Soil Solarization: Control Pests, Weeds and Soil Diseases

What is Soil Solarization?

Soil solarization is an environmentally friendly and inexpensive method to not only control pests, weeds, and soil diseases, it can also help plants thrive by releasing more nutrients into the soil.

Farmers and home gardeners can reap the many benefits of applying this simple, non-chemical, natural solution.

How Does Soil Solarization Work?

Knowing how soil solarization works is the key to success. For six to eight weeks a sheet of clear plastic covers the soil to trap the heat from the sun. Trapping the heat allows the soil to reach temperatures that are lethal for most weeds and pests.

Soil solarization can be used in raised beds and any type of soil, but lighter soils, like sand, may take longer. This method works best during the hottest months of the year, generally between June and August, and in areas of the farm or garden that get full sun.

Soil solarization has also shown to be effective in the spring before planting begins and in the fall after the harvest has ended.

How to Solarize Soil

Prepping to the soil

The main goal is to make the soil texture as fine as possible. To get this texture, use a rototiller or shovel and rake to rid the area of debris, clumps, and weeds.

Till the soil, at least six inches or deeper, then rake. Make sure the area is smooth and flat with no slopes, or as little slope as possible. The best solarization will be in areas with south to southwest sun exposure.

After tilling, irrigation of the soil to at least 12 inches in depth is needed because it will conduct more of the sun’s energy. The soil cannot be too muddy or too dry; the perfect balance is needed to help the microorganisms eliminate soil-borne diseases, pests, and weeds.

It is best to cover the soil with the plastic sheeting within 24 hours after irrigating the area or after a good rain. If preferred, a hose drip can be placed to irrigate the soil after applying the plastic.

Plastic Sheeting: Choices and Placement

The plastic sheeting can be found at nearly every garden center. The sheeting chosen should be transparent or clear to allow the sun to heat the soil below.

The thinner the better, but thinner plastic has a tendency to tear in areas where higher winds are likely and in areas that could be walked on by humans or wildlife.

To maximize the benefits of soil solarization, the plastic sheeting should lay directly on top of the soil. To keep the sheeting in place, dig a trench up to six inches in depth around the area to be solarized and bury the edges in the trench.

It is important that you keep the sheeting tight to the ground. If you are solarizing a raised bed, choose plastic sheeting that is longer than the treatment area so the edges can be buried in the trench.

Soil Solarization Duration and Replanting

To be the most effective, the temperature of the first six inches of soil should be between 110 degrees Fahrenheit to 125 degrees Fahrenheit. Using a soil thermometer will ensure the solarized soils are reaching or exceeding the recommended daily temperatures.

At these temperatures, most pests, weeds, and soil diseases should be eliminated. If soil solarization is being used during the summer months, the plastic sheeting should not be removed for four to six weeks.

If the soil temperatures reach higher than daily recommended temperatures, the solarization period could be shorter. In cooler climates or during cooler seasons, like spring or fall, the sheeting can be left on for eight weeks to ensure the soil is receiving sufficient heat. 

Another trick to help create more heat is to use two thin layers of the plastic sheeting separated by unused plastic bottles, leftover pipes, or anything that will create a space.

When solarization is complete, the plastic can be left on or a fall crop can be planted. If the decision is made to leave the plastic on, regular checks of the soil to ensure that it is not degrading.

While solarization can eliminate many harmful organisms, it can also eliminate many useful organisms. Adding hummus, compost, mulch, and other organic soil amendments will help repopulate the useful organisms. 

Benefits of Soil Solarization

Research from the University of California Integrated Pest Management Program shows that the solarization process accelerates the breakdown of organic materials in the soil which then releases beneficial plant nutrients like potassium, magnesium, calcium, fulvic acid, and nitrogen.

Their research also shows that many of the beneficial organisms, like mycorrhizal fungi, can repopulate quickly after soil solarization. Beneficial fungi and bacteria help plants grow while protecting them from pathogens.

It is also believed that earthworms are not harmed during soil solarization because they can escape the treatment area by burrowing further into the earth.

Many of the pests, weeds, and soilborne diseases may be eliminated in the first few days of soil solarization, but some require the treatment for the recommended time of four to six weeks.  

Problems that Can be Controlled When You Solarize Soil

Fungal and Bacterial Pathogens

Verticillium wilt, crown gall disease, tomato canker, Fusarium wilt, southern blight, potato scab, and Phytophthora root rot are caused by fungal and bacterial pathogens.

Pests and Insects

Insects and pests that can be detrimental to many plants spend at least one stage of their life in the soil: eggs and worms or larvae. Fungus gnats and root aphids are examples of pests that dwell in the soil. 

Nematodes

There are beneficial nematodes, like those that can infest grubs, but some nematodes cause damage to a plant’s flowers all the way down to the roots.

Solarization can be especially useful when nematodes are causing problems, but it may not eliminate all species. Nematodes can move away from the area being solarized and return when the treatment is finished.

The nematode species that are susceptible to soil solarization include: Criconemella spp., D. heterocephalus, Rotylenchulus reniformis, M. incognita, Belonolaimus longicaudatus, and P. minor.

Weeds

Some species of weeds are resistant to soil solarization, like perennials, because of their deep root systems. Annual weeds are less resistant to soil solarization because of their shallow root systems.

If rhizomes of bermudagrass and johnsongrass are close to the soil surface, solarization can be used to control or eliminate them. Mixed outcomes have resulted when trying to control field bindweed, some species of clovers, and purple and yellow nutsedge.

To successfully eliminate most weeds in the garden or growing area, all recommendations to solarize need to be followed including full sun exposure and the correct plastic sheeting.

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Reap the Benefits of Soil Solarization

Instead of choosing pesticides that are known to cause damage to the environment, choose a more natural approach to eliminating pests, weeds, and soilborne diseases.

Soil solarization can be more cost-effective for gardens and grows that are mid- to large-scale.