By using the same potting soil or garden soil repeatedly, plant seeds, fungi, and pathogens can accumulate, which can lead to garden pests. In order to protect your soil from microbial growth, you must periodically sterilize it if you choose to reuse it rather than replenish it. It provides your plants with a clean, uncontaminated growing medium.
What is Soil Sterilization?
For a long time, soil has been sterilized in commercial greenhouses and by agricultural producers. Farmers sterilize their fields when pests or diseases infest high-dollar crops – sterilization is cheaper than treating the problem or potentially losing the crop. Greenhouses perform it to avoid replacing their potting soil every year; farmers sterilize their fields when they have pest or disease problems.
In soil used for seed germination, propagation of stem or shoot cuttings, or transplanting of juvenile plants, soil sterilization is a more important step. Mold, fungal spores, and other harmful organisms can infect the soil in these conditions and cause damping off and other problems for plants. When growing mature plants, soil sterilization is less important, but it is still beneficial when performed periodically.
There is some controversy regarding soil sterilization, including whether or not it is necessary for small-scale gardeners.
For what little benefit it brings, some people are of the opinion that it is not worth the effort. This may be detrimental to the beneficial aspects of soil, wiping out beneficial microorganisms and bacteria. Many gardeners believe it is essential for plant growth, especially in containers or raised beds.
As for the pros and cons of soil sterilization, they depend on the technique used to do so. By following procedures carefully, negative effects can be minimized.
Ways to Sterilize Soil
Sterilizing soil using either chemicals or heat treatments is the most common way to sterilize garden soil or potting soil.
Commercial operations tend to use this process at a larger scale because of its high cost and potential hazards. It is easy to use chemical sterilization treatments and it can be applied quickly, even for huge amounts of mineral soils. Chemical treatments have some drawbacks, which is why they are not as popular, especially with homeowners.
Health and safety risks are associated with chemical applications, both for those doing the application and for those nearby. Chemicals today have very narrow ranges where their effects can be seen. Most of the time, one chemical is only able to eliminate one of a few diseases or pests. Immediately following application of the soil, a quarantine period applies. It is necessary to let the chemicals degrade fully or flush them out of the substrate.
Residues may accumulate in soil if they are not applied properly and be taken up by plants through root system, concentrating in plant tissues. As a result of prolonged exposure to chemicals, diseases or insect pests become more resistant to the treatment, rendering it less or even not effective over time.
Compared with chemical sterilization, heat treatments are much more common among home gardeners because of the many negative aspects listed above.
Steaming or direct, dry heat is used to raise the soil’s temperature to a level that is too high for harmful organisms, fungi, and plants to survive, and die. The word sterilization is often used to describe heat treatment, but in reality, soil is not totally sterile when heated.
The level of internal temperature reached, and the time the threshold is maintained, will determine whether or not it kills weeds or pests. In moist soil or potting mix, most instructions recommend at least 30 minutes at the given temperature to kill specific organisms.
Research shows that although sterilization can considerably reduce beneficial soil bacteria populations, it does not eradicate them completely. Within a short amount of time, certain species such as Pseudomonas and Bacillus, resume colonizing the soil after sterilization. The rapid recolonization of the area overrides the initial population reduction resulting from sterilization.
In heat treatment, if the internal temperature of the substrate gets too high, it can cause harm. In addition to soluble salts, manganese toxicity, and toxic organic compounds, excessive soil heating may increase the risk of phytotoxicity.
The organic material in soils high in readily decomposable organic matter (manure, leaf mold, compost) is more susceptible to damage when exposed to excessively high temperatures than mineral soils or potting mixes where the organic material has completely decomposed.
Methods to Heat Sterilize Your Soil
The method by which you steam or heat sterilize your soil highly depends on the amount of substrate you are working with, and the speed at which you want the process completed. The four common methods of heat sterilization differ based on their heat source: boiling water/steam, home ovens, microwaves, and solar energy.
To sterilize large areas of soil, especially whole fields or gardens, the sun’s natural heat is often used. Solarization involves covering soil layers with plastic and letting the sunlight penetrate to heat the soil over time, killing pathogens, weed seeds, and pests.
The process of solarization can be accomplished in a few different ways. Each method produces similar results, so the choice comes down to preference and budget.
- A large scale garden or field is covered in plastic to trap the energy of the sun.
- The soilless mix is spread between a bottom and a top layer of plastic so that solar energy can be absorbed.
- Mineral soil or potting soil is put into plastic bags and set out in the sun.
Choosing what plastic to use
When sterilizing, there several different advantages and disadvantages depending on the material.
It is better to use clear or transparent plastic than black for solarization. Unlike clear plastic, black plastic deflects some solar energy rather than trapping it all.
