My gardening season begins on my garage floor. Here I mix potting soil that will nourish this season's seedlings and replace worn-out soil around the roots of houseplants.
There's really no special magic in good potting soil.
The three basic ingredients in my mix are garden soil, mineral aggregate and organic matter. Used alone, garden soil is too dense for containers.
The mineral aggregate loosens up the mix to let water flow readily into and through it. Vermiculite and perlite are two lightweight aggregates, the first made from heated mica and the second from heated volcanic rock. Sand and calcined clay ("kitty litter") are heavier aggregates, so are useful for top-heavy plants such as cacti.
Organic matter in potting soils acts like a sponge to absorb water, which plants can then draw on between waterings. Organic matter also buffers soils against drastic changes in acidity, and keeps nutrients from washing out through the bottom of seedling flats and flower pots. Peat moss, sphagnum moss and coir (a byproduct of coconut processing) are organic materials that you can buy. Compost and leafmold are two organic materials that you can brew up yourself and, in contrast to the previously mentioned materials, also offer nutrients to plants.
Some people pasteurize their potting soil to reduce the threat of pests. The key is to avoid too much heat, which can bring its own problems. Bake the potting soil in the oven along with an embedded potato, and when the potato is done, so is the potting soil.
Rather than pasteurizing the soil, I prefer to avoid pests with careful watering, abundant light, and other cultural conditions that make plants happy and pests sad.
SOIL IS GOOD, BUT NOT ESSENTIAL
Good garden soil is hard to obtain in reliable and large quantities, so most commercial potting mixes are made without any real soil at all. These mixes are made only from mineral aggregate and organic matter.
You can make such a soil-less mix yourself by sieving together equal volumes of peat moss and perlite. This mix has no nutrients, so stir in a starter feed of one-half cup of dolomitic limestone and some fertilizer whose nutrient ratio is about 5-10-5. Real soil does add nutrients and other good things to a potting mix, so I favor traditional potting mixes, which contain real garden soil.
Even if you buy potting soil, keep a few bags of some type of mineral aggregate and organic material on hand. No one potting mix can suit the needs of every plant. Add extra aggregate to any mix used for cacti or succulents, and extra organic matter to any mix for plants such as African violets and begonias that like consistently moist soils.
For the ritual opening of the garden season, I give my garage floor a clean sweep and then make a pile of 2 gallons each of garden soil, peat moss, perlite and compost. On top of this mound I sprinkle a cup of lime and a half cup each of soybean meal and powdered kelp. This is a mixed bag of ingredients, but I reason that plants, just like humans, benefit from a varied diet.
I slide my garden shovel underneath the pile and turn it over, working around the edge until the whole mass is thoroughly mixed, and moistening it slightly if it seems dry. Finally, I rub the mix through a half-inch sieve, and recite a few incantations to complete this brew that nourishes my seedlings and houseplants each season.