Even the most experienced gardeners can have trouble with pests, weeds, and diseases in their gardens. These problems can crop up unexpectedly, leaving frustration and disappointment in their wake when plants die and gardens fail.
One method to prevent these harmful problems from destroying your garden is soil sterilization.
There are many pros and cons to sterilizing the soil and a number of different methods you can use, as well. Below, we will discuss not only the reasons to sterilize the soil but also how to sterilize soil successfully.
- What Is Soil Sterilization
- Why Sterilize Soil
- Why Bother Sterilizing Soil At Home
- Sterile Potting Soil
- What To Consider Before Sterilising Soil At Home
- 4 Methods of Sterilizing Soil At Home
- Final Thoughts On Sterilizing Soil
- Frequently Asked Question
What Is Soil Sterilization
Soil sterilization is a means of killing weed roots, soil mites, and other pests, viruses, fungi, and bacteria within the soil in order to prevent them from destroying your crops and plants. This can be accomplished through a chemical process or even heat processing.
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Why Sterilize Soil
The process of sterilizing soil is frequently used by commercial greenhouses, farmers, and other agricultural producers and growers because it is a preventative measure that is less expensive than losing crops or treating problems as they arise.
Opinions differ when it comes to the benefits of sterilizing soil, especially when it comes to smaller consumers such as homeowners, homesteaders, or indoor plant enthusiasts. There certainly are some beneficial reasons to do it, but there are also a few reasons why you may prefer not to.
For example; if you are only planting and growing mature plants, then soil sterilization may not be necessary. The plants may be healthy and strong enough to handle any deficiencies of old or unsterilized soil.
There’s a reasonable argument that sterilizing soil is detrimental because not only does it kill the bad microorganisms, it also kills the good microorganisms which are necessary for healthy plant growth.
Good microorganisms help move nutrients and water through the soil so the plants can consume them efficiently.
Sterilize Soil Before Planting
The most effective time to sterilize soil is prior to planting. This will help to create a clean and problem-free environment from growth begins. It may sound obvious, but it will be very challenging once plants are in situ.
Preparing the soil in this way is a good option when used to germinate seeds or when potting on seedlings or young plants. It is also a useful good option when you want to repurpose compost or soil that has previously been used before or has a higher probability of disease or contamination with weeds or pests.
It is worth noting also that sterilizing is completely counterproductive when you re-pot a plant that has unsterilized soil on the root ball. Otherwise, you risk cross-contamination.
If you are germinating seeds or growing small immature plants, you should consider sterilizing used soil or simply use new compost that has been sterilized commercially to prevent the risk of damping-off, transplant shock, or other issues caused by poor soil conditions.
Damping-off is a common problem for seeds immediately after germination. It happens when pathogens in the soil attack the seedling, causing it to suddenly wither and die on the stem. These pathogens are destroyed through the process.
It’s always tempting for gardeners when transplanting juvenile plants to make use of old potting soil or compost from the compost heap. In this situation, there is a moderate risk of contamination of the tender young plant.
Just as you should thoroughly sanitize used pots before re-using, I recommend that you should or at least could sterilize potting soil to kill off any threat, and mitigate issues such as:
- Weeds and weed seeds
- Viruses in the soil
- Bacteria that could cause serious problems such as damping off, blight, fusarium wilt, and verticillium wilt
As plants mature they build up a resistance to cope with these problems and these problems become less of a risk. However, this is dependent on the continuing health of the plant.
Why Bother Sterilizing Soil At Home
Here are a few of the advantages of sterilizing potting soil at home.
- It is easy and inexpensive
- Depending on the method, it is non-toxic
- It may be less harmful to the soil and environment than needing to use chemical applications of pesticides or herbicides after problems have already set in
- It can be done at home and on your own time
- It controls the spread of pathogens, diseases, and pests.
- It reduces the work of weeding and removing pests by hand
Sterile Potting Soil
If you only need a small amount of soil, such as a bag or two for your indoor plants, or when you’re looking to germinate a packet of seeds, it is much easier, less expensive, and faster to simply purchase a bag of sterilized potting soil.
Store-bought soil that is marked as sterilized has been through a rigorous and commercially accepted method of sterilization and so there really is no need to do it again at home.
