I've been making my own potting soil for a number of years, for a number of reasons. As previously mentioned I found that commercial mixes dried out too quickly; they formed into a dense brick when dry pulling away from the side of the container and just weren't conducive to promoting plant vigor.
Before we begin to make our mixes I will tell you about the main ingredients that make up a good potting mix:
Soil, the backbone of the mixture. Garden Soil often contains weed seeds, insects and disease pathogens. You want to buy sterilized soil to use in your mixture.
Sphagnum peat/peat moss is the basic ingredient in most mixes. This substance is mined from bogs; peat moss is a more decayed version of sphagnum. Used to absorb and hold moisture.
Vermiculite A mica based mineral that has been heat treated. Contains potassium, calcium, and magnesium. Adds volume with little weight. Allows air penetration to the root zone.
Perlite Crushed volcanic rock, very light weight. Holds 400% its weight in water; helps provide support to plants. Excellent drainage.
Compost Provides the most powerful item in the recipe organic materials, adds nourishment and it holds and dispenses moisture to roots and contributes to overall growth.
Polymer Crystals (optional) I find this very helpful in reducing moisture loss in smaller containers exposed to full sun such as flower boxes These are crystals that can how up to 50 times their volume in water. These are marketed under several brand names.
Time-release Fertilizer (optional) Various brand names available, releases fertilizer over a set period of time. Use per instructions on the container.
There are different mixtures that are used for different purposes. Seed starting mix (also known as soilless mix) as the name implies is used to start seeds. It contains no soil, it very light and airy in order to give seeds optimum conditions to produce strong roots. It retains moisture well so important is successful seed germination.
Seed Starting Mix
4 gallons peat
1 ½ quarts perlite
1 ½ gallons aged compost
¼ cup dolomite limestone
Yield about 5 gallons
General Use Potting Mix
3 parts peat
1 part sterilized garden soil
1 part aged compost
1 part bone meal
2 parts perlite
Time release fertilizer per package instructions (optional)
Polymer Crystals (optional, add to planting container per package instructions)
It's a good idea to check your mix to ensure good performance; natural ingredients tend to vary from batch to batch.
There are three areas that you will want to test:
Drainage: Fill a pot loosely with you mix, pour a quart of water into the pot. If it completely drains in about a minute the drainage is good.
Porosity: Place 1 quart of your mix into a container. Add 2 cups of tepid water. Wait about 2 minutes; you should be able to pour off about a cup of water, leaving a cup behind absorbed by the mix. This determines the correct balance of mix, water and air.
Germination: Place a few fast sprouting seeds into a container of your mix; if they germinate you mix is "good to go."
A final note, it's great to reuse containers such as cell packs, flats etc. to start seeds. Be sure to disinfect them with a 10% bleach solution before each use. This prevents the spread of disease that might affect your seedlings.