To the new homesteader or gardener, preparing potting soil before reusing it might seem to be simply removing the old plant and putting the soil into the next pot to be used.
For the experienced gardener, there are so many different reasons to squirm at that thought!
Used potting soil could carry diseases or parasites, especially if the previous plant died from something other than lack of light, water, root space, temperature, or flowering & seeding.
Killing any parasites, fungal spores, and bacterial or viral infections can be done in a few different ways; while this might not be a major concern when planting a surplus of common veggie plants (after all, we prefer natural selection, where only the hardiest seedlings in the bunch survive), it could be a vital step with rare or hard to obtain root cuts, seeds, or rooted stem cuttings.
Some are so delicate that they require sterile soil and a close eye!
Boiling Soil to Kill Adult Insects, Larvae, Eggs, and Bacterial Infections
Insects of various life stages, from egg to larvae to adult, find refuge within the soil of potted plants. While some might be absolutely harmless, others could damage the root system of the plant, eat the leaves, or spread diseases. By generously pouring boiling water over the soil, you can essentially cook the pests and any bacterial infections right out of the soil! This is a great choice for processing large batches of soil, and a large tote with drainage holes or a bathtub with a fine mesh drain stopper would work perfectly.
Sterilizing Potting Soil in the Oven
I will warn you: this method could stink you out of house and home! All you have to do is heat the soil in a 200 degree (F) oven until it reaches 180 degrees (F). Once it does, continue to bake for 30 minutes! That’s it! This will kill everything that could be residing within the soil. This is the best method that you can use for preparing seed trays in the spring, or for starting seeds in larger pots. Your pan should be around 2.5″ deep with soil, and you could process a few pans at the same time; decreasing the overall amount of soil that you will have to sterilize.
Using the Sun (Solar Heat) to Sterilize, Cleanse, and Renew Soil
Once you have all of the soil that you intend to heat in one pile, lay down plastic sheeting over the ground. Lay the soil evenly over the plastic 4 to 6 inches deep. Then, soak the soil well with water. Do not over saturate though; the water serves to keep the soil moist and to essentially help steam the pile. This method will take the longest, between 2 and 8 weeks depending on the season and its temperatures. During the sweltering summer heat, it may be ready within 2 to 4 weeks. This is not a winter friendly method, but it can be done over the course of the fall, winter, and early spring months.
Preparing Home Sterilized Soil for Re-Planting
After the soil has been sterilized, you might have to break it up into finer pieces, as it was before sterilization. Certain methods, like oven sterilization, may leave behind hard clumps of soil. Try using a hammer to make this process easier. Soil needs to be light, fine, and absorbent for seedlings to thrive.
Once soil is nice and loose, check through it for signs of any pests. Carefully inspect it with gloves, searching for the adult pests that might live within the soil. If adults are found, they should be dead due to the sterilization process. If they are found alive, the process failed and will need to be performed again. The pests you might find include:
- Slugs
- Centipedes
- Snails
- Pill Bugs
- Ants
- Fungus Gnats
- Spider Mites
- Aphids
- White Flies
- Scale
If no adult insects are found, that’s really good news! However, you must still look for any existing larvae or visible eggs. For many of these pests and parasitic insects, the eggs or larvae are the life stages that inhabit the soil. For the most part, the adults only use the soil for hiding or breeding. If you have the time to perform a sampling test, take about 4 cups of soil after mixing the batch up very thoroughly. Moisten the soil without saturating, and place it in a warm place for 7 to 21 days. Watch for any signs of life within the soil every other day. If no life is observed, chances are that your chosen sterilization method has worked for you.
The pots that you use with the soil absolutely must be sterilized and cleaned, too. Start by washing pots inside and out; you can use boiling methods, solar heating methods, or bleach methods to sterilize the pots, depending on what they are made of and which method you’re most comfortable with using. If you skip this process, you may only end up reintroducing all of the potential dangers to your potting soil including bacterial or viral infections, eggs or larvae of pests, and fungal spores. They will simply pick up where they left off, with an improved chance of taking over due to the free and clear soil.