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If you use the same soil over and over again, it might be in the best interests of your garden to sterilize it from time to time. Repeatedly using the same potting mix or garden soil can lead to a buildup of harmful pathogens, garden pests, weed seeds, and fungal spores.
Soil sterilization is the process of treating your growing medium with heat to eradicate these soil contaminants for better conditions for germinating seeds, propagating cuttings, or growing juvenile plants.
Although soil sterilization is regular practice by commercial gardening centers and farmers to save on replenishing soil each year, there is a debate on whether it is necessary for home and small-scale gardeners.
So, while we will take you through the three main methods of soil sterilization – solarization, oven/microwave, and steam – we will also tell you how you can avoid the process altogether.
Why Should I Sterilize My Soil? What Are the Benefits of Soil Sterilization?
If you have a large garden it can be time-consuming, expensive, and hard work to replenish your soil every year.
However, if you continue to use the same growing medium over and over again, you may experience what gardeners call ‘soil sickness’.
This is a term used to describe the rise of negative conditions for plant growth resulting from the build-up of pathogens and a depletion of nutrients.
The purpose of soil sterilization is to quickly eradicate mold, fungal spores, nematodes, and other harmful organisms from your growing medium if you don’t replenish your soil each season.
This helps to provide the best possible growing conditions for seed germination, transplanting juvenile plants, or the propagation of stem or shoot cuttings.
Soil sterilization is less important when you’re only growing mature plants, but if you are reusing the same growing medium, you may benefit from periodical sterilization.
Additional advantages of soil sterilization include an improved crop yield, relief from soil fatigue, increased availability of nutrients, and reduced damping-off of your new plants.
Plus, evidence suggests that soil sterilization may be beneficial to friendly soil bacteria and other microorganisms.
Steam and oven sterilization of soil is ideal for small growing areas, such as house plants and pots. Solarization is your best option to sterilize large areas or to clear grass and dense weeds in larger growing spaces.
Solarization works best at the surface of the soil and decreases in effectiveness the deeper you go.
However, the effects of solarization increase the temperature of the soil enough in the first 12 to 18 inches for it to be lethal to the majority of soil-borne pests. More on the different methods of soil sterilization later.
Let’s take a more in-depth look at the benefits of soil sterilization.
Sterilizing Soil to Kill Weeds
Soil solarization is more effective in controlling annual weeds than perennial weeds, as perennials usually have deeper vegetative structures, such as tubers, rhizomes, and roots, that may resprout following initial sterilization.
You’ll get better results controlling weeds with optimal solarization conditions, such as tight-fitting plastic tarps, high solar radiation, and good soil moisture.
Turning over the soil before solarization may also help, by bringing harder to kill rhizomes – such as those of johnsongrass and bermudagrass – to the surface where the heat is more intense.
Sterilizing Soil to Kill Pests
You should be aware that the level at which pests can be controlled in your soil is dependent on the depth, duration, and intensity of the elevated temperatures in the sterilization process.
While some pests can be eradicated in just a few days, it may take weeks to kill off others.
Plus, eggs, nematodes, and other pests living below the top inches of soil may survive solarization, by escaping the high temperatures.
You increase your chances of eradicating pests and their eggs by cultivating the top 12 inches of soil before carrying out solarization.
Soil solarization has been shown to be effective in reducing the populations of several species of soil nematodes, which is particularly useful for home and organic growers.
However, as nematodes are mobile, they can move deeper into the soil to escape the heat.
This means that following solarization, nematodes made recolonize the soil and roots, causing damage to your plants. You can minimize the effects of this by avoiding cultivating the soil too much following sterilization.
While solarization is not as effective against nematodes as it is with fungi and bacteria, it’s certainly useful in the top 12 inches of soil.
However, the ability of nematodes to burrow and recolonize quickly means that your soil may require yearly treatment to keep the pests at bay.
Sterilizing Soil to Kill Pathogens
Solarization is effective in controlling many harmful soilborne fungi and bacterial pathogens, including but not limited to those that cause damping-off, tomato canker, Southern blight, Verticillium wilt, and Fusarium wilt.
However, heat-tolerant pathogens, like those which cause charcoal rot and melon decline, are more difficult to eradicate with solarization.
Sterilizing Soil Can Increase Beneficial Soil Organisms
You don’t need to worry about killing off beneficial soil organisms with solarization, as many are either able to survive sterilization or recolonize the soil quickly afterward.
By increasing the population of beneficial bacteria and fungi, you aid plant growth, as well as making your soil more resistant to harmful pathogens.
Furthermore, although there is a lack of detailed evidence, earthworms are believed to burrow deeper in the soil to escape the high temperatures.
Sterilizing Soil Can Increase Nutrient Availability
Sterilization initiates changes in the chemical and physical features of the soil that improve the development and growth of plants.
