Posted on

purifying soil to get rid of weed seeds

How to sterilize soil: The ultimate guide for every treatment

Repeatedly using the same garden soil or potting mix over and over again can lead to a buildup of weed seeds, fungal spores, harmful pathogens, and other detrimental garden pests. If you opt to reuse your soil continuously instead of replenishing it, it’s in your best interest to periodically sterilize it. This provides a clean, uncontaminated growing substrate for your plants.

What is soil sterilization?

Sterilizing soil is a process that has been done in commercial greenhouses, and by agricultural producers for quite some time. Greenhouses perform it to save them from replenishing their potting soil every year; farmers sterilize their fields when they have pest or disease problems in high-dollar crops – sterilization is cheaper than treating problems or potentially losing an entire crop.

Soil sterilization plays a bigger role when soil is used for seed germination, the propagation of stem or shoot cuttings, and the transplanting of juvenile plants. These growing conditions increase the susceptibility of damping off and other plant problems if the soil is infected with mold, fungal spores, or other harmful organisms. Soil sterilization is less critical when only growing mature plants, but is still beneficial when performed periodically.

There is a mix of opinions on whether or not soil sterilization is necessary or even good practice for homeowners and small scale gardeners to follow.

Some believe it isn’t worth the time invested for what little benefits are seen; some are concerned it negatively impacts the beneficial aspects of the soil, wiping out good bacteria and microorganisms; others believe it’s critical for good plant growth, especially when gardening in containers or raised beds with potting mixes.

The truth is, pros and cons exist, depending on the method of soil sterilization used. Negative ramifications can be minimized when procedures are carefully followed.

The enthusiast’s guide to herbs

We’re proud to present our new e-book, The Enthusiast’s Guide to Herbs! Learn everything you need to know about growing and caring for herbs indoors, including in-depth info cards for the 35 most commonly grown herbs.

Click the link below to find out more!

Ways to sterilize soil

There are two basic methodologies used to sterilize garden soil and sterilize potting soil: chemical treatments and heat treatments.

Chemical Treatments

This practice is utilized on a larger scale, typically by commercial operations because of the cost and hazards involved with the process.

On the upside, chemical sterilization treatments are easy to use and application can be done quickly, even when sterilizing a large amount of mineral soil or potting soil.

There are some major drawbacks though, which is partly why chemical treatments aren’t as popular, especially with homeowners.

Chemical applications pose a health and safety risk to the person doing the application, and those in close proximity.

Modern chemicals have a very narrow range for which effects will be seen. In most cases, a single chemical will only eradicate one of a slight few diseases or pests.

After application, there is a quarantine period before the soil can be used again. This is necessary to allow the chemicals to degrade fully or be flushed out of the substrate.

If not applied properly, residues may linger in the soil and potentially be taken into the plants through the roots, concentrating in plant tissues.

Long-term use of chemicals can increase the resistance of a disease or insect pest to a given treatment, rendering it less effective or even completely ineffective over time.

Heat Treatments

Due to many of the negative aspects of chemical sterilization treatments listed above, heat treatments are much more commonly utilized by home gardeners.

Heat treatments raise the temperature of the soil through steam or direct, dry heat to a threshold where the harmful organisms, fungal spores, weed seeds, etc. are unable to survive, and die. Heat treatment is often called sterilization, but this is a misnomer as the soil isn’t completely sterile at the temperatures induced.

It does kill weeds and pests depending upon the internal temperature reached, [1] and the length of time the threshold is maintained. Most instructions recommend at least 30 minutes at the given temperature to kill specific organisms (see Table 1) in moist soil or potting mix.

Research has shown that even sterilization significantly reduces the population of beneficial soil bacteria, it does not eradicate them completely. After sterilization, certain species such as Pseudomonas and Bacillus, quickly recolonize the soil and their populations reach high levels within a short amount of time. [2] This quick recolonization overrides the initial reduction in populations through sterilization.

| Table 1. Target Temperatures for Soil Sterilization of Moist Soil | |
| | |
| Target Temp | Organisms Killed when Temp Sustained for 30-minutes |
| 120℉ | water molds (oomycetes) |
| 145℉ | most plant pathogenic fungi, bacteria, and viruses, worms, slugs, centipedes |
| 160℉ | plant pathogenic bacteria, soil insects |
| 180℉ | weed seeds |
| 212℉ | heat resistant plant viruses and weed seeds |

Heat treatments can be harmful if the internal temperature of the substrate is allowed to get too high. Excessive soil heating may increase the chance of phytotoxicity due to soluble salts, manganese toxicity, [3] and toxic organic compounds.

