It involves putting old potting soil in lidded, five-gallon buckets or black plastic bags that are tightly tied shut and leaving them in the sun for 4-6 weeks. The heat builds up inside the buckets or bags just enough to kill bugs and pathogens. You also can sterilize old potting soil in your oven.
As long as used potting soil still looks fairly fluffy and doesn't emit a rotten odor, gardeners should be able to use it again with good results. However, if the plants formerly grown in the potting soil struggled with disease or insect problems, it's probably best to discard the mix and start fresh next season.
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.
Heat. Heating the soil is very effective and the soil can be used immediately after cooling, unlike chemically treated soil. Many plant pathogens are killed by short exposures to high temperatures. Most plant pathogens can be killed by temperatures of 140°F (60°C) for 30 minutes.
However, if you know your plants ended up hit with downy mildew, or other plant disease, you'll want to either toss the soil or spend some time sterilizing it. Many fungi and viruses CAN overwinter in soil, making them especially hard to eradicate from year to year.
2. Foul Odor. A foul, rotten eggs odor indicates that your potting soil bag has gone bad. This bad smell results from anaerobic bacteria that grow in old, damp, and compacted soil.
Compost heap - Here is the easiest option of all: Simply toss the old soil or unused half-bags of soil right on the compost bin. A good compost pile should be a balanced mix of green material, brown material and soil.
Soil can harbor diseases, insect pests, and also weed seeds. If you purchase new potting soil, it's considered sterile because it has been treated with heat, steam, or chemicals to kill weed seeds and pathogens.
Boiling water or steam
Steam is a great way to efficiently sterilize your soil. It can be done with or without a pressure cooker. If using a pressure cooker be sure to follow all of the manufacturer's safety precautions. Fill the pressure cooker with a couple of cups of water and insert the rack into the cooker.
Solarization: Soil can be sterilized in a plastic sheet and put under the sun. This method may take about six to ten weeks to be effective.
Potting media mixed by home gardeners should be pasteurized to kill weed seeds, soil-borne insects, and pathogens. Pasteurization destroys most organisms and is done by applying heat until the soil reaches 180°F (82°C) for 30 minutes.
The good stuff doesn't come cheap, though, so at the end of the season you may be left wondering: Can you reuse potting soil? With a little prep work, you sure can—and it's not hard to do, either.
Reuse of contaminated soils is permissible only to intermediate levels of contamination and only if the operator can demonstrate that there is no resultant degradation of stormwater quality.
If you're not up for sterilizing and refreshing old potting soil, you can still put it to use instead of throwing it out. It can be dumped directly out of your containers and into established beds and borders. You can use it in raised beds or wherever your yard has holes or eroded areas.
Potting soil is sterile, which is good for indoor plants because it prohibits fungus. But outdoors, it won't retain any nutrients, in part because water leeches through it so quickly, taking what little nutrients there are with it. Grass will not thrive without rich nutrients in your garden soil.
Fight the urge to flush excess potting soil down the kitchen drain. I'm admittedly guilty of this one. The kitchen sink seems like the perfect place to transfer an indoor plant from pot to pot, but soil and other dirt types can very easily clog your drain.
Yes, you can use potting soil that is two years old. However, the nutrients in the soil may have depleted over time, especially if the soil has been used before. To ensure good plant health, you might need to replenish the soil's nutrients by mixing in fresh compost or a slow-release organic fertilizer.
That potting soil is worn out because the peat moss has decomposed. That peat moss can decompose even if you never take it out of the bag. If your potting soil has been sitting in your shed since last year in an opened bag and it's gotten wet, toss it. If it somehow stayed bone dry, it should be OK to use.
Give fungi an ideal environment - moisture, nutrients and a confined space - and you might soon be growing mushrooms alongside your indoor plants. You might first notice this type of fungi as clusters of fuzzy white balls in the substrate or a white, fuzzy 'mould' on the surface of the soil.
Cinnamon fungicide control
Damping off disease is a fungus-based problem that hits small seedlings just as they begin to grow. Cinnamon will help prevent this problem by killing the fungus. It also works with other fungal problems exhibited on older plants, such as slime mold and with deterring mushrooms in planters.
Replenish lost nutrients by adding slow-release fertilizers, vermiculite or compost. Because compost is dense, it can lead to compaction and should be used judiciously (a ratio of one part compost to three to four parts potting soil is most effective).
Saturate the soil with 1/2 cup hydrogen peroxide 3% per 1 litre water. Put wet soil in a watertight container and leave overnight before planting. This kills pathogens such as fungi and bacteria including insect eggs and nematodes (roundworms).