It speeds up the breakdown of organic material in the soil, releasing soluble nutrients such as potassium, nitrogen, magnesium, and fulvic acid, making them more available to plants.
Why You Should Never Use Bleach to Sterilize Your Soil
While using chemicals to sterilize soil is sometimes used by commercial operations, because it can be done quickly on a large scale, it is not a method that is recommended for the home gardener, primarily due to the hazards involved in the process.
Disadvantages of Chemical Soil Sterilization
There are health and safety risks – such as respiratory distress and skin irritation – to the person applying the chemicals to the soil and those in close proximity. Plus, the soil will be toxic to pets and children, making the garden an unsafe environment.
If chemical sterilization is not carried out properly, residues may linger in the soil, which can be drawn into the plants, concentrating in the tissues and causing damage.
Following chemical sterilization, there is a quarantine period before the soil can be used for growing. This is to allow the chemicals to fully degrade or to be flushed from the soil.
The chemicals used have a very narrow range of effects. In many cases, a single chemical will only eradicate a few pathogens or pests.
Long-term or repeated use of chemicals can increase pest, pathogen, or disease resistance, making it less effective over time.
Bleach is a chemical that should never be used to sterilize the soil. While it will destroy weeds, harmful fungi, bacteria, and nematodes, as well as other pathogens, it will also kill off beneficial soil organisms too. It can take many months for these organisms to recolonize the soil after being exposed to bleach.
Plus, unfortunately for plants, bleach will create an inhospitable growing environment for seeds and plants by drastically altering the pH. Your soil may be barren indefinitely if the bleach doesn’t leach from the soil.
When the bleach is flushed from the soil it has the potential to spread into waterways and the water table, where it can have a significant effect on wildlife.
How to Sterilize Soil
Safer and more soil-friendly soil sterilization methods involve using heat to control the populations of harmful organisms to provide the best environment for growing seeds, seedlings, and mature plants. Heat treatments raise soil temperature via direct, dry heat or steam to the threshold where harmful organisms are unable to survive.
For sterilizing small amounts of soil, such as a seed starter mix or soil for household plants, the oven, microwave, or steam method is sufficient. For whole garden beds, the only suitable method for sterilizing large areas is solarization.
How to Sterilize Soil in the Oven
- Preheat your oven to 200°F.
- Place around three inches of soil into an oven-proof container and thoroughly moisten the soil with water.
- Cover the container with aluminum foil and pop it in the oven.
- When the internal temperature of the soil reaches 180°F, ‘bake’ for 30 minutes.
- Allow the soil to cool to room temperature and keep covered until it’s time to use.
The oven method of soil sterilization tends to pose less of a risk to the microwave and steam methods. However, you may not enjoy the odors generated during the sterilization process, so make sure you have plenty of ventilation, so they can dissipate quickly.
See the table below to see the target temperatures you want to sustain for 30 minutes to kill the different kinds of harmful soil organisms and pathogens.
| Temperature (°F) | Organisms Killed |
| 120 | Water molds (oomycetes) |
| 145 | Pathogenic fungi, bacteria, viruses, nematodes, centipedes, and slugs |
| 160 | Heat-resistant pathogenic bacteria and insects |
| 180 | Weed seeds |
| 212 | Heat-resistant plant viruses and weed seeds |
How to Sterilize Soil in the Microwave
- Place about two pounds of damp soil into a clean zip-top plastic bag.
- Pop in the microwave and heat until the internal soil temperature reaches between 180°F and 200°F. This should take around 90 seconds, but it is dependent on the power of your microwave.
- Remove from the microwave (carefully) and seal the bag.
- Allow to cool and keep the bag sealed until you’re ready to plant.
The microwave method is a good choice if you have just a small amount of soil to sterilize. Just be sure that there is no metal whatsoever in the soil.
How to Sterilize Soil With Steam
- Put a few inches of water into a large pan and place a rack at the bottom.
- Place no more than four inches of soil in heat-proof containers and cover tightly with foil.
- Pop the containers on the rack, and put the lid on the pan, slightly cracked to allow steam to escape.
- Boil the water for 30 minutes.
- Once cooled remove the containers and keep the soil covered with foil until you’re ready to use it.
Steam sterilization can also be done with a pressure cooker, just be sure to adhere to the manufacturer’s safety instructions.
How to Solarize Soil
There are a number of ways you can solarize your soil – all of which produce the same results – and the one you choose is dependent on the amount of growing mediu
- Large garden beds are covered with plastic to utilize the energy from the sun.
- Big quantities of potting soil – too much for the oven, microwave, or steam methods – is sandwiched between two layers of plastic to trap the solar energy.
- Smaller amounts of potting or mineral soil are placed in plastic bags and left out in the sun.
When Is the Right Time for Solarization?
