Standard commercial potting soil isn’t good for the planet. It usually includes peat, vermiculite, and perlite, none of which is sustainable. Harvesting peat is particularly destructive. When ancient peat bogs are drained, they release carbon into the atmosphere. You can buy a sustainable, organic mix, but not if you’re on a tight budget.
Soil-free seed-starting mixes became popular to protect vulnerable seedlings and clippings from diseases and fungi, but sterilizing a homemade potting soil mix is an effective preventative.
The mix should be lighter than garden soil to drain well and allow delicate new roots to spread. A good recipe to start with is in thirds: one part compost or good garden soil for support and nutrients; one part light-textured leaf compost, loam, sawdust, or bark compost to prevent compaction and encourage water retention; and one part coarse sand or grit for drainage.
You can use your oven to sterilize the mix in small batches. Put the soil in a roasting bag or covered dish and heat it for 30 minutes at 160 F, using a cooking thermometer to check that the temperature of the soil doesn’t exceed 180 F.
An even greener method is to sterilize the mix outside in a cardboard solar cooker or by solarization under glass or plastic.
Organic gardening sources offer more advice on using available ingredients to create your own soil mix recipes.