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Soil Solarization for Control of Soil-Borne Pathogens & Weeds

HGIC 1261

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Soil solarization is a non-chemical method for controlling weed seeds, bacteria, fungi, and nematodes. As a non-chemical control method it fulfills many gardeners’ wishes for organic or low toxicity control. It is a potential alternative to chemical fumigation.

Soil solarization is usually only considered as a remedy for challenging problems in the home garden. This is because the treated area will be out of production for nearly 2 months during the summer growing season. The basic procedure is to cover the soil with a clear plastic and trap the sun’s heat to raise soil temperatures high enough to kill weed seeds and harmful plant disease organisms.

Solarization can be applied in many areas of the home landscape, but no plant material can be present in the area to be solarized. The extremely high soil temperatures will kill any plants left in the solarized area, and plant remains will interfere with even heating.

Vegetable garden plots and annual flowerbeds are the best areas for control. The larger the plot area the better control achieved. Treated areas greater than 30 inches wide seem to have the best results. Areas smaller than 30 inches have a cooling affect on the outer edges.

Procedure

Soil solarization works well in South Carolina because of the high temperatures during the summer months. The warmer the climate during the growing season and the longer you have to leave the plastic in place, the better results you will obtain. Long periods of heating are necessary to achieve good control of hard-to-kill weeds like crabgrass and nutsedge.

Soil Preparation: The first step in the solarization process is to cultivate the bed and prepare it for planting to avoid disturbing the soil afterwards. Disturbing the soil afterwards will bring weed seeds to the surface that have not been affected by the heat treatment. Only the top 2-3 inches of soil receive the greatest results. Apply fertilizer or irrigation equipment that will be needed prior to solarization.

Till the soil thoroughly, 6-8 inches deep, to destroy all large clods and remove any existing plant material. Leaving clods or plant material will interfere with uniform conduction of heat through the soil, allowing some pests to remain protected from the effects of the treatment. A well-tilled, smooth surface provides good contact between the plastic mulch and the soil surface.

Irrigation: Next, after the bed is cultivated and raked smooth, irrigate the area. This is a very important step in the process. Soil moisture is required for two different reasons, (1) to increase the transmission of heat through the soil, and (2) to make seeds and other resting structures more sensitive to high temperatures. Seeds are very heat resistant when dry.

A sprinkler or drip-line irrigation line gives the best results. Water the soil heavily so that it is wet, yet without standing puddles. Once the soil has been irrigated there is no need to irrigate after laying the plastic.

Trenching: Dig a trench approximately 6-8 inches deep and wide around the perimeter of the plot. The trench will be used to bury the edges of the plastic to be placed over the plot.

Covering: Cover the soil with clear UV stabilized plastic (polyethylene or PVC), and bury the edges in the trench. Stretch the plastic as tightly as possible. Once the plastic is in place, refill the trench, covering and sealing the plastic edges. This will ensure that the plastic is held in place and will prevent heat from escaping or allowing wind to get beneath. Bricks or stones can be used to further protect the edges against wind or other disturbance.

A double layer of plastic will increase the effectiveness of solarization. An air gap should be created between the two layers of plastic giving extra insulation. This air gap can be maintained by placing objects such as empty soda pop cans between the layers of plastic.

The time needed to get high levels of control is dependent on temperature. Using clear polyethylene allows sunlight to pass through directly to heat the soil. Thin clear plastic (1-2mil) allows more sunlight to pass through and penetrate the soil. Thin clear plastic favors more rapid and deep heating of the soil than does black plastic.

Leave the plastic in place for 4-8 weeks during the hottest part of the summer. In South Carolina the best time to solarize your soil is from June through August. The effectiveness of soil solarization is determined by the combination of soil temperature and exposure time. With increasing temperature, less time is needed to reach a lethal combination of time and temperature.

Excess time under solarization will not be harmful to your plot. If you are not planning to plant immediately, leaving the plastic in place can only maximize the treatment and will act as a barrier against recontamination.

Removing Plastic and Planting: After your plot has been exposed to high temperatures for 4-8 weeks, the plastic mulch can be removed. Removal of the plastic should be done very carefully without disturbing the soil. Do not re-till the soil. This will bring weed seed and plant disease organisms from depths that were not affected by the treatment and recontaminate the plot.

This is why any soil preparations like fertilization or irrigation systems should have been completed prior to covering with the plastic.

Disposal of the plastic can be considered an environ-mental pollution problem. The clear polyethylene plastic is able to withstand the high temperatures it is exposed to better than black plastic. The plastic can be taken up and saved for a second treatment. A second use for the plastic would be to paint the clear plastic white as mulch for tomatoes, peppers and other plants, which would benefit from the higher soil temperatures in the fall. If you are planning to plant crops that need cooler soil temperature, the plastic should be completely removed in time for the soil to cool before planting.

Once you have removed your plastic mulch you are ready to plant. If you are not planning to plant until the following spring, cover the area with weed-free mulch and avoid activities that will disturb the soil.

Factors That Influence Success

Soil characteristics such as color, structure, and depth have an effect on the effectiveness of soil solarization. Dark colored soil tends to adsorb more solar radiation in comparison to light colored soils. Loose, friable soils enable heat to be transmitted throughout the soil and allow heat to go deeper. Control is much better in the top few inches of the soil and decreases with soil depth.

The level of control achieved depends on a combination of the duration of the solarization process, the high temperatures reached and the susceptibility of the weed species to be controlled. Fungi that live in the top 2-3 inches of soil, like Rhizoctonia or the Southern blight fungus, are easier to control with solarization than other plant disease organisms.