QUESTION: Can I put old potting soil in my compost? I found several bags in my garden shed. Ted K
ANSWER: Old potting soil makes a great addition to your compost if you take just a few precautions. Soil that’s already been used can harbor pathogens that carry plant disease and could infect next season’s plants when you use the compost in your garden.
These pathogens may include viruses, fungi, bacteria, nematodes, and more. We know that introducing disease to your garden when you’re trying to make it more healthy is the last thing you want to happen, so just follow the steps below to make sure the pathogens are rendered risk-free in your compost.
First things first: You should never add potting soil where a diseased plant has been growing to the compost pile in an attempt to recycle it. (The same goes for diseased plant debris, by the way.) You know the soil where a diseased plant has been housed is bound to carry disease, and we want to minimize the risk of passing that disease along in the future. Instead, discard this soil in the trash so it can’t infect any of your plants.
By pasteurizing your old potting soil, you can deactivate any pathogens it may contain and, at the same time, kill the seeds of any invasive plants that may be sprinkled throughout it. (Yes, pasteurizing soil is something you can do at home on your own!) To start, just load any potting soil you wish to reuse in your compost into black plastic garbage bags. (Make sure when you’re gathering supplies that you choose black bags, which will soak up more heat than other bags and pass that heat along to the soil inside.) Then move the bags to the sunniest spot in your yard so they can bake in the sun.
Leave the bags in their sunny spot for at least a week. This replicates the pasteurization that occurs in your compost pile as it heats up in a much shorter time period and makes the potting soil safe to add to your compost so you won’t have to worry about contaminating the mixture.