Yes, you can make your own potting soil. In fact, depending on the quantity required, it's typically far less expensive to make your own than it is to buy. And you can customize your own mix to your own specifications. You might want to read through few of the posts regarding "mixing your own" on the Container Gardening forum. There are lots of great recipes out there and anything suggested or endorsed by Al ('tapla') comes highly recommended. He has a great deal of experience and a great deal of science behind his mixes.

The type of fertilizer you use has no bearing on if your mix is considered a "potting mix" - a potting mix is only a mix designed to address growing plants in containers as opposed to in the ground and the very different conditions these circumstances require. Containers, regardless of size, DO need fertilizing. The extra watering they require and the porosity of the mix allows nutrients to leach out rapidly and these need to be replaced. Whether that fertilization comes from incorporating modest amounts of compost or composted manures into your mix, applications of other organic fertilizer sources or synthetic, usually water soluble fertilizer, is up to you.

Many container gardeners incorporate some sort of slow release fertilizer during the initial planting and then supplement throughout the season with a dilute liquid application periodically. "Weekly, weakly" is often the way it's approached :-)

I almost always make my own potting mix. In the large pots I generally include a bottom layer of leaves mixed with horse manure (slightly aged) so that the roots have something nutritious to grow down into. Higher up is finished compost mixed with some regular soil, which most folks will tell you is forbidden. Works well for me, though. I am growing a lot of flowers in pots this year (cannas, dahlias, cosmos, petunias, gladiolas, marigolds, and spider plants mostly) and in past years have grown tomatoes and peppers in that sort of mix.

Marcia

Most often when we need something to fill a pot or a box we use the compost I make, just that and nothing else. Commercial soiless potting mixes will most likely be just some peat moss, a non renewable resource, and either perlite or vermiculite to improve drainage.