LSU AgCenter Horticulturist
It’s not a bad idea to be conservative. Whether we are talking about environmental conservation or being fiscally conservative, as gardeners, there are decisions we can make to conserve money and resources.
One such way is reusing potting soil. Many home gardeners today grow annuals, herbs and vegetables in containers and are very productive, but these plants complete their life cycle in one year’s time. If plants were healthy and just died at the end of their life cycle, the pots and media can be reused and replanted with next season’s plants.
If it’s time to clean and store the pots, you will need to do something with the spent soil. Many people simply throw the soil in the garbage, but it can be used in many other ways. First, the soil can simply be used to fill holes in the yard or top off landscape beds and other potted plants. The soil also can be spread thinly over the lawn or added to compost.
Some warn against the reuse of soils and for good reason. If plants died from bacterial, fungal or viral disease, then you definitely do not want to reuse the soil, as soilborne pathogens can still be present and active. However, if the plants that grew in the soil before were healthy, it is generally OK to reuse the potting soil.
One way to be sure that soil is clean is to sterilize the soil. There are a couple of ways to sterilize the soil. One is to solarize the soil in the sun, and the other is to heat up in an oven or microwave.
Start by removing the dead plants and dumping the used potting soil into a bin to sort out any large roots and grab any grubs or insects. If you wish to sterilize the soil with the sun, dump the soil into strong, black contractor plastic bags or lidded plastic containers. Seal the bag or bucket and leave them in the sun for four to six weeks.
The solar heat will kill any pathogens in the soil. The heat will also increase the rate of decomposition of the soil. Therefore, it is a good idea to add compost or another type soil amendment that can replenish the organic content.
The second method to sterilize the soil is to heat in the oven. This can make the house smell very earthy, so keep this in mind. Be sure to remove any insects, place the soil in an oven-safe pan, and cover with foil. Bake at 175 to 200 degrees Fahrenheit for 30 minutes. You also can microwave 2 pounds of soil in microwavable containers at full power for 90 seconds.
Now that your soil is sterile, you can use it again. You will want to replenish any nutrients that were depleted during the sterilization process. You can do this by adding compost and other soil amendments.
Here is a brief list of common soil amendments that can be found at local nurseries or garden centers: compost, peat moss, mycorrhizae, topsoil, composted manure, worm castings, wood ashes, mushroom compost, biochar and many others.
In addition to adding organic matter back, you can bulk up the potting media by adding new potting soil in a 1-to-3 ratio (one part new to three parts old). Additionally, you can renew old media with sphagnum peat moss, which has a coarse texture, contributes to good aeration and has excellent water-holding capacity — but add sparingly.
You also can add sand to improve drainage and aeration, but do so sparingly because sand can make containers very heavy. Perlite is another soil amendment that provides good drainage and is lighter in weight than sand. Lastly, vermiculite can be used to provide good soil aeration, nutrients and water-holding capacity.
Reusing potting soil gives a new meaning to the term dirt cheap. There are many options for reuse of potting soil. Save a few dollars so you can buy more plants!
When cleaning up container plants, consider reusing soil for reuse or filling holes. Photo by Heather Kirk-Ballard/LSU AgCenter
Used potting soil can be used to fill holes in the yard. Photo by Heather Kirk-Ballard/LSU AgCenter
Remove dead plants and large roots from used media and refresh with new media or soil amendments for reuse. Photo by Heather Kirk-Ballard/LSU AgCenter