Soil mixtureshigh in readily decomposable organic matter (manure, leaf mold, compost) are more likely to give injury when exposed to excessively high temperatures than mineral soils or potting mixes where the organic matter has completely broken down.

Supplies needed for home soil sterilization

Depending on the method you choose to use, your supply list will vary slightly. Overall the following items are things you already have in your home.

  • Baking pans or other heat-proof containers.
  • A thermometer capable of reading upwards of 200℉. Meat thermometers work well since they can be inserted into the center of a mass of soil, and are easy to clean when you are finished using them.
  • Plastic wrap or aluminum foil.
  • Zip top bags.

Methods to heat sterilize your soil

Choosing which method to steam or heat sterilize your soil is highly dependent on the amount of substrate you are working with, and how quickly you want the process done. The four common methods of heat sterilization different based on their heat source: boiling water/steam, a home oven, a microwave, or natural energy from the sun.

Solarization

Using the natural heat of the sun is a common way to sterilize large amounts of soil, especially entire gardens or fields. The basic principle with solarization is soil is covered with layers of plastic and left to absorb the rays from the sun, raising the temperature over a period of time to kill off problematic pathogens, weed seeds, and pests.

Sterilization through solarization can take place in a couple of different ways. The methods produce the same results; which one you choose is a matter of preference and budget.

Large scale gardens or fields are covered with plastic to trap the sun’s energy.

Larger quantities of soilless potting mixes are spread between a bottom and top layer of plastic to trap the solar energy.

Smaller quantities of either mineral or potting soil are placed in plastic bags and set in the sun.

Choosing what plastic to use

The type and thickness of the plastic used for sterilization has some distinct advantages and disadvantages.

Clear or transparent plastic is a better choice than black for solarization. Black plastic deflects some of the solar energy instead of trapping all of it as clear plastic does.

Thinner plastic (1 mil) allows for better heating but is more susceptible to tearing by wind or animals. Medium thickness plastic (1.5 to 2 mils) works better where conditions are windy. Thick plastic (4 mils or more) should only be used in small areas.

How to solarize soil

Regardless of which specific method of solarization you are following and which type of plastic you choose, similar basic steps should be followed.

Whether working with mineral soil or potting soil from containers, initial prep is important to increase the efficiency of the heat treatment. Start by breaking up all clods and removing litter from previous plants.

If you are working with potting soil, lay down the bottom layer of plastic and spread the potting mix evenly over the top, staying at least 6-inches away from the edges of the plastic.

Irrigate the substrate until they are slightly moistened. Garden soils should be wetted to a depth of 12-inches.

Cover garden soils and potting mixes previously spread out with a layer of plastic. Pull the plastic tightly across the soil surface, securing it with rocks or a layer of soil along the edges. If using plastic bags, fill them with soil and close tightly, setting them in a sunny location in the yard.

Four to six weeks of solarization during the hottest part of the year should be sufficient to sterilize the soil. Areas with cooler, windy, or cloudy climates may be upwards of eight to ten weeks of solarization.

It’s believed that many beneficial soil organisms are able to either survive solarization or they recolonize the soil very quickly afterward. Earthworms are thought to move deeper in the soil profile to cooler areas. This results in a sterilized soil that is quickly recolonized with both beneficial soil organisms and helpful earthworms.

Boiling water or steam

Steam is a great way to efficiently sterilize your soil. It can be done with or without a pressure cooker. If using a pressure cooker be sure to follow all of the manufacturer’s safety precautions.

Pressure cooker method

Fill the pressure cooker with a couple of cups of water and insert the rack into the cooker.

Set heat-proof containers on the rack above the water with no more than 4-inches of soil in each container.

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

Set up your steam container by putting an inch or two of water in the bottom of a non-pressure cooker; then place a rack in the bottom of the sterilizing container.

Set heat-proof containers on the rack above the water with no more than 4-inches of soil in each container.

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.

Sterilizing soil in your oven works well if you have small to medium batches to process, and it poses less danger than using boiling water/steam or the microwave. Keep in mind though as the sterilization occurs your kitchen will be filled with what may be deemed an unpleasant odor. It’s best to do this when you can open doors and windows for adequate ventilation.