What To Consider Before Sterilising Soil At Home
There are four main techniques used to sterilize the soil in order to reduce pathogens and pests, and the method you choose will be determined by a number of factors. These are:
The volume of soil you have
It might be contentious in your home, but if you only have a small quantity of soil that you need to take care of, you can use the oven, microwave, steam, or pressure cooker methods.
Consider these methods if you are only processing enough soil for a few pots or seed trays. If you have large amounts of soil to process, such as a flowerbed, garden, or even large fields, you will need to use the solarization method.
The materials needed
Some methods of sterilization require equipment, such as a pressure cooker, microwave, or oven. If you do not have access to these items (or you just don’t want to use equipment you would otherwise use to prepare food!), you may need to adjust the method you use to take care of your soil.
A plastic sheet is all you need to solarize large quantities or areas of soil. You may have seen these large plastic sheets. They are frequently used to cover agricultural land prior to planting.
The labor involved
How much time and effort will it take you to sterilize your soil? Well, some methods are more labor-intensive than others, and this may affect the method you choose. It would be ineffective to try to process an entire field’s worth of soil in the microwave right, but you could easily process a small amount.
4 Methods of Sterilizing Soil At Home
The four most common techniques are:
- Oven sterilization
- Steam sterilization
- Microwave sterilization
Read on for detailed instructions for each technique before deciding what will work best for you.
1. How To Sterilize Soil In an Oven
If you are only processing small or even medium-sized batches of soil, you may want to choose to process it in the oven where you can fit several containers or trays at a time.
Of course, the drawbacks of this method include filling your kitchen with the odor of cooked soil. It’s not the best smell in the world, and you will need adequate ventilation before attempting this.
You will need a large oven-proof container, your soil, aluminum foil, an oven-safe thermometer, and water.
- Preheat your oven to 200 degrees Fahrenheit.
- Fill your container to about three inches deep with soil. Don’t make it too deep or the middle layer of soil will not get hot enough. Remove any rocks or leftover plant matter.
- Wet the soil just enough to moisten it completely. The water will create steam that will kill contaminants in the soil, however, too much water will slow down or stop the process.
- Cover your container with aluminum foil to prevent the soil from drying out ahead of the sterilization process occurring.
- Measure the temperature of the soil with a heat-safe thermometer. Once the soil temperature reaches 180 degrees Fahrenheit, bake the soil for thirty minutes, keeping the oven door closed the entire time.
- Turn off the oven.
- Wait for the soil to cool down to room temperature. Once the soil is cool, it is ready to use.
- Serve with lashings of cream…joke!
How To Sterilize Soil Without Baking
If you don’t want to bake your soil, you can still sterilize it with other means. This is helpful if you don’t want your oven or kitchen to smell like earthy, warmed-up soil. You can sterilize the soil with the sun or the microwave, or, you can steam it.
2. Sterilizing Soil Using The Sun
This method works through ‘solarization’, and by covering your soil with layers of plastic. The heat from the sun will build up the temperature of the soil, resulting in bacteria, diseases, weeds, and pests being killed during the process. According to the University of California, temperatures need to reach 110º to 125°F for 6 hours or more.
Sterilization through solarization can take place in a couple of different ways. The methods you choose will produce similar results; which one you choose is a matter of preference and budget.
If you are preparing small quantities of soil, you could use plastic bags to create a mini-greenhouse effect by placing the bag in strong sunlight to absorb the sun’s rays. It’s important to lay the bag flat so the soil forms a thin layer of a couple of inches.
For large areas, you will need to shovel soil into thin layers spread between large sheets of transparent plastic. The basic principle is to heat through the soil. For this reason, thinner layers work better.
If you are sterilizing entire borders or gardens, you simply need to cover them with plastic and allow the sun to heat up over time. Clearly, this process requires strong direct sunlight for as long as possible.
The best plastic to use is clear or transparent as it will heat up the quickest and allow the sun’s direct radiation to penetrate into the soil.
Very thin plastic will heat up fast but is prone to damage and can get easily torn. I recommend a good quality medium-thickness sheet, that can be washed down afterward, folded, and reused another day.
