Cleanse Last Season's Potting Soil By Leaving It In The Sun. Here's Why

If you're one to write off last season's potting soil when the spring comes around, pause for a second. Yes, it may look a little clumpy and a bit tired, and there is always the risk some unwanted invader has moved into it, but there is a simple fix that could revive it. Hot sunlight can create a low effort heat treatment that can knock out any pests and their eggs lingering in the soil. The process is called solarization and all you have to do is bag your soil up in a heavy duty garbage bag and set it out in the sunlight. It is a bit slow, taking four to six weeks, but that's part of the appeal as well as you don't need to monitor it or stir it, just simply let the warmth do its thing. If you're looking for a simple way to sterilize potting soil, then this method is definitely doable.

This process is best suited to used potting soil from previous growing seasons, which is more likely to contain pests or depleted nutrients. But obviously, heat alone won't solve every problem, if the soil is completely depleted or full of mold, it will not bounce back the way you want. But as a simple first step, this can give your soil a second chance at life and since good quality potting soil is essential to keeping both your container beds and houseplants healthy, it's always worth trying to revive what you already have.

It should be noted that once the soil has been through its time in the sun, it will be clean but also a bit lifeless and it will never have the same nutrient profile it started with. In the same way that the heat clears away pests, it also strips away any helpful microbes, so it will need a little boost before you can start using it again. Mixing the treated soil with fresh potting soil and organic matter such as compost will help restore nutrients and improve its overall quality. And mixing in some perlite can also improve the texture somewhat, as older potting soil might have a more dense and compacted nature. This is a problem that needs solving because plants need fresh airflow around their roots. 

If after trying these tricks, it still isn't salvageable, there are other ways to use your potting soil before you throw it out like filling in low spots in the yard. Not every batch is worth saving for your plants but that doesn't mean it has no use at all.