I'm cheap. There, I've said it. I hate spending money on something so basic as soil, so I heat it up and kill the pathogens so I can reuse it.
So, why pasteurize soil, and what exactly is it?
Pasteurizing is heating it to a medium temperature, not hot enough to completely sterilize it, but enough to kill off harmful pathogens like some kinds of fungus, and bacteria. Generally, it isn't hot enough to burn the soil, or kill the beneficial types of micro organisms.
There are two ways to pasteurize soil; both entail heat, or more precisely, steam. My favorite way, being a pyromaniac, is over a carefully controlled bonfire, usually with scrap wood from some demolition project or other.
I use a large metal boiler, with an air cooler from a truck as the lid. The lid is important, as it holds the steam in, which is how this whole thing works.
The fire is built underneath the rack, and I usually aim for at least a foot (30cm) under the pot or boiler to give room for stoking more wood onto the fire. The whole process takes about an hour or so, generally not any longer than that.
Lift the lid to check once in a while. The goal is to get steam puffing out all over the surface of the soil. It's important that the soil is damp, which prevents it from actually charring or smoldering. This isn't a bad thing, necessarily, but it's a waste of soil.
The boiler has to be removed or you can put the fire out if it's pretty much done. The soil will take about 24 hours to completely cool off, so don't plan on planting it right away.
The soil here is used (previously grown in) Sunshine Mix #4, and it's been through two winters at least. The water holding polymer that it's sold with has broken down by this time, so it most likely won't be as good at holding moisture. I mix it with a couple of shovels of aged compost, which cooks at the same time.
The other way of pasteurizing soil is to pour boiling water on it, as it sits in a flat or pot with drain holes. I don't use this method so much because it tends to make a lot more mess, and I'm all about keeping it neat and tidy - not.