Soil Solarization

Also known as soil tarping and soil pasteurization, soil solarization consists of trapping the heat of the sun under a clear plastic tarp to kill many soil pests. The tarp is placed over the soil surface for from four to six weeks during the warmest part of the year. How hot your soil gets depends on the weather conditions and where you live. Under the intense sun of the Sacramento and San Joaquin Valleys of California in early summer, the soil 2 inches deep usually reaches 140 degrees and the soil 18 inches deep is generally elevated to 102 degrees at the end of solarization.

The process has proved capable of controlling verticilium wilt, some fusarium wilts, clubroot, southern blight, lettuce drop, root rots, and other diseases. Although it won’t control nematodes completely, it does reduce infestations. In combination with other methods, soil solarization allows shallow-rooted crops to be grown in nematode-infested soil. It also kills many kinds of weeds and weed seeds, especially those of weedy grasses.

Although researchers aren’t quite sure why, plants often grow better in solarized soil, even when no pests were known to be present. Solarization seems to make nutrients temporarily more available, and it may also allow greater proliferation of some helpful soil organisms. The positive effects of solarization last for two to three years.

Soil solarization is most effective during hot, sunny weather. Cloud cover, fog, and wind are detrimental to the heat-capturing process. Heavy rains are also a handicap, because standing water cools the tarp and must be swept off. Begin solarization when you expect four to six weeks of clear, warm weather. Mid-July is probably the best time to start, although local conditions may allow success at other times between May and September.

Steps to Solarizing

The area to be solarized should be at least 6 by 9 feet, since smaller areas will lose some heat, especially at the edges. The soil will heat more uniformly if you weed the area, till it well, and break up any large clods. Fertilize as if you were about to plant. If you intend to water with a buried soaker hose or a drip irrigation system, install it now. Avoid air pockets between the soil and the plastic by raking the surface of the ground so that it is level and smooth.

Water the area to be solarized, using your irrigation system or a sprinkler. Water for an hour or two, or until you are sure that the moisture extends at least 2 feet deep; then water again two days later. This double watering encourages some of the disease spores to germinate. It also ensures that the soil will be moist before you put on the tarp. Moist heat is more lethal to soil pests than dry heat.

Although 1-mil polyethylene tarp is the most effective in building up heat, it is easily punctured by handling and strong winds. For a more durable tarp, use 1-1/2 to 2-mil plastic. Ultraviolet-stabilized polyethylene will last the longest; unstabilized types will begin to break down and crumble in six to eight weeks. Spread the plastic over the prepared area and tuck all the edges firmly into the soil. To cover a large area, join tarps together with transparent tape or duct tape, or glue them with a long-lasting, heat-resistant glue. (The tape is also handy for mending the tarp if it tears during solarization.)

If it rains while you are solarizing, sweep any standing water off the tarp, since it will reduce the heating of the soil. Remove the tarp when you are ready to plant. If the soil is too moist for planting when you uncover it, let it dry for a few days.

Be careful not to undo what you have accomplished. Don’t bring soil from untreated areas into the solarized area. Mulch the paths to the treated area with wood chips or sawdust, so that you won’t track contaminated soil into the clean area. Don’t transplant seedlings from untreated seedbeds into the treated area. Also, avoid disturbing the solarized soil as much as possible; try to plant within the top 2 inches.