You have a few planters that still have last year's potting soil in them and you're wondering, can you reuse old potting soil? Does potting soil go bad after using it once or can I use the soil I already have for planting this season? Yes, you can reuse potting soil, but only if you make sure it is pest and disease free - or if you have sterilized it. This post will tell you everything you need to know.
How to sterilize old potting soil
You can sterilize potting soil for later use. However, most gardeners recommend using the sterilized soil as a supplemental mix rather than as the primary potting soil for a container. Here's how to sterilize potting soil:
A simple method is solarization
The easiest way to sterilize potting soil is to harness the power of the sun. This is called solarization. Solarization is a non-toxic method of killing pests and diseases in the soil using heat from the sun. This requires two things: clear plastic and sunlight.
You can use this method to sterilize larger amounts of potting soil. How to proceed: First remove all plant residue and break up the clumps. Then lay out a clear plastic sheet and spread the potting soil on top to a depth of about 30 cm.
Water the soiluntil moist but not soaking wet. Then cover the soil with a second sheet of clear plastic, making sure to push the top layer under the bottom before securing it with rocks, stakes, soil or other means. Leave the potting mix covered with plastic wrap in full sun for 4 to 6 weeks. In areas with more cloudy days, you can leave them outside for up to 10 weeks.
You can also place the moist but not wet soil in clear plastic bags, heavy-duty black plastic bags, or sterile buckets. Seal them tightly and place the bags or buckets in the sun.
Bake small amounts of potting soil
If you need a smaller amount of potting soil, you can sterilize it in the oven by bringing it to a temperature high enough to kill potential pathogens. Most soil-borne diseases cannot survive for 30 minutes at approximately 60 degrees Celsius. However, for some viruses this temperature is not high enough. Baking planting substrates at 90 degrees Celsius for 30 minutes definitely results in more consistent success.
Here's how to do it: To bake potting soil, spread it evenly in an ovenproof container (such as a baking pan), cover with aluminum foil, and bake for about 30 minutes, measuring the temperature with a meat thermometer.
Preparing the soil in the microwave
Since many people use microwaves instead of traditional ovens for cooking, it stands to reason that a microwave could sterilize potting soil. While this is technically true, this method is not recommended for several reasons: First, the potting soil may contain metal residues that can damage microwaves and even cause a house fire. Additionally, a microwave can only hold a small amount of potting soil at a time.
If you still like this method, you can place moist potting soil in microwave-safe containers with vented lids. This includes microwave-safe plastic bags, but make sure they have a means of ventilation. Check the bottom temperature immediately afterward with a meat thermometer and repeat at 30-second intervals until the bottom temperature reaches 180 degrees Celsius.
Like reusing old potting soil after sterilization
Once sterile, the used potting soil needs to be amended to restore appropriate structure and nutrient levels for the plants. Before reusing sterilized potting soil, you need to make sure you replenish the medium with plenty of nutrients. It's not difficult to make your own potting soil, but the best potting mixes have just the right mix of ingredients.
Moisture retention and drainage:Perlite creates a more airy soil and improves drainage. Sand also provides additional drainage. Vermiculite ensures better aeration of the plant soil and retains moisture, which is why it is the most commonly used ingredient in potting soil and planting mixes.
The addition of peat moss or sphagnum mossAdding potting soil also helps retain moisture. All of these substances are usually added in equal proportions to planting mixes, along with an organic substance for nutrients.
PH value: Sterilized potting soil should be tested after sterilization. The optimal pH value of the potting soil should be neutral, i.e. a pH value between 6.5 and 7.5 is optimal. Adding lime to acidic soil (below pH 6.5) can return it to neutral, while to alkaline soil (pH above 7.5) you should add acidic ingredients such as additional sphagnum moss or sulfur.
Fertilizers:Since container plants tend to leach out nutrients, they require slow-release components such ascomposted organic matterand regular fertilization during the growing season. The best potting mixes often also contain additional nitrogen, phosphorus and potassium (potash) in the mix.
Store used potting soil in clean, airtight containers.
Once the potting soil has been sterilized and amended, you should store it in an airtight container to prevent it from attracting new pests or pathogens. These containers should also be clean and sterile as diseases and pests are often transmitted through tools and containers.