Add approximately 3-inches of soil to an over-proof container.

Mix in enough water to thoroughly moisten the soil, but not so much that it becomes runny or overly saturated. Too much water will slow down the process substantially. The water is needed to create steam as it is 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.

Microwave

Using the microwave to sterilize your soil is a good option if you only have a small amount of soil to work with. Carefully inspect the soil before placing it in the microwave to make sure it doesn’t have any metal in it.

Place about two pounds of moist soil inside a clean zip-top, plastic bag. The soil shouldn’t be waterlogged or runny but damp enough it holds together in a clump when you squeeze a handful.

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.

Carefully remove from the microwave, close the top of the bag, and place inside a cooler until the temperature naturally comes back down to the ambient conditions.

Conclusion

Soil sterilization eradicates harmful organisms, weed seeds, and pathogens from mineral soil and potting mixes through chemical or heat treatments. This is advantageous in conditions where soils are used repeatedly to germinate seeds, propagate cuttings, or grow juvenile plants. Providing a “clean” growing environment prevents damping off and encourages strong, healthy growth. A variety of heat treatments can be used by homeowners to cheaply and efficiently sterilize their soil.

Join our email club—get printable info cards free!

Sign up to receive our newsletter and get access to 10 printable plant info cards from our e-book for free. Also receive:

Tips To Sterilize Potting Soil, Garden Soil And Soil For Seeds

Since soil can harbor pests, diseases, and weed seeds, it’s always a good idea to sterilize garden soil before planting to ensure the most optimal growth and health of your plants. While you can go out and purchase sterile potting mixes to meet your needs, you can also learn how to sterilize soil at home quickly and efficiently.

Methods for Sterilizing Soil for Seeds and Plants

There are several ways to sterilize garden soil at home. They include steaming (with or without a pressure cooker) and heating the soil in the oven or microwave.

Sterilizing Soil with Steam

Steaming is considered one of the best ways to sterilize potting soil and should be done for at least 30 minutes or until the temperature reaches 180 degrees F. (82 C.). Steaming can be done with or without a pressure cooker.

If you’re using a pressure cooker, pour several cups of water into the cooker and place shallow pans of level soil (no more than 4 inches (10 cm.) deep) over top of the rack. Cover each pan with foil. Close the lid but the steam valve should be left open just enough to allow the steam to escape, at which time it can be closed and heated at 10 pounds pressure for 15 to 30 minutes.

Note: You should always practice extreme caution when using a pressure for sterilization of nitrate-rich soil, or manure, which has the potential of creating an explosive mix.

For those not using a pressure cooker, pour about an inch (2.5 cm.) or so of water into the sterilizing container, placing the soil-filled pans (covered with foil) on a rack over the water. Close the lid and bring to a boil, leaving it open just enough to prevent pressure from building up. Once the steam escapes, allow it to remain boiling for 30 minutes. Allow the soil to cool and then remove (for both methods). Keep foil on until ready to use.

Sterilizing Soil with an Oven

You can also use the oven to sterilize soil. For the oven, put some soil (about 4 inches (10 cm.) deep) in an oven-safe container, like a glass or metal baking pan, covered with foil. Place a meat (or candy) thermometer into the center and bake at 180 to 200 degrees F. (82-93 C.) for at least 30 minutes, or when soil temp reaches 180 degrees F. (82 C.). Anything higher than that can produce toxins. Remove from oven and allow to cool, leaving the foil in place until ready to use.

Sterilizing Soil with a Microwave

Another option to sterilize soil is to use the microwave. For the microwave, fill clean microwave-safe containers with moist soil– quart size with lids are preferable (no foil). Add a few ventilation holes in the lid. Heat the soil for about 90 seconds per every couple pounds on full power. Note: Larger microwaves can generally accommodate several containers. Allow these to cool, placing tape over the vent holes, and leave until ready to use.

Alternatively, you can place 2 pounds (1 kg.) of moist soil in a polypropylene bag. Put this in the microwave with the top left open for ventilation. Heat the soil for 2 to 2 1/2 minutes on full power (650 watt oven). Close the bag and allow it to cool before removing.

Soil Solarization in Raised Beds

For a while I was lucky with my raised bed garden, and had few problems with pests and diseases. But this summer, a vicious case of wilt (I’m guessing Fusarium wilt) weakened or stunted several of my plants (mostly peppers) and a hungry colony of flea beetles had taken up residence in my tomatillo crop.

