I rarely take a chance on potting soils because so much depends on it, and on several occasions when I have taken a chance I’ve been disappointed. Worse, makers can call their potting soil organic without carrying a label stating that it approved by OMRI (the Organic Materials Review Institute). You can check the ingredients on a bag, but this can lead nowhere.
So I make my own. Why gamble? (The below is with a bow to my friend Eliot Coleman and his book “The New Organic Grower.”)
The mix I use is the same as is commonly used for the soil block system of seeding (see Soil block entry), but it works just as well in pots. I mix ours in a home-made 55-gallon plastic composting drum, but a big plastic tub works just as well. The following recipe makes about two bushels, so cut it accordingly if you need less. (By bucket, I mean a standard ten-quart bucket.)
3 buckets of brown peat
½ cup of lime (to offset the acidic peat
2 buckets coarse sand or perlite (for air spaces.) I prefer perlite; it’s lighter.
3 cups base fertilizer (see below)
1 bucket of garden soil
2 buckets of compost
Combine the peat and lime, then mix in the sand or perlite. Then add the base fertilizer, which is one cup each of blood meal, greensand, and rock phosphate (or colloidal phosphate).
This will carry your plants all the way through until it’s time to transplant outside, with no additional nutrients.
A word on sterilizing your potting mix or compost: Don’t. Organically fertile soil is comprised of billions of beneficial bacteria, fungii, and other microbes which among other things turn organic matter into a form plants can use. Sterilize the soil, and you’re killing what I call this beneficial “microherd.”