Potting soil is readily available at most garden centers, but it gets expensive if you are starting many seeds or a container garden. Creating your own potting soil mixes saves money and ensures that the soil is sterile and contains no weed seed. The soil in your garden is not suitable for potting soil mixes, as it is full of weed seeds and will quickly compact and lose its ability to absorb water. A good potting soil mix supplies some nutrients to the plant, retains water without becoming soggy and provides good aeration to the plant's roots.
Fill a roasting pan half full with slightly moist compost. Cover the pan with foil and stick a meat thermometer through the foil and into the compost.
Place in a preheated 250-degree Fahrenheit oven. Allow the soil to reach a temperature of 180 degrees and then bake for 30 minutes. Remove from the oven and allow it to cool to room temperature.
Mix one part of the sterilized compost with one part peat moss and one part vermiculite, perlite or coarse builder's sand. Mix together in a large bin or bucket, and break up any clumps in the mixture as you combine it.
Moisten the soil evenly before using in pots. Wet the soil down thoroughly with warm water and allow to sit for 30 minutes so it absorbs as much of the water as possible. Properly moist soil will hold together when squeezed but no excess water will squeeze out.
Add a slow-release, general-purpose fertilizer to the soil mix if using with established plants. Fertilize with a half-strength liquid fertilizer two weeks after seedlings have emerged if using the potting soil for seed starting.