Fumigation

Fumigation, which kills most insects, disease, nematodes, and weeds, may be the most practical solution for a valuable, pest-prone crop. Because fumigation kills by using toxic chemicals, it is important that care be given to each stage of the fumigation process to ensure the safety of the fumigator and the effectiveness of the treatment.

Three commonly used fumigants are metham, dazomet, and methyl bromide. Although metham is a liquid and dazomet is in granular form, the active ingredient in both metham and dazomet is methyl isothiocyanate (commonly referred to as MIT), which is released as a gas after the materials are applied to the soil. Since metham is a liquid, it may be easiest to use on small areas as a liquid drench. For best results, cultivate the soil thoroughly, and irrigate to uniformly moisten the soil. Then apply metham at a rate of 1 pint per 50 square feet of surface area. After application, irrigate the soil with enough water to saturate the surface. As with all common fumigants, you will achieve maximum effectiveness if you cover the treated area with plastic sheeting. The plastic helps ensure that the fumigant remains in the soil long enough to be effective before escaping into the atmosphere. Cultivate the treated area seven days after application to a depth of 2 inches. Do not plant until 14 to 20 days after treatment. If the soil is cold and wet you will have to wait longer. Always refer to the product label for details and precautions.

Since dazomet (Basamid) is a granular material, it is easy to apply with any spreader. Although not mandatory, you will achieve the best results if you cover the treated area with plastic sheeting immediately after incorporation. You can also irrigate or roll the surface to seal it instead of using plastic sheeting. If you choose not to use plastic sheeting, keep the soil surface moist for several days to slow the loss of methyl isothiocyanate from the soil. Again, refer to the product label for details and precautions.

Methyl bromide is a pressurized liquid and becomes a gas when released. Since methyl bromide is highly volatile, you must cover the treated area with plastic and seal all edges with soil before treatment. Methyl bromide is well suited for large acreage. Mechanized units are available that inject the fumigant 6 to 8 inches below the soil surface and cover the entire area with plastic sheeting in one operation. Wait for two weeks after removing plastic film before planting. Methyl bromide is very toxic and should be used only by professional applicators. As always, refer to product label for details and precautions.

Regardless of the fumigant you use, soil preparation is the key to successful sterilization. Soil should be cultivated twice to a depth of 6 to 8 inches: once 7 to 10 days before fumigation and once immediately before fumigation. Tillers and rotavators are excellent for this purpose. At treatment time, the soil should be free of clods and fresh organic debris, moist enough for seed germination, and have a temperature greater than 55deg.F at the 6-inch depth. Since most fumigants are inactivated by high levels of undecomposed organic material (such as roots, stumps, leaves, and grass), remove organic debris or allow it to decompose before fumigation.

If the soil is not properly prepared and free of fresh organic matter, there may be insects, diseases, nematodes, or weeds present that the fumigant will not kill. When using MIT (metham and dazomet) products, soils can be cooler, but longer waiting periods are needed before planting. Although the soil should be moist, wet soils are not sufficiently aerated to allow free movement of the fumigant within the soil and will result in poor pest and disease control. Seven days after fumigation, cultivate the treated area to a depth of 2 inches. Till only 2 to 3 inches deep to prevent weed seeds and other pests from being moved into the previously treated soil. Do not plant until 14 to 20 days after treatment. If the soil is cold and wet you will have to wait longer.

Fall is an excellent time to fumigate because soils are warm and proper moisture levels are easier to attain. Rooting beds and seed beds frequently are used during the cold winter months. If they have not been fumigated in the fall it is very difficult to achieve the required temperature and moisture conditions needed to fumigate in the winter or early spring.

Your county Cooperative Extension Agent is a good source of advice on which fumigation technique would be best for you. If you have never fumigated soil before, have an experienced pesticide applicator help the first time you fumigate. Fumigants are highly toxic chemicals that must be handled properly to be both safe and effective.