Soil Solarization

How to rid your garden soil of early blight & septoria leaf spot pathogens

By: Sandy Feather 2015
Penn State Extension

Q. I have downsized my vegetable garden now that our children are grown and my wife is less interested in canning the produce. Now that my garden is smaller, crop rotation is not very practical, especially because we grow mostly tomatoes and sweet peppers. I have had terrible problems with my tomatoes, and wonder if the soil is contaminated with diseases. I try to garden organically. Is there anything I can add to the soil to help with these diseases?

A. Although you do not specify which disease problems you are having with your tomatoes, several have been problematic for the past few years. They are typically worse when we have wet growing seasons because wet foliage creates a very favorable condition for fungal diseases to flourish. Diseases such as early blight and septoria leaf spot overwinter on bits of plant debris.

Late blight typically does not overwinter in northern states because it requires live tissue to overwinter.

late blight on tomato leaf

The main way it survives in northern states is on potato tubers that were overlooked at harvest or in cull piles. It does overwinter in southern states, and the spores get blown our way on weather fronts and/or storms.


While commercial growers use soil fumigants to minimize insect, disease and weed problems where the same crop is grown every year, home gardeners do not have access to these products. Soil fumigants are restricted-use pesticides, and can only be purchased and used by licensed pesticide applicators. They are extremely toxic and home gardeners should not even think about trying to obtain or use them. There is nothing else you can add to your soil that would have the same effect.

Solarization of Garden Soils

One option home gardeners do have is to solarize their soil. This is a simple process, but it does mean than you will not be able to grow anything in your garden plot for most of the growing season because it must be done during the warmest, sunniest part of the year. Solarization entails covering your plot with clear plastic for 14-15 weeks between June and September. The sun will warm the soil to temperatures that will kill disease-causing organisms, weed seeds and insects.

Organic Matter

Start by preparing your plot next spring as though you were going to plant. Research has shown that incorporating organic matter compost, animal manure, or green manure (cover crops) increases the efficacy of soil solarization. As they break down, they increase the heat trapped under the plastic and release volatile compounds that help kill pathogens. The process of soil solarization does have a negative impact on soil microbes; the addition of organic matter can help protect them.


Till the plot thoroughly, and be sure to break up any large clods of soil. You want the site to be as smooth and level as possible to maximize contact between the soil surface and the plastic. Next, you want to be sure the soil has adequate moisture, which is essential for solarization to be successful. You may be lucky enough that we have a good rain once the plot has been tilled. If not, use a sprinkler or soaker hoses. It is ideal if you can wet the soil to a depth of 2-3 feet. Remember, the plastic will prevent additional moisture from entering the soil once it is in place, so do not skimp on the water.

raking soil to level it
Grading topsoil

Use a durable clear plastic, 2-4 mils thick, that contains ultraviolet inhibitors so that the long exposure to the sun does not cause it to break down until the process is complete. Once the plot has been watered thoroughly, dig a 6- to 8-inch-deep trench around the perimeter of the plot. Lay the plastic by placing one edge in the trench, and then covering it with the soil removed to dig the trench. Stretch the plastic tightly over the ground and cover the remaining edges. You may wish to anchor the edges with bricks or cement blocks so that it does not blow off or lift in strong winds. If that happens, the moisture and heat build-up will be lost, and you will have to start all over again.

How long?

Leave the plastic in place all summer. Remove it in late summer or early fall, and then cover the plot with a mulch such as clean oat straw or plant a cover crop such as winter rye to protect the soil from erosion over the winter. If you choose to use winter rye, be sure to cut it back and till it into the soil as early in spring as possible so that your plot is ready to plant later in spring.

While your plot is covered, perhaps you can grow your tomato plants in large containers. Old nursery pots from trees or large shrubs, or 5-gallon buckets with holes punched in the bottom for drainage work well.


Blighted tomatoes

Late blight on tomatoes

Composting for the garden


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