Carrots in healthy soil.
image by Jeremy Doorten/sxc.hu
Sterilized soil has none of the insects, weed seeds, fungus or diseases that plague garden plants. It also doesn't have the beneficial insects and bacteria that help break down organic matter into healthy soil. Yet if your garden is plagued by disease or you are trying to start a new bed in an area overgrown with weeds, sterilizing the topsoil is an organic way to solve the issue. Solarization heats the soil to a temperature that sterilizes the topsoil, in this case at least 114 degrees F for a minimum of four weeks.
Till the top 12 inches of the garden bed. Remove any large pieces of organic matter such as mowed-down weeds after tilling.
Water the bed thoroughly so that the water penetrates down at least 12 inches and the entire bed is moist. In dry areas an overnight deep watering might be necessary.
Cover the bed with heavy clear plastic sheeting, such as a painter's tarp. Weigh down the edges with heavy rocks or by staking.
Allow the soil to cook under the plastic for four weeks in summer time and six to eight weeks in spring or fall. Check weekly to ensure the soil has not dried out under the tarp and water if it has.
Remove the plastic after the required time has passed. Work in 1 to 2 inches of compost to add beneficial organisms back into the soil before planting.