Plastics with a thinner (1 mil) thickness can be better insulated, but are more susceptible to tearing by wind or animals. Windy conditions are better served by plastics of medium thickness (1.5 to 2 mils). The use of thick plastic (four millimeters or more) should be restricted to small areas.
How to solarize soil?
Whatever method you use for solarization and whatever type of plastic you use, the basic steps remain the same.
- No matter what type of soil is used, the initial prep is paramount in increasing the efficiency of the heat treatment. Break up all clods and remove the litter from previous plants.
- Using potting soil, put down the plastic on top, then spread the mix evenly over the top, staying at least six inches away from the edge of the plastic.
- The substrate should be lightly moistened by irrigation. You should water the soil in your garden to a depth of 12 inches.
- A plastic sheet should be placed over the previously spread garden soil and potting mix. Wrap the plastic tightly around the soil surface, securing it with rocks or a layer of soil along the edges. You can also use plastic bags filled with soil and placed in a sunny position in your yard.
- The soil should be sterilized within four to six weeks of solarization during the hottest part of the year. It may take up to 10 weeks for areas with cooler, windy, or cloudy climates to reach solarization.
According to some research, many soil organisms are able to survive solarization or become recolonized quickly afterward. It is believed that earthworms move into cooler soil layers as they move deeper into the soil profile. Soil treated in this way quickly recolonizes with beneficial organisms and helps to regenerate healthy earthworms.
Boiling water or steam
Using steam to sterilize your soil is a great way to achieve maximum results. You can do it with or without a pressure cooker. Use a pressure cooker only if it is labeled with all of the manufacturer’s safety precautions.
Pressure cooker method
- Place the rack inside the pressure cooker along with a couple of cups of water.
- A rack with heat-proof containers above the water should have no more than four inches of soil in it.
- Cover each container tightly with foil.
- Place the lid on the pressure cooker, leaving the steam valve open slightly to allow steam to vent until the pressure begins to build.
- Adjust the heat source under the pressure cooker, allowing it to build steam.
- When the steam valve closes, process the soil at 10 pounds of pressure for 15 to 30 minutes.
- Remove from the heat source and allow the pressure to subside completely before removing the lid.
- Keep sterilized soil covered with the aluminum foil until it’s time to use.
Non-pressurized container method
- Start by filling the bottom of non-pressure cookers with about an inch of water. Then place a rack at the bottom.
- The heat-proof containers should not contain more than 4 inches of soil in each. Place the rack over the water.
- Cover each container tightly with foil.
- Place a lid on the container, leaving it cracked slightly to prevent steam from building up.
- Once the water comes to a boil, allow it to boil gently for 30 minutes.
- Remove from the heat source and allow the temperature to drop before removing containers.
- Keep sterilized soil covered with the aluminum foil until it’s time to use.
If you have small or medium amounts of soil to sterilize, you can do so in the oven at a lower temperature with less danger than if you used boiling water or steam or used a microwave. The sterilization process, however, will result in an unpleasant odor filling your kitchen. You should do this when you can open doors and windows for adequate ventilation.
- Add approximately 3-inches of soil to an over-proof container.
- The soil should be thoroughly moistened, but not drenched in so much water that it becomes runny or saturated. A large amount of water will significantly slow down the process. In order to create steam, water needs to be driven off.
- Cover the tops of the containers with aluminum foil and place in an oven preheated to 200℉.
- Monitor the internal temperature of the soil with your thermometer. When it reaches 180℉, allow it to “bake” for thirty minutes without opening the oven door.
- Shut the oven off and allow the soil to cool to room temperature.
The microwave can be a good way to sterilize soil if you have only a small amount. Be sure that the soil does not contain any metal before you microwave it.
- Fill a clean zip-top bag with about two pounds of moist soil. It shouldn’t be waterlogged or runny, but damp enough to hold together in clumps when squeezed.
- Leave the top of the bag open and place it in the center of the microwave.
- Microwave on high until the middle of the soil reaches a temperature between 180℉ and 200℉. The length of time this takes to achieve will depend on how powerful your microwave is.
- Place the bag inside a cooler as soon as the temperature naturally returns to ambient levels after being removed from the microwave.
Supplies Needed for Home Soil Sterilization
Your supply list will vary slightly depending on the method you choose to use. All of these items can be found in your home already.
- Baking pans or other heat-proof containers.
- A thermometer that can measure up to 200°F. They are convenient to use since they can be inserted into the center of soil masses, and can be cleaned easily once you are done using them.
- Plastic wrap or aluminum foil.
- Zip top bags.
Through chemical or heat treatment, soil sterilization removes harmful organisms, weed seeds, and pathogens from mineral soil and potting mixes. It is advantageous when soils are repeatedly used to germinate seeds, propagate cuttings, or grow juvenile plants. By “cleaning” the growing environment, damping off is prevented and strong, healthy growth is encouraged. Homeowners can use a variety of heat treatments to sterilize their soil cheaply and efficiently.