If there were one sure way to destroy virtually every kind of harmful insect egg and larvae in your garden soil, would you be interested? How about if the process were easy, cheap, and carried a host of other benefits along with it? Solarization is a simple, five-step process that kills insects, plant diseases, nematodes, harmful fungi, and weed seeds. At the same time, helpful microorganisms within the soil apparently benefit, possibly form the lack of competition. Soil that has been solarized allows plants to draw on the nutrients, especially nitrogen, calcium, and magnesium more readily. Seeds germinate more quickly. Plants grow faster and stronger, often maturing earlier with substantially higher yields than in unsolarized soil.
Solarization works in the same way as a greenhouse, where a transparent covering, in this case 3 or 6 mm plastic sheeting, traps the sun's heat. After several days of sunshine, soil temperatures rise to as high as 140 degrees at the surface and well offer 100 degrees as far down as 18 inches. It takes four to six weeks of sunny weather to pasteurize the soil. For most of the USA, that means planning to spread plastic somewhere between the end of June and the first of September.
Any size plot, down to a 3-foot wide bed, will retain enough heat to do the job, although the larger the area, the more heat is generated and maintained, and the longer lasting the effects. It's easier to lay plastic down in a narrow strip than a wide patch. so that is a major consideration. And therein lies the solitary expense in using the sun's energy to improve your soil: a strip or roll of clear plastic large enough to cover your area and overlap on all sides by at least a foot.
Five Steps to Healthier Soil
- Prepare the Soil. Pull any weeds or old crops. Turn in any soil amendments and then rake the surface smooth. It's important to remove any stones or clumps that might raise the plastic and create air pockets that could cause uneven heating.
- Water thoroughly. Leave a sprinkler on for several hours or overnight to soak the soil. This creates 100 percent humidity under the plastic, which acts with the heat to kill all those unwanted critters.
- Dig a trench all around the bed or plot 6-8 inches deep.
- Lay a clear plastic sheet, 3-6 mm thick, over the area, overlapping the trench on all sides. Fill in the trench, weighing down the plastic while pulling it as tight as possible.
- Sit back, relax, and wait. Although cloudy weather will slow things down by cooling the soil under the tarp, a few weeks of sunshine will improve your soil dramatically, easily, and inexpensively. If you live in an area with cool or cloudy summers, or if you just don't want to wait all season, you can speed up the process by adding a second sheet of plastic. Using the hoops commonly used to elevate row covers or bird netting, raise the second sheet of plastic over the ground-level sheet. The airspace between acts as a temperature buffer zone during cloudy weather and the combination of the two sheets of plastic serves to raise the soil temperature an additional 6 degrees.