Soil solarization is most effective when done during the hottest and longest days of the year, which are usually between June and August. In warmer climates, you can get good results from solarization as early as late May or as late as early September. You should be aware that in shady areas, solarization may not be effective.
Preparing the soil
Solarization is much more effective on a smooth seedbed, as this allows the plastic to sit directly and tightly against the soil for uniform sterilization of the entire area. Air pockets between the soil and plastic will greatly reduce the temperature the soil will reach, meaning not all pests and pathogens will be eradicated.
If you’re working with rough ground, cut down the existing weeds and grass as low as possible and leave the cuttings on the top of the soil, where they will be broken down during the solarization process.
Improve the uniformity of heating by laying your beds north to south. Solarization is most effective when there is minimal or no slope, or where the slope has south-facing exposure. On north-facing slopes, the soil temperature is unlikely to reach the highs needed to control pests.
Form raised beds prior to the solarization process to minimize soil disturbance afterward, which may bring to the surface viable weed seeds and pathogens from deeper in the soil.
Irrigating the Area
If the soil is too dry, weed seeds and pathogens may not take up enough water to make them vulnerable to the increased heat. For best results, wet the top 12 inches of soil and immediately cover with plastic to minimize evaporation.
Unless the soil gets dry, don’t irrigate again, as this will lower the soil temperature and therefore, increase the time needed for successful control of pests and pathogens.
Solarization may be less effective on sandy soil types as it drains faster and therefore, produces less steam. Drip irrigation of sandy soils will help maximize the benefits of solarization.
Laying the Tarp
Transparent or Black Plastic?
Transparent or clear plastic is generally most effective for solarization as the sun’s rays will pass through the tarp and become trapped in the soil below.
As black plastic absorbs and deflects some of the sun’s heat, it is usually less effective, with the exception of in coastal and cooler climates. In these areas, black plastic prevents weed growth beneath, which can’t be killed when the air temperature is too low. If you do use black plastic, the soil solarization process is likely to take longer.
Thickness of Tarp
There are several thicknesses of tarp and what you choose depends on the environment and size of the area to be treated.
- Thin plastic (1 mil thick) provides higher soil temperatures, but it is more susceptible to tearing from the wind or animals walking over it.
- Medium thickness plastic (1.5 – 2 mil) is more resistant to tearing in windy areas.
- Thick plastic (4+ mil) can be used to solarize small areas.
UV Protection
Plastic tarps for large-scale solarization are treated with a UV inhibitor to prevent them from breaking down. For gardening use, clear tarps from the hardware store will be sufficient as they will last the 6 weeks of solarization without degrading.
However, you should regularly check your tarp for damage or degradation, and any holes should be patched with durable tape as soon as possible. A significantly damaged tarp should be immediately replaced to prevent contamination of the soil and the environment. For smaller areas – such as a small raised bed – you can replace the damaged plastic with a fresh tarp.
In cooler climates or for solarizing smaller areas, you can use a double layer of plastic with an air space between, which can raise soil temperatures by up to 10°F.
Tarp Placement
It’s important the tarp is held as tightly and flush to the soil as possible for maximum effect. One way to achieve this is by burying the edges of the tarp all around the bed. You could also weigh down the tarp, but it may ‘sail’ when it’s windy.
Let Solarization Occur
The length of solarization is temperature-dependent. The aim is to maintain a daily soil temperature of around 125°F or above in the top 6 inches. A soil thermometer or temperature probe will help you monitor the temperature of the soil without needing to disturb the tarp.
If you solarize during the hottest months of the year, four to six weeks should be long enough to control most pathogens and pests. If you live in warmer climates or are experiencing unusually high temperatures, four weeks of solarization may be efficient. In colder or windier climates, you’re better off leaving the tarp in place for 8 weeks to ensure the maximum effect.
After Solarization
When removing the plastic after solarization, be careful not to disturb the soil too much, as this may bring up viable weed seeds or eggs to the surface. Also, if you cultivate your soil prior to planting, make sure it’s shallow (about 2 inches deep), as this too can bring pathogens to the topsoil.
You can plant immediately into your newly sterilized soil, with seeds or transplants, for a winter crop. If you’ve used black plastic, it can be left on the soil as mulch. Simply cut holes in the tarp and transplant your plants through the plastic.
Alternatively, you can leave the tarp in place until you’re ready to plant. If you’ve used a clear tarp, you should paint it white or silver to keep the soil cool and repel insects to prevent recolonization. Keep an eye on the plastic for any serious degradation.
The Disadvantages of Soil Sterilization
Excessive Temperatures
Heat treatments can be harmful if the temperature of the soil is allowed to get too high. Excessive high temperatures may increase the likelihood of phytotoxicity due to manganese toxicity, soluble salt, and an increase in toxic organic compounds.