Combined with the drought in California, a particularly hot season, and a month-long vacation looming, I decided to put all those problems to bed — under a sheet of plastic for the remainder of the summer.

Soil solarization is a highly effective, nonchemical method for controlling soilborne diseases. While it’s commonly used on commercial farms, it’s not as prevalent in home gardens because it does require part or all of the soil to lay fallow during peak summer. In a home garden where space is often limited, it’s hard to give up a raised bed for the four to six weeks it takes to treat the soil.

But if you plan ahead (or, like me, you know you’ll be away from the garden for an extended period), soil solarization is an ideal solution for killing weed seeds, controlling nematodes and pests, eliminating soilborne plant pathogens, and improving tilth and soil biology. Think of it as a solar oven in the garden, baking everything underneath it — and what comes out is sterile soil, free of the problems that used to plague your plants.

The Benefits of Soil Solarization

Simply by using the power of the sun, solarization can rid the soil of most weed seeds, especially those from annual weeds (some perennial weeds, like Johnson grass, may have deep roots or rhizomes that the heat won’t reach).

The intense heat also speeds up the decomposition of organic material in the soil, thereby releasing soluble nutrients such as nitrogen, potassium, calcium, and magnesium and making them more available to plants.

Solarization disinfests the soil of the fungal and bacterial pathogens that cause Fusarium wilt, Verticillium wilt, potato scab, Southern blight, early blight, tomato canker, club root, crown gall, and damping off. (Note that it doesn’t work on airborne diseases, like garlic rust or late blight.) Certain fungi, such as the spores that cause Fusarium wilt, can live for many years in the soil and even in the soil clinging to your garden tools, so in these cases crop rotation is not an effective means of control.

Solarization also reduces populations of nematodes (like root knot and dagger) and destroys the eggs, larvae, and pupae of destructive pests (like cucumber beetles and squash vine borers).

But What About the Good Guys?

It’s believed that earthworms simply burrow deeper into the soil to escape the heat, and come back to the surface when conditions are ideal. As for beneficial soil organisms, many of them are able to either survive solarization (such as mycorrhizal fungi, which are highly heat tolerant) or rapidly recolonize the soil (such as the bacteria and fungi that parasitize plant pathogens and stimulate plant growth).

In fact, the increased numbers of beneficial microbes can make the soil more resistant to pathogens after solarization (as opposed to nonsolarized or fumigated soil). In turn, plants grow faster, get stronger, and stay healthier.

Timing Is Everything

Soil solarization works by trapping radiant energy from the sun under a thin plastic tarp to heat the soil at temperatures high enough to kill soil organisms.

Soil temperatures of at least 99°F, held steady for about four weeks, will prevent the emergence of many annual weeds and mesophilic fungi, which are the majority of common plant pathogens. That means soil solarization works best in the hottest month of the year when days are long and skies are clear, which for many zones is around the summer solstice in June or July.

Out on the Southern California coast where I live, our hottest months are September and October, when Santa Ana winds blow in from the high desert and bring us sweltering Indian summers (as well as notorious California wildfires).

Even with a relatively mild high of 75°F in early September, before the heat wave’s hit, the first 3 inches of soil in my raised bed (that gets full sun for most of the day) heats up to 116°F. In just a few weeks, that temperature will steadily rise and cook whatever’s still lingering under the plastic.

Soil solarization is less effective in the spring, even if your garden is bathed in sunshine all day. The mild weather does little to control soilborne diseases, so it’s not worth the effort to try early in the year before you start your planting.

For best results, determine the hottest four- to six-week window in your climate and plan to solarize your soil in that time.

Preparing the Soil

Before you begin, remove all the plants and mulch from the bed. If they were affected by disease, bag and trash them.

For successful solarization, the soil should be smooth and flat to allow the plastic to lay snug against it. Till or turn over the soil and remove or break up any clods, rocks, weeds, and plant debris. You don’t want anything in the soil that could potentially tear or puncture the plastic. I typically dig to a depth of a fork tine, but for heavy clay soils or soils that haven’t been cultivated in a while, going at least 1 foot deep is recommended.

Studies have shown that adding organic animal- or plant-based amendments (such as aged animal manure or cover crop residues, especially Brassicaceae cover crops like mustard) before solarizing improves its efficacy. The amendments not only increase the rate of heat generation in the soil, but also its heat-carrying capacity. And when it comes to soil solarization, more heat is always a good thing.

