Soil Treatments to Control Soilborne Pests

The objective of soil treatment is to reduce the numbers of weeds, insects, and disease-causing organisms in the soil and thereby promote plant vigor, increase yield and quality, and decrease the need to use pesticides later in the production cycle. Soil treatments complement good disease control but do not replace the use of disease-free seed, cuttings, and transplants nor lessen the need for proper sanitation practices.

Timing

The best time to fumigate is in the fall while soil temperatures are above 55 degrees F (13 degrees C) at a 6 inch (15.2 cm) depth and soil moisture is 50 to 85% of field capacity. Cold, wet soil reduces the effectiveness of fumigants and greatly increases the time required for the chemical to escape. Dry soils allow the fumigant to escape too rapidly. Pests in dry, cold soil are usually in a resting state, which increases resistance to fumigants.

Treatment Selection

The best soil treatment is one which fits into the grower's or landscaper's overall operation, has been selected to control the target pest, and which can be applied safely and effectively, leaving no residues toxic to the crop. See Crop Sensitivity to Soil Fumigants. Each material has recommended uses printed on its label and warns of possible damage to certain crops. Always read the label completely, particularly if herbicides or fertilizers are to be used on the same soil.

Treatments must be carried out by applicators licensed in category. Almost all chemical fumigants are restricted use pesticides and must be applied by a certified applicator.

Application

Each material is labeled with recommended dosages and methods of application. The use of rates lower than recommended will result in poor pest control. The higher doses are employed in soils high in clay or organic matter or in soils heavily infested with the pest.

General Procedure in Plant Beds

In the fall, disk the soil and allow some time for the organic debris to rot. Till the soil 7 to 10 days before treating. Soil moisture should be 50 to 85% of field capacity and soil temperature above 55 degrees F (13 degrees C) at a 6 inch depth (15.2 cm). Remove about a pint of soil at this time to be used in later testing for residual fumigants. See the "Lettuce Seed Test" below.

Use all the recommended safety equipment and procedures and do not work alone. Apply the fumigant as directed, sealing the surface with a plastic cover (bury the edges 4 to 6 inches) by watering, rolling, or dragging. Do not inject fumigants below the depth to which the soil has been prepared. Soil below the level of injection or outside the sealed surface will not be fumigated. The surface must remain sealed for a period specified on the label.

After the fumigation period, remove the cover. Tilling the soil the next day aids in aerating the soil. Do not recontaminate the soil with infested soil on equipment (Read the Recontamination section below). Moisture, temperature, and the material used determines the length of the waiting period before planting. If deep rooting trees or shrubs are to be planted, dig the planting hole and allow it to aerate for 3 to 6 months. All the fumigant must be gone before planting. See the "Lettuce Seed Test" below.


General Procedures for Potting Soil

Remove about a pint of soil at this time to be used in later testing for residual fumigants. See the "Lettuce Seed Test" below.

Treatment must be done in a well ventilated area. Soil should be on a surface which will allow the soil to be moved without recontaminating it with untreated soil (on a wooden, metal, asphalt or concrete surface or plastic tarp). The layer of soil must be no deeper than 6 inches (15.2 cm) at any point. Chemical fumigants must not be used near valuable plants.

Use all suggested safety equipment and procedures. Do not work alone. A mistake could cost you your life. Apply the material and seal the soil surface as recommended on the label. Following the specified fumigation period, remove the surface seal and aerate soil until no trace of the fumigant remains.

Lettuce Seed Test for Traces of Soil Fumigants

(Adapted from Plant Pathology Information Note No. 160. C. W. Averre, J. C. Wells, and S. F. Jenkins. Dept. of Plant Pathology, North Carolina State University.)

If soil is treated with a chemical fumigant to control weeds, insects, and disease-causing organisms, all traces of the fumigant must be gone before planting. The time required for the chemical to completely leave the soil depends on the chemical used, the soil temperature and moisture, depth of application, soil type, and the amount the soil has been worked after fumigation.

The applicator can perform the following test himself to be certain it is safe to plant in the fumigated soil:

Remove 5 to 10 small samples of soil which had been treated with the fumigant, including some from the deepest point of injection. Immediately, place a small portion of this sample in an airtight jar so that the fumes do not escape. Any jar with an airtight lid will work (mason jar, jelly jar, peanut butter jar). Sample the treated soil adequately.

In a similar jar place some soil that was not treated. This is an untreated check to be compared with the first jar.

Moisten the soil in both jars if necessary for germination and sprinkle lettuce seeds on the soil surface.

Close the jar tightly, being certain there is no grit under the seal.

Since lettuce seed usually requires light to germinate, place the jars in a bright spot where it will be above 65 degrees F. Do not place the jars in direct sunlight because they may overheat.

If the germination is good in both jars, it is safe to plant in the treated soil. If germination is good in the untreated soil jar and poor in the treated soil jar, allow the soil to aerate longer and repeat the test later. Only plant when the germination is about the same in both jars.

Recontamination

Pests inadvertently added to treated soil have no competition and may cause more damage than they normally could in untreated soil. Disinfest all tools, flats, and equipment that will be used to handle the treated soil. Do not walk or drive on treated soil.

Crop Sensitivity to Soil Fumigants

Some crops are very sensitive to certain chemicals and cannot be grown successfully in soil treated with those materials. The following is a summary of information taken from product labels, state bulletins, and the scientific literature.

SOIL TREATMENTS

Consult product labels for recommended rates, methods of application, and proper safety measures to follow. Most fumigants temporarily raise the level of ammonia and soluble salts in the treated soil. Proper aeration and, sometimes, leaching will restore the proper balance of elements.

Material	Target Pests	Comments
Steam	W/I/N/F/B	Heat to 180-200 degrees F and hold for 30 minutes.
Aerated steam	W/I/N/F/B	Heat all soil to 165 degrees F and hold for 30 minutes.
Dry heat	W/I/N/F/B	Heat all soil to 180-200 degrees F and hold for 30 minutes.
Sodium methyl dithiocarbamate (Vapam*)	W/I/N/F	Do not use where fumes may contact valuable plants.

Target pests: W = Weeds; I = Insects; N = Nematodes; F = Fungi; B = Bacteria

*Trade name