How to Reuse Old Potting Soil

Most gardening and horticulture experts don’t usually recommend reusing old potting soil, because old potting soil has less nutritional content and can harbor soil-borne diseases. However, it is possible to reuse old potting soil by "cleaning" it, although most techniques are cost-prohibitive or environmentally unfriendly. One new technique to clean and reuse old potting soil involves a solarization method that heats the potting soil to 140 degrees F to kill weed seeds and some disease spores.

Old potting soil

Black plastic sheet

Wooden pallet

2 clear plastic sheets

Double-sided, heavy-duty tape

Wire hoops

Thermometer

Build the solarization tent. Lay a sheet of black plastic on a flat, solid foundation. Lay a wooden pallet on top.

Place the old potting soil in piles, pots or bags on top of the wooden pallet. The soil should not be more than 1 foot deep. Water the soil to moisten it thoroughly.

Lay a sheet of clear plastic on top of the soil. Seal all the edges of the clear plastic to the black plastic using double-sided, heavy-duty tape to make it air-tight.

Tent a second layer of clear plastic sheeting on top of wire hoops. Seal the edges of the second clear plastic sheet to the bottom plastic sheeting using the double-sided tape.

Pry open a corner of the plastic sheeting and insert a thermometer into the soil to check the temperature every one or two days. After the soil achieves and maintains a temperature of 140 degrees F, you can remove the soil and use it.

If you have sunny, warm weather with air temperatures around 70 degrees F, your soil will be "clean" after only one or two days. In cooler and overcast weather, the cleaning process will take longer.

Add some peat moss or organic compost to your old potting soil to replenish lost nutrients.

Don’t remove and reuse the potting soil until it reaches 140 degrees Fahrenheit and stays at that temperature for at least a half day.