Target specific pests. By knowing which insects, weeds or diseases dominate your soil, you'll be able to choose the fumigant best suited to your needs. A few common multipurpose fumigants include chloropicrin, dichloropropene, Telone C-17, Telone C-35, Vapam HL, K-PAM HL and MC 33.
Wait until the soil temperature is between 50 and 80 degrees Fahrenheit at a depth of at least 6 inches. If you attempt to fumigate soil when it is too cool, the fumigant chemicals may vaporize too slowly and fail to kill weeds and disease in the soil. If the temperature is too hot, the chemicals may vaporize too quickly and the application may not last long enough to be effective. Late summer or early fall is usually the best time to fumigate soil, because the long summer days provide plenty of time for the soil to absorb heat, and the upcoming winter season allows plenty of time for the chemicals to break down before spring planting.
Work the soil to the required root depth characteristic of the plants you wish to grow. Break up any clumps in the soil, as these may be too large for chemicals to penetrate. Check to see that all compost and other organic matter have decomposed fully. Remove any old roots, decaying wood or other large pieces.
Squeeze a handful of soil to check for moisture. If the soil forms a ball that breaks apart easily, it contains the proper amount of moisture for fumigation. Soil that cannot form a ball should be watered until it reaches the right moisture level. If the ball of soil does not readily break apart, you'll need to wait for the soil to dry out somewhat before you fumigate.
Apply the fumigant to the soil according to the manufacturer's instructions. Fumigants have a number of application methods, depending on the type of chemical and the needs of the soil. Granulated fumigants work well for small areas such as lawns and home gardens. These are typically distributed using a shaker or seed spreader. In other cases, liquid or gaseous fumigants are poured or sprayed over the surface of the soil. Some large farms and commercial growers use specially designed farm equipment to inject fumigant below the surface of the soil.
Seal the fumigated soil with a tarp, sheet of plastic or layer of water, based on the directions on the package. Sealing the soil ensures that the fumigant remains in the soil for the necessary length of time.
Wait for the chemicals in the soil to dissipate. The duration of waiting times vary widely, depending on the type of fumigant and the application method. While certain treatments affect the soil only for a few days, others remain for several months.
Test a soil sample before you plant the area. In one jar, plant various types of seeds in a layer of treated soil. Plant the same types of seeds in a jar of untreated soil as a control for your soil test. Screw the lids on tightly and wait a few days. If seeds sprouted in the jar with treated soil, the area is safe for planting.