Excuse me while I interrupt our normally scheduled weekday silence. It seems I left a rather important step out of our last post. Oops! I had completed the task a few weeks prior and it completely slipped my mind by the time I started the post last week.
Several yeas ago, when I attempted to start seeds for the first time, the majority of the soil I used became “fuzzy” and many of the seedlings quickly failed due to something called, “dampening off”. All soil contains soilborne pathogens, bacteria, fungus, diseases etc. Many of them are good; however, in the right conditions, the “badies” will harm your plant.
To help prevent any issues, you should sterilize your soil. Essentially, you are going to “bake” your soil. Much like microwaving or baking your eggshells (previous post) gets rid of bad pathogens, baking your soil does the same thing. It’s a process called, sterilization (also called pasteurization).
This process will give your plants the best environment possible by killing off any bad organisms and weed seeds that may still be in the soil. It’s really important to do if you are reusing potting soil or using dirt gathered from your garden or even if you are using purchased soil. You may hear commercially sold soil is sterile. I’m not sure how true that it – the soil I used that sprouted fuzz was a brand new bag, with a well know name on it.
All this sounds complicated, right? It’s really not and your soil can be safely used as soon as it cools. There are several methods you can use to sterilize your soil, We’ll start with the easiest and end with, what I feel is, the hardest method and one that I’m not sure I’m ready to try… just yet.
1. Use the SUN!
This method works best in warm weather. Select a very sunny spot in your yard that gets at least 8 hours of sunlight. Lay out a tarp or plastic sheeting and cover with about 4 inches of soil. Spray with water until very moist but not muddy. Cover with plastic sheeting or a dark colored tarp, securing the edges with rocks, lumber, garden fabric pins, etc. Each week, rake the soil around to make sure the heat is being evenly distributed. Allow the soil to bake in the sun for 4-6 weeks (longer if the weather is cool).
This is a small batch method. Fill a microwave safe, glass container with 4 inches (or less) of MOIST soil and place it in the microwave. Heat it at full power for about 2-5 minutes or until the interior temperature reaches about 180°F. Take the container out and wrap in towels to allow the soil to cool slowly.
This is a small batch method. Preheat oven to 200°F. Fill an ovenproof container with 4 inches (or less) of MOIST soil and cover with aluminum foil to keep the heat in. Bake until the internal temperature reaches about 180°F. This takes about 30 minutes. Take the container out and wrap in towels to allow the soil to cool slowly.
Sterilizing soil with steam can be done with or without a pressure cooker.
- Pressure Cooker – pour in enough water that it is just below the rack. Place a shallow pan of soil with 4 inches (or less) of MOIST soil on top of the rack. Cover the pan with foil. Close the lid of the cooker, leaving the steam valve open just enough to allow steam to escape until it’s ready to be closed. Heat at 10 pounds of pressure for 15-30 minutes. Allow soil to cool and then remove from pot.
- Regular Pot – if you’re not using a pressure cooker, a regular cooking pot will work. Pour enough water that it is just below the rack. Place a shallow pan of soil with 4 inches (or less) of MOIST soil on top of the rack. Cover pan with foil. Close the lid of the pan and allow to the water to boil for 30 minutes, making sure you leave the lid cracked enough to prevent pressure from building up. Allow soil to cool and then remove from pot.
- To moisten the soil quickly, use warm water – it absorbs better.
- DO NOT overbake the soil as it can release harmful toxins into the air. It can also kill beneficial organisms. Stop baking if the soil begins to emit a strong odor.
- Sterilize your used pots, trays and garden tools by soaking them for 30 minutes in 10% solution of household bleach & warm water.
- Store sterilized soil in a clean bucket or container.
Do you sterilize your soil? Have any tips? Leave them in the comments below.
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