We gardeners are a bit obsessed with acquiring new plants. From the hard-to-find succulents to exotic flowers, the quest never ends. You can fund this obsession by reusing your potting mix and spending the extra cash on a new plant instead of more soil — just keep these few things in mind.
Photo from HomeFixated
New potting mix contains nutrients your plants feed on. Over time those nutrients can be drained from the soil by healthy plants, leaving you with less than an ideal mix of soil to be used again.
By adding a few amendments to used soil, you can have it ready to use again. Adding Perlite will help lighten the soil and provide drainage, and you can bump up the nitrogen levels in the soil by adding fertiliser with compost. (Be sure to remove visible weeds, bugs, leaves, rocks and other undesirables, too.)
Avoid reusing soil that has been used to plant tomatoes as they are high-energy plants that sap most nutrients from the soil, and if you are planting seedlings it's best to go with with new soil that has balanced pH levels and leave the used soil for more mature plants.
Check out the following articles for more details on how to reuse your soil.
Recycle Potting Soil - A Money Saving Option [HomeFixated]
Reuse Potting Soil from Containers [Gardens Alive]