Released June 2, 2008

ALBANY, Ore. – During the past decade agricultural scientists all over the world, including Oregon, have been experimenting with a simple, non-chemical means of combating plant diseases and weeds called soil solarization.

Soil solarization is a fancy term for the simple process of putting transparent plastic sheeting over moist tilled soil during the warmest and sunniest months of the year, explained Oregon State University crop scientist Dan McGrath. The clear plastic traps the heat of the sun, causing physical, chemical and biological changes to occur in the soil.

Under plastic sheeting, the top few inches of soil can be as much as 20 degrees warmer than uncovered soil. In this extra-warm environment, disease-causing organisms tend to flounder while more heat-tolerant "beneficial" microbe species increase in numbers. Also, the heat eliminates some kinds of pests, weed seeds and seedlings.

"Solarization is like the cleansing process that takes place in a compost pile," explained McGrath, staff chair of the Linn County office of the OSU Extension Service. "When the soil temperature rises under the plastic, microorganisms that prefer warmer temperatures flourish and attack many of the harmful organisms.”

Scientists term this change in the soil microbial community after solarization as a "microbial shift."

"Warming the soil speeds up organic matter decomposition and makes more nutrients available to plants, explained McGrath. "Agricultural scientists are finding, in some cases, that soil solarization may be a more environmentally friendly alternative to soil fumigants or other fungicides.”

McGrath and colleagues studied the effects of solarization on soils for six years in Oregon's Marion County agricultural fields and at OSU's North Willamette Research and Extension Center in Aurora. They found that solarization helped to reduce weed numbers, but it does not eliminate all weeds. Some weeds, including purslane, are resistant to solarization, he said.

The OSU researchers found that solarization reduced weed populations, disease-causing organisms such as Verticillium wilt, crown gall, root-lesion nematode and Phytophora root rots, to a depth of almost a foot under the soil surface. They found fewer harmful invertebrates and insect pests in the top three to six inches of soil. Solarization also increased populations of warmth-loving beneficial soil organisms.

Solarization is something you can do in your own garden. McGrath suggests trying solarization on one garden bed per year, in a "summer fallow" rotation process, just as farmers do. The success of solarization will depend on the intensity of sunlight, soil moisture, weather and length of time the plastic is left on the soil.

If you live in a foggy, cloudy or windy climate, the process may be less effective as in a hot dry climate – and the plastic might have to be left on longer than in a sunny hot area.

May, June and July are the best months in North America to start a solarizing project because the heating power of the sun is at its peak. If June is cloudy, as it can be in western Oregon, wait until clear weather prevails to solarize. In the Pacific Northwest, it is probably best to keep the plastic on about two months to ensure that enough heat works for adequate time to profoundly affect soil microorganisms, weeds, and weed seedlings, suggests McGrath. In areas farther south, four to six weeks of summer solarization may suffice.

To solarize, you will need a roll of plastic sheeting. Any thickness less than five millimeters will do, as long as the sheeting will cover the area of your bed plus about a foot and a half extra around all sides. Thin plastic works better than thick plastic, according to solarization researchers, and you don¹t need UV-resistant plastic.

Next, remove or mow down the weeds in the area you want to cover with plastic. Remove and compost or dispose of all the loose plant debris from mowing. This is the best time to add any amendments such as lime, compost or fertilizer that you want to have in the soil before post-solarization planting, because the more you stir up the soil, the more you will bring up weed seeds from deeper layers that aren't solarized.

Rototill the soil and amendments. Break up large clods and work in any loose plant debris. Then, rake the surface of the bed as smoothly as possible to ensure close contact of the plastic against the soil. Dig a six- to eight-inch deep trench outside and all around the edge of the bed. Put the clods of soil aside, outside the raised bed.

Once the garden surface is smooth, place a soaker hose or a sprinkler the length of the bed and soak the soil for three or four hours so soil is wetted to a depth of at least a foot. Moist soil responds better to solarization than dry soil, as the water conducts the heat. The moisture tends to "wake up" soil pathogens as well, making them more metabolically active, and more susceptible to the heat.

Finally, cut a piece of plastic the size of the bed, plus an extra foot and a half all around. Put the plastic over the bed and pull it tight and then anchor it snugly by weighing the edges down in the surrounding trench with dirt clods, rocks or bricks.

Next comes the easy part. Wait eight to 10 weeks for the sun to work its magic.

If you take the temperature under the plastic on a hot day during the process and you will be impressed. OSU researchers have measured temperatures as hot as 127 degrees in the top four inches of soil under plastic sheeting.

After eight to 10 weeks, remove the plastic and plant a green manure, overwintering crop or leave the plastic on until spring. Or plant crops and use the plastic sheeting as mulch. Just cut small X’s in the plastic and plant your starts. Take care not to mix up the soil from lower layers, as the any of the effects of the process reach down only about a foot below the surface of the plastic.

Solarization may be used to eliminate pests and pathogens from homemade potting mixes too. Spread moist potting mix in a three- to four-inch layer on a clean asphalt or cement driveway and cover it with clear plastic sheeting and leave for the recommended eight to 10 weeks.

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http://extension.oregonstate.edu/news/story.php?S_No=1037&storyType=garden

Contact: Carol Savonen, 541-737-3380