Soil Solarization How-To
A key question in organic gardening is how to control weeds, insects and diseases in the garden. First and foremost in organic strategy is building healthy, living soil that is rich in organic matter and teeming with beneficial microbes. This type of environment allows healthy plants to grow and produce with little stress. Unfortunately, weed control and combating pests and soil borne diseases are still issues. Occasionally it is necessary to use physical controls in place of chemical treatment to “clean” the garden bed. A highly effective method for this is soil solarization.
What Is Soil Solarization?
Soil solarization is simply a way to heat the soil for a period of time, in essence sterilizing it. This treatment will severely reduce populations of insects, weed seeds (and perennial weed roots), virus, bacteria and fungus. The only equipment needed is a garden spade, possibly a rototiller—depending on the size of the area—and a piece of clear plastic sheeting (4 mil is a good thickness) large enough to completely cover the area in question. The process is simple:
- In early summer, spade or till the garden bed to a depth of six inches. Remove plant material. Make sure the clumps are broken up.
- Water the soil. Apply enough water to make it moist but not to the point of runoff.
- Cover the area with clear plastic, burying the edges of the plastic in the soil to hold in as much heat as possible. Leave the plastic in place for six to eight weeks.
- Plant something, either a crop to be harvested or a cover crop, that will germinate and grow quickly, after uncovering the bed.
- For container gardens, simply group the containers of soil together in a full sun location, elevated from direct ground contact, and cover with a double layer of clear plastic. Seal the plastic to the ground with blocks or bricks to allow heat to build up. Leave it in place for six to eight weeks.
Growing something immediately after soil solarization is important to avoid re-infestation with weeds and to rebuild organic matter levels. In an area with a long growing season, cowpeas would be a good crop because they not only sprout and grow quickly, but also fix nitrogen. In areas with short summers, this would be a good time to work in well finished compost and plant fall crops that have been started for transplant.
How Does It Work?
For soil solarization to be effective, the soil temperature must reach, and be held at, 140 degrees Fahrenheit. In most of the U.S., July is the best time of year to achieve temperatures in this range. Leaving the plastic in place for several weeks allows this level of heat to penetrate to six inches. Doing so will effectively control the organisms that cause Verticillium wilt, Southern blight, crown gall, Fusarium wilt, Phytophthora root rot, and many other diseases, and the aforementioned weed seeds; as well as nematodes and perennial weeds to a somewhat lesser degree.
Some Negative Effects
The down-side of soil solarization is that the same effect it has against soil pests also works against beneficial microbes. It also significantly reduces organic matter content in the soil. For this reason, soil solarization should be used in the context of a larger soil management program that includes cover cropping, crop rotation and composting, all of which will help mitigate the negative effects.
Try It Yourself
With planning and proper technique, soil solarization will help you up your organic gardening game. The benefits of reduced stress on your plants will leave you with a bountiful harvest of delicious home grown fruits and veggies without resorting to harsh chemicals.