Step By Step
- Purchase your plastic sheet in large single panels if possible, and find some pegs or stones to secure the sheet to prevent it from blowing away, or letting cool wind creep underneath the edges and cool the soil.
- Break up any clumps of dirt and remove any leftover plant matter and large rocks that could tear the plastic or slow down the process.
- If you are using a smaller amount of soil or potting soil, lay down your first layer of plastic. Spread your soil over the top of it. Keep the soil a few inches away from the edge.
- Wet the soil until it is moist. If you are solarizing the soil in a garden or field, you will need to make sure the top foot of the soil is damp before you start the process. Either water it manually or begin the process after rain.
- Cover the prepared soil with plastic and make sure it is flat. You can secure the plastic with rocks to hold it in place. I also suggest burying the edges of the plastic under the soil to seal in the heat build-up. If you are placing the soil in plastic bags, make sure they are tightly closed and the soil is spread in a thin layer.
- It can take approximately four to six weeks during the hottest part of the year to complete the process of solarization in large areas. It may take longer in cooler or more shaded areas.
3. Steam Sterilization Of Soil
Another means of sterilizing soil is to use boiling water or steam. This is a quick and effective method if you have the right equipment. You may want to use a pressure cooker for this process. However, make sure you follow the directions that go with your pressure cooker and always be careful.
Pressure cooker method
You will need a pressure cooker with a rack, and heat-proof containers that are safe to use in your pressure cooker. You will also need, soil, aluminum foil, and water.
- Start by putting your pressure cooker in a safe location. Add a couple of cups of water and the rack.
- Fill containers with soil ready for placing them into the cooker. The soil can be up to 4 inches deep, but no more than that.
- Cover the containers with foil and place them in the pressure cooker.
- Place the lid on your pressure cooker and follow the manufacturer’s directions so that it begins to build steam.
- For sterilization to happen, the soil will need to be processed at 10 pounds of pressure for somewhere between 15 and 30 minutes.
- Once the soil is processed, remove the heat and allow the pressure to subside according to the directions on your pressure cooker.
- Once the soil has returned to room temperature, it is ready to use. Alternatively, keep it tightly covered until you are ready to use it.
Non-pressurized steam method
For this method, you will need a large pot with a lid, a wire rack, heat-safe containers, aluminum foil, soil, and water. A large water bath would work well. It’s basically the same process as cooking a steam pudding.
- Place the rack and an inch or two of water into your pot.
- Fill your heat-proof containers no more than four inches deep with soil
- Cover them with aluminum foil, then place them on the rack inside the pot. Do not stack the containers on top of each other.
- Put the lid on the pot, allowing steam to escape.
- Turn on the heat and bring the water to a boil.
- Once the water is boiling, allow it to continue boiling gently for thirty minutes.
- Remove the heat and allow the soil to cool completely before use.
4. Sterilizing Soil In a Microwave
If you do not have a large amount of soil to process or you are short on time, you can process it in a microwave. Again, this may cause an unpleasant odor so be sure to use good ventilation. Or even consider temporarily moving your microwave outdoors or into your garage.
You will need a microwave, zip-top plastic bags or plastic containers, a heat-safe thermometer, soil, and water.
- Make sure your soil does not have any pieces of metal in it.
- Fill your bag with about 2 pounds of moist soil. The soil should be wet enough to form clumps, but not soggy.
- Do not close the bag or container fully, allow steam to escape to avoid a pressure build-up.
- Place the container in the middle of the microwave. Microwave your soil on high until the soil reaches 180 to 200 degrees Fahrenheit.
- Once the soil reaches the appropriate temperature, carefully remove the container, release any air and then seal the bag or container. Allow it to cool.
- Once the soil has returned to room temperature, it is ready to use. Keep the container closed or bag sealed until the soil is needed.
Final Thoughts On Sterilizing Soil
Okay, so there we have it. Sterilizing soil may not be a critical requirement, but on some occasions, it is a great way to give yourself assurance that you have done all you can to protect vulnerable plants or seedlings.
I have covered the main four techniques used depending on the amount of soil and the equipment you have available to you. Each method I’ve covered includes a step-by-step and easy-to-follow guide that makes sterilizing soil yourself straightforward and achievable.
Why not give it a go and see what it does for your growing adventures?