how to sterilize soil

How To Sterilize Soil 4 Easy Methods Anyone Can Try At Home

Even the most experienced gardeners can have trouble with pests, weeds, and diseases in their garden. 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

Soil sterilization is a means of killing weed roots, 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.

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.

Why Sterilize Soil

There is a wide variety of opinions on whether or not to sterilize the 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 way to sterilize soil is prior to planting. This may sound obvious, as you will find it challenging to do once you have plants in situ. What I really mean is sterilize the soil to create a clean and sterile environment.

It is counterproductive to add a potted plant with unsterilized soil on the root ball to a pot full of sterilized soil as you risk cross-contamination. It’s most effective when used to germinating seed or potting on seedlings or young plants. Or where you wish to repurpose compost or soil that has previously been used before or has a higher probability of disease or contamination with weeds or pests.

Seedlings

If you are germinating seeds or growing small immature plants, you should sterilize used soil or simply use new compost that has been sterilized commercially to prevent the risk of dampening off or other issues caused by poor soil.  

Damping-off is a common problem for seed 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 sterilization process.

 Juvenile Plants

It’s always tempting for gardening to transplant juveniles into pots or containers and make sure of old potting soil or compost from the compost heap. In this situation, there is a moderate risk of contamination to the tender young plant.

Just as you should thoroughly cleanse used pots, there is a notion that you should or at least could sterilize potting soil to kill off any threat, for example

  • Weeds and weed seeds
  • Viruses in the soil
  • Nematodes
  • 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 it should present less risk. However this is depending on a good level of plant health, a sick plant is a weak and vulnerable plant at any stage.

Here’s 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, disease, and pests.
  • It reduces the work of weeding and removing pests by hand

Sterile Potting Soil

You don’t really need to sterilize newly purchased potting soil, especially if it is marked as sterilized. However, you may prefer to purchase potting soil that has already been sterilized.

If you only need a small amount, such as a bag or two for your indoor plant or wish to germinate seed, it would be easier, less expensive, and faster to simply purchase a bag of sterilized potting soil.

We will now share four techniques on how to sterilize soil that will greatly reduce pathogens and pests.

How To Sterilize Soil 4 Methods

There are a number of methods to sterilize the soil, and the method you choose will be determined to be a number of factors.

The volume of soil you have

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, 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 areas. You may have seen these large plastic covering on agricultural land prior to planting.

The labor involved

How much time and effort will it take you to sterilize your soil? 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

The four most common methods for sterilizing soil are:

  1. Oven sterilization
  2. Solarization
  3. Steam sterilization
  4. Microwave sterilization

Lets talk about each method so you can see what will work best for you.

1. How To Sterilize Soil In 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, aluminium foil, an oven-safe thermometer, and water.

  1. Preheat your oven to 200 degrees Fahrenheit.
  2. 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.
  3. 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.
  4. Cover your container with aluminum foil to prevent the soil drying out ahead of the sterilization process occurring.
  5. 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.
  6. Turn off the oven.
  7. All the soil to cool down until it reaches room temperature. Once the soil is cool, it is ready to use.
  8. Serve with lashing 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 do not want your oven or kitchen to smell like earthy, warmed-up soil. You can sterilize the soil with the sun, the microwave, or steam it.

2. How To Sterilize Soil In The Sun

This method works through solarization, by covering your soil with layers of plastic. The heat from the sun will build up the temperature of the soil under to kill most of the bacteria, diseases, weeds, and pests.

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 sterilizing 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 try shoveling soil into thin layers spread between large sheets of transparent plastic. The basic principle is to heat through the soil, so thinner layers will work better.

If you are sterilizing entire borders or gardens, you simply cover them with plastic and allow the sun to heat up over time. Clearly this process requires strong direct sunlight for a long as possible.

The best plastic to use is clear or transparent as it will heat up the quickest and allow the sun direct radiation to penetrate into the soils.

Very thin plastic will heat up fast but is prone to damage and tears. So, I would recommend a good quality medium thickness sheet, that can be washed down afterward, folded, and reused another day.

Step By Step

  1. Purchase your plastic sheet in large single panels if possible, and find some pegs or stones to secure the sheet to prevent it blowing away, or letting cool wind creep underneath the edges and cool the soil.
  2. 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.
  3. 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.
  4. Wet the soil until it is somewhat moist. If you are solarizing the soil in a garden or field, you will need to make sure the top foot of soil is damp before you start the process. Water it or start after rain.
  5. 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 into a thin layer.
  6. 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 under cooler or less ideal circumstances.

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, heat-proof containers that are safe to use in your pressure cooker, soil, aluminum foil, and water.

  1. Start by putting your pressure cooker in a safe location. Add a couple of cups of water and the rack.
  2. 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.
  3. Cover the containers with foil and place them in the pressure cooker.
  4. Place your lid on the pressure cooker and follow the manufacturer’s directions so that it begins to build steam.
  5. For sterilization to happen, the soil will need to be processed at 10 pounds of pressure for somewhere between 15 and 30 minutes.
  6. Once the soil is processed, remove the heat and allow the pressure to subside according to the directions on your pressure cooker.
  7. Once the soil has returned to room temperature, it is ready to use. Or keep it tightly covered until you are ready to use it.

Non-pressurized steam 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.

  1. Place the rack and an inch or two of water into your pot.
  2. Fill your heat-proof containers no more than four inches deep with soil
  3. Cover them with aluminum foil, then place them on the rack inside the pot. Do not stack the containers on top of each other.
  4. Put the lid on the pot, allowing steam to escape.
  5. Turn on the heat and bring the water to a boil.
  6. Once the water is boiling, allow it to continue boiling gently for thirty minutes.
  7. Remove the heat and allow the soil to cool completely before use.

4. Sterilize Soil In Microwave

If you do not have a large amount of soil to process or you are a little short on time, you can process it in your microwave. Again, this may cause an unpleasant odor so be sure to use good ventilation. Or even consider temporarily moving you microwave outdoors or into your garage whilst performing this routine.

You will need a microwave, zip-top plastic bag or plastic container, heat-safe thermometer, soil, and water.

  1. Make sure your soil does not have any pieces of metal in it.
  2. Fill your bag with about 2 pounds of moist soil. The soil should be wet enough to form clumps, but not runny.
  3. Do not close the bag or container fully, allow steam to escape to avoid a pressure build-up.
  4. Place the container in the middle of the microwave. Microwave your soil on high. Because microwave ovens vary, your actual microwave time may vary. Keep heating the soil until it reaches a temperature of 180 to 200 degrees Fahrenheit.
  5. Once the soil reaches the appropriate temperature, carefully remove the container, and close it. Allow it to cool.
  6. Once the soil has returned to room temperature, it is ready to use. You may keep the container closed and store the soil for future use as long as you keep the bag sealed.

Final Thoughts On How To Sterilize Soil

Okay, so there we have it. Sterilizing soil may not be a critical requirement, but in some scenario’s it can be a great way to reassure yourself that you have done all you can to protect vulnerable plants or seedlings.

We have explored the four ways you can try to sterilize your soil at home with our step by step guides. Why not give it a go and see if what it does for your growing adventures.

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