So, if you have some composted chicken manure, worm castings, bat guano, green manure, or well-aged kitchen compost, go ahead and incorporate that into the top 3 to 4 inches of soil, and rake it in well.

Once your soil is smoothed over, set your garden hose or irrigation system over the bed and water deeply. You want the top 12 inches of soil to be moist. The moisture conducts heat faster and deeper into the soil, while making soil pathogens more sensitive to the heat.

Choosing the Plastic

Surprisingly, clear plastic is actually more effective than black plastic at heating up soil. The sun’s rays easily pass through clear plastic, only to be trapped inside to heat the soil. By contrast, black plastic tends to absorb and deflect part of the heat.

And though it might seem counterintuitive, the thinner the plastic, the better it captures and traps the heat. Very thin plastic (1 mil) is the best option, but can be susceptible to damage from birds, critters, or the elements. A good compromise is 1.5 mil to 2 mil plastic with weatherproofing or UV resistance, as it will last long enough until you remove it from your bed.

I use a 2 mil plastic dropcloth (found at my local hardware store), but for larger gardens, you can find bulk rolls of polyethylene from farm supply stores or online.

Covering Your Raised Bed

Cut the plastic to fit, leaving at least 8 inches of overhang on all sides. There are a few ways to secure the plastic:

  • You can dig a trench along the inside perimeter of the raised bed and then bury the edges of the plastic 6 to 8 inches deep.
  • You can hold the plastic down with heavy objects (like 2x4s, pipes, planks, paving stones, or whatever you have around the house) placed around the perimeter of the bed, making sure they’re tucked tightly against the walls.
  • You can staple gun the plastic to the bed itself if the walls are made of wood.

Staple gunning the plastic is the way I do it, and I simply wrap the corners around the bed as if I was wrapping a package. Staples go in every 6 to 8 inches to ensure a secure seal, and then a couple of weights are laid on top of the plastic to keep it stable under any winds. (You don’t want your tarp to become a sail during a summer storm!)

The weights can be bricks, beams, sandbags, stones, or even dirt (but make sure you remove this dirt before you take the plastic off, as you don’t want to contaminate your newly sterile soil with it). You also want to be careful with sharp or rough-edged weights that could cause pressure points, leading to tears.

The idea is to keep your plastic as flat against the soil as possible, with little to no flaps or openings that could let in outside air (and decrease the temperature under the plastic). Patch up any holes in the plastic immediately with duct tape, and keep an eye on your beds throughout the four- to six-week period in case the plastic needs patching. (One morning, I found a few holes in one of my tarps that had apparently been caused by a raccoon running through the garden.)

If your daytime temperatures are cooler than normal, you can increase the amount of heat generated in your raised beds by adding a second layer of plastic over the first layer. Separate the layers slightly with PVC pipes, plastic bottles, or other smooth objects that can run the full length of the bed; that small pocket of air can increase the heat in the soil by as much as 10°F.

Maintaining the Proper Temperature

Soil solarization is most effective when the top 6 inches of soil is maintained at or above a daily temperature of at least 110°F for four to six weeks, as most pathogens reside in this upper layer. If you don’t have a soil thermometer to test this, a meat thermometer works well. I like to get a reading in the middle of the day right in the middle of the bed; afterward, I simply patch the hole with duct tape.

See all this condensation under the plastic? When I rest my hand on the surface, I can feel how how it is under there. That’s what you want, every single day.

Post-Solarization

After removing the plastic, you can sow seeds or plant transplants like normal. To avoid bringing any surviving weed seeds to the surface, stick with shallow plantings.

There’s no need to further cultivate the soil. You’re already starting fresh with soil that’s gained additional nutrients from solarization, so fertilizer can wait until midway through the season (simply do a side dressing, soil drench, or foliar spray as needed).

To improve your chances of not reinfecting the soil, always start with clean pots and new soil for seed starting and transplanting, and wash away the old (possibly contaminated) dirt from your garden tools, gloves, and other accessories that frequently come in contact with your plants.

Linda Ly

I’m a plant lover, passionate road-tripper, and cookbook author whose expert advice and bestselling books have been featured in TIME, Outside, HGTV, and Food & Wine. The No-Waste Vegetable Cookbook is my latest book. Garden Betty is where I write about modern homesteading, farm-to-table cooking, and outdoor adventuring — all that encompass a life well-lived outdoors. After all, the secret to a good life is. Read more »