Soils commonly contain diseases, pests, and even weed seeds. This is why sterilizing the soil is recommended before planting new crops. Most soil sterilization methods are ideal for small amounts of potting soil. However, there is one method that you can use to sterilize larger areas of garden beds outdoors.
We’re going to look at each of these methods in more detail and give you step-by-step instructions.
1. Solar Sterilization
This method uses the heat from the sun to sterilize the soil and kill any pathogens that may be present. It’s the only method that you can use on outdoor garden beds unless you want to add unnecessary chemicals to your soil.
This method can also be used to sterilize smaller amounts of soil for potting mixes, but there are quicker ways to do this, which we’ll go into a little later.
Using solarization does require a little patience as you need a few weeks of hot sunshine to complete the process effectively.
Here are step-by-step instructions on how to sterilize soil in a large garden bed.
- Obtain a large sheet of clear plastic that will cover the area required. Make sure that the plastic is durable enough so that it isn’t going to deteriorate with all that sunshine.
- Remove any plant debris or stones that could tear the plastic once it’s laid. You should also break up any clumps of soil while you’re at it.
- Water the soil and ensure that the top 12 inches of the soil are completely damp. This will help in the sterilization process.
- Lay the sheet of plastic over the soil and secure the edges with bricks, rocks, or lengths of timber. Make sure that the plastic is completely flat so that the heat build-up will be sealed in. You might even want to bury the edges of the plastic in the soil to get a really good seal. Also, make sure that the plastic is well secured and is not likely to blow away if you get a sudden gust of strong wind.
- Leave the plastic in place for around four to six weeks during the hottest part of the year. This will ensure that the soil has been sterilized effectively and is now ready for planting.
You can also use this method for smaller amounts of soil that you plan to use in your own potting mixes by following these steps:
- Lay down one sheet of plastic in an area that gets full sunlight all day.
- Spread a thin layer of soil over the plastic making sure to keep the soil away from the edges.
- Wet the soil to make sure that it’s damp.
- Lay a second sheet of plastic over the soil and secure it with bricks, rocks, or pieces of timber. Make sure that the edges are secure well so that the plastic can’t blow away.
- Alternatively, you can put your soil in a large plastic bag that you tie up securely and lay on the ground in the sun. Make sure that the soil is in a thin layer inside the bag.
If you have an area in your yard that is out of the way but receives plenty of sunshine, you can have a succession of soil-filled bags that you’re sterilizing, one after the other, during the hottest months of the year.
It may interest you to know that this soil sterilization method is commonly used by large-scale organic crop farmers where they have entire fields under plastic for a couple of months at a time. Then they rotate their crops and sterilize another field to continually ensure that their soil is free from pathogens.
The disadvantage to this method is the time that it takes to fully sterilize the soil. But, the advantage is that the method is easy and clean and keeps the soil out of your kitchen.
On the other hand, for small quantities of potting mix, there are easier methods you can use that produce results much faster. Let’s discuss these in more detail.
2. Sterilize Your Soil In The Microwave
This is probably one of the fastest methods that you can use to sterilize small amounts of soil. In fact, if you’re going to be sterilizing soil often to make your own potting mixes, it might be worthwhile investing in a second-hand microwave that you can use solely for this purpose.
This way, you can keep the microwave in your shed or garage and keep your kitchen free from soil-cooking odors. Plus, you’re only going to be using the microwave for sterilizing the soil and not for cooking food in.
Here’s what to do:
- Most importantly, make sure that the soil does not have any bits of metal in it. Remember that metal and microwaves do not mix.
- Use zip-lock bags and fill each one with around 2 pounds of damp soil. Make sure that the soil is damp enough that it can easily form clumps.
- When sealing the bags, leave a little air hole so that excess steam can escape. This means your bags of soil won’t blow up in the microwave and make a huge mess.
- Put the bag of soil in the middle of the microwave plate and microwave it on high. Only cook one bag at a time to ensure adequate sterilization.
- You’ll have to use a thermometer to check the temperature of the soil until it reaches 180 to 200 degrees Fahrenheit (85 to 95 degrees Celsius). Once you’ve done this the first time, you’ll be able to determine how long to cook your soil until it reaches this temperature.
- Once the soil is at the right temperature, very carefully remove the bag and seal it completely. Remember that the bag will be hot, so use a pair of oven mitts to handle it. Allow the bag of soil to cool down completely.
- When the soil has reached room temperature, you can use it straight away or keep the sealed bag in a cool spot until you’re ready to use the soil.
