I was messing around on the interwebs, as I am often want to do, and I found out that there are other ways to recycle soil other than simply throwing it onto the compost pile, which is what I normally do.
Now, yes. You can simply break up the mass and throw it on your compost pile. This will reinvigorate the soil in time. It will also neutralize any bacteria/diseases that have built up in the used soil if the compost gets hot enough, but it will take time.
Another way to do it is to mix it with some fresh compost and add in some form of fertiliser. But this won’t get rid of any bacteria/problems. To get rid of diseases you can always sterilize it by baking it. Yep… Bake it!
Sterilizing the soil is really rather easy. But there are a couple of things you need to do. First off remove the old plant, roots and stem. You can riddle it if you want, but there is no real need.
Then transfer the soil to a bucket and add in twice the amount of fresh compost and mix it well. Then you will need to get a large disposable baking tray, like this one and a piece of foil big enough to cover it with.
Fill the tray to within an inch of the top so the tray is still easy to handle. Use multiple trays or work in small batches if you have a lot of potting soil to recycle.
Place the tray in the preheated oven at 85-90C (190-195F) for about 30 minutes to kill any harmful bacteria. You also need to be careful that the soil doesn’t reach 95C (200F) because this could cause plant toxins. If you are using a fan assisted oven you may need to reduce the temperature a little. If you are not sure you can use a meat thermometer to check.
Remove the trays from the oven and dump the potting soil from the baking tray into a wheelbarrow or somewhere safe to cool slowly. Repeat the previous steps until all of the soil is baked and cooled. Feel free to reuse any trays and foil as needed until the job is done.
Note… Be careful with the soil when you bring it out to cool. It WILL be hot for longer than you expect. Do not use it until it is completely cool.
Once the soil is cool you can mix in some granular fertilizer if you wish and it is good for potting.