Soil Prone to ‘Infection’
Sterilized soil is not immune to pathogens and pests in the future, In fact, it may be more prone to invasion in the future because competing harmful organisms have been killed off.
Limited Soil Penetration
Soil solarization is only effective in the top 12 inches of the soil. If you cultivate the soil after sterilization, you run the risk of bringing weed seeds, eggs, and nematodes that have escaped the high temperatures, to the soil surface.
Eradicating Beneficial Organisms
Solarization takes at least four weeks but can take as long as ten weeks in colder climates. This can drastically cut down on your growing season. However, preparing the soil in this way will make for healthier and happier plants in growing seasons to come.
It’s a Long Process
Just as soil sterilization kills off harmful soilborne pathogens, it also has the potential to harm beneficial microbes and insects. However, certain species of beneficial bacteria have been shown to repopulate the soil within a relatively short space of time.
How to Avoid Needing to Sterilize Your Soil
There are several ways you can avoid needing to sterilize your soil, allowing you to enjoy a longer growing season with healthy plants and crops.
Healthy Soil Management
- Test the soil before fertilizing
- Improving the texture and nutrient content of your soil with compost
- Limiting cultivation of the soil
Crop Rotation
A proper crop rotation cycle is a great way to reduce or prevent pest infestation, therefore, limiting disease transmission to your plants
Sterilizing Your Gardening Equipment
It is a lot easier to sterilize your garden tools than it is the soil. At the end and the start of the growing season (or at the sign of plant disease), sterilize them by soaking in a white vinegar solution. This will prevent the transfer of pathogens and pests. Cleaning your seed trays before planting will also reduce the risk of disease transmission.
Companion Planting
Companion planting is the process of growing mutually beneficial plants next to one another. There a numerous advantages to companion planting, including:
- Protecting neighbor plants from pests e.g. fragrant herbs can repel certain insects
- Boosting soil fertility e.g. nitrogen-fixing plants can benefit non-nitrogen-fixing plants.
- Weed control e.g. growing ground-covering plants next up to thin, upright plants can discourage weed growth
Read more about companion planting here.
Regular Weeding
Although it is labor-intensive, if you have a problem with weeds in your garden, digging them out is an alternative to sterilization. Companion planting and fertilizing your beds with green manure can help control the weed population.
Summer is the best time of year to solarize grass, as the air is warm and the sun is at its greatest heating potential. It takes about 6 weeks for solarization to kill off and breakdown grass, with minimal effort and no herbicides.
Summer is the best time to sterilize soil, when the sun is at its hottest and the days are at their longest.
You can solarize soil in winter, but it will take significantly longer. Furthermore, while it is likely to be effective in killing off areas of grass, it may be less effective in eradicating pests as the sterilization process relies on heat.
Black plastic may not raise temperatures in the soil as high as clear plastic, but it is particularly effective in eradicating weeds as it blocks sunlight. This means that weeds can no longer photosynthesize, and they will die and break down under the plastic.
However, some sturdy weeds may be hard to control with black plastic solarization. Hardier species can puncture the plastic, and drought or heat-resistant species can enter a dormant state and regrow when the tarp is removed.
Grass and annual weeds are the first to die off, but hardy or woody weeds may take longer. In the summer months, it can take as little as two months for weeds to die and rot away. During the winter months, you will need to leave the black plastic for around six months.
Soil solarization is effective in killing nematodes- tiny worms that feed on the roots of plants – in the top 12 inches of soil. Although evidence is lacking, it’s believed that earthworms – which are beneficial to the soil – will burrow deeper into the soil to escape the heat. The majority of earthworms in your soil should escape relatively unscathed.
Yes, harmful bacteria and other soilborne pathogens are killed by the extremely high temperatures during soil solarization.
Research has shown that although solarization significantly reduces the population of beneficial soil bacteria, it does not kill them off entirely. Following the sterilization process, beneficial species, like Bacillus and Pseudomonas, recolonize the soil quickly.
Quite the opposite actually. Soil sterilization, speeds up the breakdown of organic material in the soil, releasing more soluble nutrients. Furthermore, since competing pathogens are eradicated during sterilization, there are more nutrients available to be absorbed by plants.
While freezing soil may kill some pests, it is not an effective method of killing fungi, mold, and several types of bacteria and viruses, which will simply enter a dormant state until the soil warms up again. Ensure proper sterilization of your soil by using a heat treatment method.
Soil sterilized by heat, speeds up the decomposition of organic matter, creating a more airy texture. This open structure is ideal for seed germination as stems can easily grow upwards and roots downward. Plus, the absence of weeds in sterile soil, allows seeds to thrive without competition for nutrients.
Additionally, the breaking down of organic matter in the soil increases the availability of potassium, magnesium, and nitrogen, which results in better plant growth and health. A healthier plant is better equipped to fight off disease, if the soil should once again become infected with pathogens and pests.
