for starters:

Here is a link that might be useful: the real dirt

If you *really*, *REALLY* want to get into it, it has to do with the porosity of growing medium which in turn relates to how much water it can hold.

It also relates to the size of the container that is being used, because this will limit how far the roots can reach to gather available water. The container size also limits how much potential water that container can hold (and by extension how often you are going to have to check on whether it has enough water).

Finally, it's going to relate to the specific gravity (related to the density) of the growing medium, which is going to relate to how much space the plants have to grow. But then also to the particle size of the soil in question, and whether those particles tend to stick together (like clay does). That will also affect how much space the roots have to grow, since the roots have to grow in the little air spaces between particles (or force the particles aside).

Your garden soil--say you have nice silt garden soil. The porosity of that soil is going to be around 46%. Got clay or sandy soil? About 42%. If your lucky enough to be blessed with loess, it's 49%.

The porosity of peat (peat is the primary ingredient in most available good potting mixes) is 92%. LOTS more holding potential for retaining water. Lots more potential for the roots to be able to grow and expand without resistance.

Let's talk about container size. The plants in the flower and vegetable beds (unless there's some serious compaction) are growing in a pretty BIIIIIIG container. As such, their roots have the potential to develop to their full reach and depths.

The cucumber has a long, thin taproot, down to four feet, and a spread of about six feet in diameter. pi*d^2/4 = 28 square feet and a 4 foot depth at the taproot.

The carrot's fibrous roots shoot down about eight feet with a five foot diameter, so 20 square feet and eight feet deep.

The pepper (tomatoes and eggplants are comparable) has a short taproot and dense fibrous root system, so it roots extend three to four feet deep and seven feet in diameter. So 39 square feet and four feet deep.

Plants in the garden have LARGE containers.

Then there's the density of the mediums in question. Soils generally range in specific gravity from 2.6 to 2.9 (meaning 2.6 to 2.9 times as dense as water). Water is already kinda heavy, as you find when you try to carry even partially-filled six-gallon buckets around your gardens. They sent me to the chiropractor.

I couldn't find a reference for the specific gravity of dry peat moss, but when I ran the numbers to convert a weight of 60 to 95 grams per liter dry into a specific weight I came up with a number like 0.1. One tenth the density of water is a lot lighter than 2.7 times the density of water. I wouldn't have ended up at the chiropractor if I were carrying six-gallon buckets of dry peat moss.

Of course, when that peat moss is thoroughly wetted, its specific gravity is going to be closer to 1 (nearer to matching the density of water, because with a porosity of 92% it's going to be mostly water). But a density of 1 is still much lighter than a density of 2.7.

Imagine if you were a plant root confined in a container. Would you rather be trying to grow in a container of peat moss (and some lightweight vermiculite and perlite), or a container of heavy (but perfectly nice) garden soil. Imagine trying to push your roots into the stuff. Imagine that in order to make room in the limited space of the garden soil having to push 2.7 times the weight of water up against the sides of the container and lift it just to make room for your roots to grow.

..................

And THAT is why we use a quality potting mix in our containers, and not garden soil.

To simplify: too much water retention, too little air in the soil, too much compaction.

For more on container soils and how water behaves in soils, try the link below.

Al

Here is a link that might be useful: Click me to see what he's talking about