Plants in pots have needs that contradict each other. Specifically, they want 1) a steady, copious supply of water, and 2) soil that's light and airy. With garden dirt, you can't have both things at once (unless your garden grows on almost pure sand, which is more a difficulty than a boon since it obliges you to water every day). When you saturate most kinds of dirt, you drive out the air. Without air, roots suffocate. Then, deprived of roots, the top of the plant can't get enough water (making for a strange condition in which roots drown and leaves die of thirst). As the leaves turn yellow and then wither, stems die. In the worst case, roots and top both die (as they often do in the landscape when weeks of rain keep the ground sopping wet, or put it underwater).
In a pot, garden dirt would stay continually saturated in the bottom few inches, for the same reason that a wet sponge held on end is soaking wet at the bottom and only damp at the top. Your plant would root only above the saturated zone and its growth would be restricted. On the other hand, with potting mix, even the wettest portion at the bottom of the pot has enough air for roots to grow vigorously (as you'll discover at the end of the growing season when you unpot geraniums onto the compost pile and see how thickly the roots have grown at the bottom of the root ball).
Use homemade potting soil to propagate your own houseplants -- click here to learn how!
Potting mix resolves the contradictions by combining two ingredients with opposite qualities, one that holds water and one that keeps the mix open and light.
In the best potting mixes, the ingredient that holds water is milled sphagnum peat moss. It is harvested from bogs that have been drained, so the moss has dried and turned a light brown color. (You can often tell whether a commercial mix contains peat moss by its color -- if the mix is black, it probably doesn't.) After harvest, the peat goes through a mill that breaks the long strands of sphagnum into short pieces, which pack tightly together and so hold more water than unmilled moss, volume for volume. Under a microscope, the structure of the fibers shows lots of empty cells. Like the pores in a sponge, these drink and hold water. In fact, a pot of milled sphagnum peat moss will hold so much water and pack so tightly that roots risk running short of air.
Potting mix includes perlite to separate the fibers of peat moss and make spaces for air. Perlite is made by heating bits of a glasslike mineral until they expand into puffy, lightweight particles. It holds no water, aside from the little that clings to the surface of each particle.
Some recipes for potting mix substitute vermiculite for perlite, or combine the two. Like perlite, vermiculite separates the fibers of peat moss so the mix is more porous, but vermiculite also holds a lot of water (it's made from particles of a mineral that has many thin layers, which expand like an accordion when heated and capture water between them). I prefer perlite alone, but for plants that like a lot of water, or for pots that might dry out too fast because they will sit in hot places (such as on a brick patio, against a south-facing wall) I sometimes reduce the perlite by half and make up the difference with vermiculite.
There are other ways to vary the ingredients, but here's the most common: If you pot a tall plant, such as an ornamental grass, that a strong wind might blow over, pot and all, you can add weight to the potting mix by swapping sand for some of the perlite. I reduce the perlite by one-fourth, then add the same volume of sand or a bit less. Sand makes good ballast, and it also creates pores in the potting mix.
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