Basics of Soil Solarization, Fully Explained

garden soil solarization new garden beds

By Erin Marissa Russell

Soil solarization uses high temperatures to kill insects, weeds, and pathogens in moist soil, using the heat of the sun instead of the chemicals often employed. It requires enough plastic sheeting to cover the desired area, water, and a bit of time and energy.

Why You Should Use Soil Solarization

Soil solarization is a nontoxic pest control method that does not use any chemicals or leave chemical residues in your garden. It simply uses plastic sheeting to trap the power of the summer sun.

This is a relatively affordable method of pest control, compared to the other options available. Your only expense will be the plastic sheeting and perhaps some UV-resistant glue, if you need to glue the edges of the plastic together to cover a large area.

Pest and weed control isn’t the only benefit to soil solarization. Soil structure is improved after treatment, and nitrogen as well as other nutrients become more attainable for plants growing in soil that has been solarized. Plants grown in solarized soil tend to grow better, yield more crops, and yield higher quality produce than plants grown in soil that has not been solarized.

Although the population of beneficial organisms in your soil may be negatively affected by solarization, this is not a permanent state. Some of these organisms will survive solarization, while others will recolonize after the process is over. The long term effect of soil solarization results in population growth for beneficial organisms such as fungi that are natural enemies of plant pathogens and have a positive effect on plant growth.

How to Know If Soil Solarization Is Right for You

You must live in a place that gets warm enough for soil solarization to be effective. People in climates that don’t get warm enough aren’t able to use sun solarization to treat their soil.

Similarly, it must be the right time of year for sun solarization. The best time is the hottest part of the year, from May to September. Look for a six-week period during the hottest part of the summer when winds won’t be too strong.

Solarization requires an empty field or bed where nothing is growing. You cannot solarize soil where plants are growing without killing the plants.

How to Use Soil Solarization

Create as smooth a surface as you can in the area you will solarize. Get the soil ready for its solarization treatment by tilling or turning the soil by hand, breaking up any clods you find and leaving the soil’s surface smooth. Remove objects large enough to raise the plastic or puncture it, such as rocks and weeds.

One method of soil solarization, called complete coverage, requires covering the entire area with plastic sheeting. Another method called strip coverage installs plastic only over the beds or certain areas in the field. Strip coverage is not as effective against pests because the untreated soil around the edges of the strips can recontaminate the treated areas.

You can install the plastic sheeting either using a machine like a tractor or by hand. If you install the plastic sheeting by hand, you will need to either use a UV-resistant glue to glue the edges together over your desired area or bury the edges of the plastic sheeting. Dig a shallow trench to hold the edges of the sheeting. You can avoid untreated areas by laying adjacent strips and burying both edges of the adjacent strips.

You must either water the soil where you are doing soil solarization before laying out the plastic or afterward. If you water the soil before, you should lay the plastic out immediately afterward so you do not lose water from the soil. Your soil needs to be moistened to 70 percent of field capacity, down to a depth of 24 inches, for sun solarization to work effectively.

Keep in mind that if you use machinery to lay down the plastic, the soil must be dry enough that it is not compacted in the process. If you use machinery to lay the plastic, it may be best to irrigate the soil after installing the plastic. You can use the indentations left by the tractor wheels as irrigation ditches.

The plastic must be kept in place for six weeks to be effective. There are some pathogens and weeds that will die within days of treatment, but many others require the full six-week treatment period.

After the treatment period, remove the plastic from your garden, making sure to get the buried edges and any pieces of plastic that may have come off the sheets.

Choosing Plastic for Soil Solarization

The success of your solarization treatment depends on using the right plastic for the job. First of all, this means using clear plastic instead of white, colored, or even black. Clear plastic does the best job of capturing heat and sending it into the soil. Although it may seem that black would work well, it actually absorbs and holds some of the heat, so the soil doesn’t get the full effect.

As far as thickness goes, look for the thinnest plastic you can get. Polyethylene plastic with a thickness of 1 mil solarizes well and is affordable. However, you may find it is not very durable and can be torn easily by the wind or by animals. If it is windy in your region, you should use a thickness of 1.5 to 2 mils.

For best results, look for plastic that has ITV-inhibiting additives in it. These will make the plastic more durable and prevent it from becoming brittle and making removal harder than it would be otherwise.

If you are solarizing on a relatively small scale, such as in a home garden, you will easily be able to find plastic sheeting at your local nursery, garden center, or hardware store. The plastic is sometimes sold under the name “drop cloths” and is 1.5 to 4 mils thick. If you are solarizing on a larger scale, as with agricultural installations, you can buy rolls of plastic that are six feet to 12 feet wide and about 4,000 feet long. The exact size varies depending on where you purchase your plastic.

More Tips for Soil Solarization

If your plastic is torn during the solarization process, you can repair it easily using clear packing tape.

If you lay your plastic sheets out by hand, you may be able to reuse them for soil solarization. However, if the plastic has gotten dirty or dusty during the solarization process, it will not work as well if used again.

You may see polyethylene sheets that have an additive allowing them to absorb infrared radiation (IR). This is supposed to make the plastic better able to retain heat. However, experts say these additives are not really effective for soil solarization.

Do not turn over, till, or aerate the soil after solarization treatment. This can cause pathogens at deeper levels of the soil to recontaminate treated soil at the surface.

Soil solarization can also be used on soil that is in containers or bags.

Now you’ve learned enough about soil solarization to determine whether it’s right for you. As long as you live in a warm enough region, it’s the right time of year, and the area you’d like to solarize is empty, you should be successful. Just be sure to use clear plastic that is as thin as you can get, and leave the plastic on the soil for the full six week treatment period.