By using this method to sterilize your soil, you can save yourself some money by not having to constantly buy bags of potting mix. This means that you can keep re-using all the old mix without worrying about soil-borne pathogens that may be present in the soil from previous plants that were growing in it.
3. Use Boiling Water To Sterilize The Soil
This is another super easy method to sterilize your soil and works really well for small batches.
Here’s what to do:
- Grab a large bucket or heat-proof container.
- Fill with soil.
- Fill your kettle with water and boil it.
- Pour the boiling water over the soil in the bucket.
- Use an old wooden spoon to stir the soil so that every particle has been moistened with the boiling water.
Boiling water will kill any insects that are in the soil as well as their eggs. You’ll also find that heat will kill a myriad of other pathogens such as bacteria.
An even more effective method is to freeze the soil first before using the boiling water. To do this, place the soil in a sealed bag into the freezer and leave it there for about five days. Then take it out and follow the method above using the boiling water.
Freezing helps to kill certain pathogens, while boiling water will usually take care of the rest. You might even like to invest in a second-hand freezer that you keep in the garage or workshop primarily for freezing your soil.
The advantage of this method is that it’s fairly quick and easy, and you don’t even have to take the soil into the kitchen.
4. Baking The Soil In The Oven
This works best if you only have a small amount of soil to sterilize, and you don’t mind filling your kitchen with the odors of cooked soil. It’s a good idea to open all your windows before attempting this process.
Here’s what to do:
- Turn on your oven and preheat it to 200 degrees Fahrenheit (95 degrees Celsius).
- Grab a large oven-proof container and fill it three inches deep with soil.
- Moisten the soil completely to create steam while in the oven.
- Cover the container with aluminum foil to seal in the moisture.
- Using an oven-safe thermometer, check the temperature of the soil until it reaches 180 degrees Fahrenheit (85 degrees Celsius).
- Once the correct soil temperature has been reached, bake the soil for thirty minutes.
- Turn off the oven and allow the soil to cool down with the door closed.
- Once the soil has cooled down to room temperature, it’s ready to use.
5. Sterilize Your Soil Using The Barbecue Or Grill
If you have a gas grill or barbecue, you can use it to sterilize small quantities of soil. Here’s what to do:
- Grab one of those aluminum baking trays that you use to cook a roast in.
- Fill this with the soil and make sure that you moisten the soil well.
- Cover the baking tray with some aluminum foil. This stops the soil from drying out and creates adequate steam.
- Grab a meat thermometer and insert it into the soil by poking it through the foil cover. Make sure that the thermometer is inserted in the middle of the soil only and doesn’t touch the bottom of the baking tray.
- Turn on the barbecue or grill and heat the soil until it reaches a temperature of 180 degrees Fahrenheit (85 degrees Celsius).
- Cook the soil for another thirty minutes ensuring that it maintains this temperature. Don’t let the temperature rise above this level to avoid damage to the soil or to produce toxicity that may be harmful to your plants.
6. Use Steam To Sterilize The Soil
This is a quick and effective way to sterilize small amounts of soil. There are two ways that you can do this. Either with a pressure cooker or just using a large pot on the stove.
Here’s the pressure cooker method:
- Add two cups of water into the pressure cooker as well as the rack.
- Fill a number of heat-proof containers with soil. Make sure the soil is no more than 4 inches deep.
- Cover these soil-filled containers with aluminum foil and place them inside the cooker.
- Put your lid on the pressure cooker and turn it on. You’ll have to follow the directions related to your individual unit so that it builds up steam.
- The soil needs to be sterilized at 10 pounds of pressure for around 15 to 30 minutes.
- Once this is done, turn off the heat, allow the pressure to reduce, and let the soil cool down to room temperature before using it.
- If you’re not ready to use the soil straight away, place it in an airtight container for later use.
If you don’t have a pressure cooker, you can use the following method instead to get the same results.
- Grab a large pot that has a close-fitting lid.
- Put around an inch or two of water into the bottom of the pot and place a wire rack inside.
- Fill some heat-proof containers with the soil making sure that the soil is no more than four inches deep.
- Cover these containers with aluminum foil and place them in a single layer on the rack that is inside the pot. Make sure that you don’t stack them on top of each other.
- Cover the pot with the lid making sure that some steam is able to escape.
- Turn on the stove and bring the water in the pot to the boil.
- Boil continuously for thirty minutes on a gentle simmer.
- Turn off the stove and allow the soil to cool to room temperature before using it.
Now you know exactly how to safely and easily sterilize your own soil. Use any of the methods above, and you’ll be able to ensure that the soil that you use for your plants is always free from pathogens.