How to Reuse Potting Soil Safely
How to Recycle Potting Soil
There's no need to buy potting soil every time you make a container garden. You can reuse the old stuff!
Will you try soil cycling?
That unwieldy cart of potting soil seems like enough when you're at the garden center. (In fact, it often seems like more than enough when you're at the register). But once you start filling pots, it's often frustratingly inadequate.
“Did I really use an entire bag for one container?” you think. Then it’s back to the store and the checkout line — and back to shelling out more money on gardening supplies.
This year, it doesn't have to be that way. You can lower the cost of filling seasonal containers by reusing last year’s potting soil.
Ways to Reuse Potting Soil
The easiest way to reuse old potting mix? Simply remove old plants from their containers, fluff up the soil and replant.
If you've reused the same soil for several years or it's developed a white surface crust, you may have to cut it with 50 percent new potting soil and/or apply fertilizer. (See "How to Reduce Risks" below.)
Of course, you don’t have to reuse potting soil solely in your flowerpots.
You can also
- work old potting soil into flower beds,
- work it into vegetable gardens,
- use old potting soil to fill up holes in your yard,
- add it to compost piles &
- bury used potting soil in compost holes.
Before you start recycling old soil, however, be sure that you know the risks so that you can guard against them.
The Risks of Soil Recycling
There are two main reasons reusing potting soil can put plants at risk.
- First, used soil sometimes contains pathogens--viruses, fungi, bacteria, nematodes and other organisms that carry disease. These pathogens can cause container plants to sicken and die.
- Used soil may also be deficient in the minerals that plants require. This, too, can cause plants to become diseased and die.
Reducing the Risks
How can you reuse potting soil without killing your plants? Try these simple strategies.
Super-Safe Recycled Soil
Want to make extra sure that your used potting soil is safe? Don't just bake it in the sun--bake it in the oven or microwave!
Laura Pottorff, Colorado State University Cooperative Extension Horticulturist and Plant Pathologist, provides specific directions for pasteurizing moistened batches of used soil at 180 to 200 degrees F for at least a half hour at a time.
1. Never reuse soil from a pot in which a diseased plant has grown.
The plant may be dead and gone, but the pathogens and other problems in the soil remain, making it likely that the next occupant will also sicken and die.
2. Pasteurize old potting soil before using it.
Soil that remains in pots exposed to the elements often harbors weed seeds, pathogens and/or insects, none of which are desirable in growing medium.
To kill off these harmful elements, bake the soil in the sun. First, empty the used soil into black plastic bags. Then place the bags in a sunny location.
The same pasteurization process that occurs during regular composting will occur inside the bags as temperatures within the mix rise, rendering the pathogens, weed seeds and other unwelcome elements in the soil harmless.
Although you can also put old soil in garbage cans or seal-able 5-gallon buckets, garbage bags are particularly easy to drag around, and the black plastic ones hold heat well.
3. Fertilize your containers after planting.
Exposure to the elements weathers the soil, leaching out nutrients. To counter this, apply fertilizer to assure that your container plants get the nutrients they need.
This is particularly important if you're using recycled potting soil, as many of its essential minerals may have been taken up by plants the previous year or leached out.
Use a slow-release fertilizer that lasts all growing season. Or, apply a liquid fertilizer (such as pee tea) every two weeks.)
4. Mix some compost and/or new potting mix into the old, especially if you've used the same soil for several years.
Exposure to the elements also makes soil more compact. Adding new potting mix to the old will not only increase its fertility, but it will also make it more friable and improve its ability to retain moisture.
If you’ve used the same soil for several years, it’s probably wise to create a 50-50 mix of half old and half new potting soil.
5. Water with rain water to reduce salt buildup.
Does the soil in your planters have white crusting on the surface? Then it may be suffering from salt buildup, which can slow plant development.
To prevent this problem in the future, water your container plants with rain water. Rain water usually has a lower salt content than tap or well water. As for reusing salty soil in containers as is? Don't. Add it to your compost pile. Or, if you really want to use it in containers, cut it with 50 percent new potting mix.
Use less soil by adding old nursery pots into the mix.
About the Author
The Dirt Farmer has been an active gardener for over 30 years.
She first began gardening as a child alongside her grandfather on her parents' farm.
Today, The Dirt Farmer gardens at home, volunteers at community gardens and continues to learn about gardening through the MD Master Gardener program.