Potting mix costs money and it uses limited natural resources such as peat moss. Here are some ideas for reusing your old potting mix.
First, if your old potting mix had known disease/contaminants, just get rid of it.
Remove debris such as leaves and sticks. Let it dry out.
Sterilize/pasteurize it to kill unwanted insects and fungus. The best way depends on amounts of old mix.
- For a large amount: solarize in black plastic bags (double bag) in the sun for 4 – 6 weeks. You would need to do this when the sun is hot and it needs to get at least 6 hours of sun a day. Cool spring weather would not be hot enough.
- For a small amount: put potting soil in a pan (such as an aluminum roasting pan). Place in an oven at between 180 and 200 degrees for 30 minutes. Use a meat probe to monitor the temperature. It may smell earthy which you might or might not like.
- Tiny amount: put it in the microwave at 30 second intervals. Monitor the temperature with a meat thermometer. Make sure there is no metal in the potting soil.
Replenish the potting mix
- Mix in even amounts of new potting mix.
- Add high quality compost not more than 25% or worm castings 10% - 20% to supply nutrients and beneficial microbes. Too much compost and the potting soil won’t drain properly – soggy roots.
- Perlite and sand aid in drainage which would be helpful if compost was added.
- Vermiculite both aerates and retains moisture.
- Add a slow release fertilizer or if you added compost then nutrients and microbes have been added already
- Check the pH, remember 6.5 – 7.5 is optimal. If lower than 6.5 add lime. Add sulfur or sphagnum moss if it’s higher than 7.5
Be careful when starting seeds. The slightest contaminations will inhibit germination.
If you don’t want to go through all this work you can still add old potting soil to garden beds. It is not recommended to add to compost piles because you want to maintain a good C:N ratio.
Some other things to add instead of peat moss are coconut coir, pine sawdust, red pine, or shredded wood.