Soil Pasteurization, Fumigation, and Solarization
Soil Fumigation
Soil fumigation is a chemical control strategy used independently or in conjunction with cultural and physical control methods to reduce populations of soil organisms. Soil fumigants can effectively control soil-borne organisms, such as nematodes, fungi, bacteria, insects, weed seeds, and weeds. Different fumigants have varying effects on the control of these pests. Some are pest-specific, while others are broad spectrum biocides and kill most soil organisms. Because of treatment costs, applicators use soil fumigants primarily on high value crops, such as vegetables, fruits, and ornamentals. Very careful attention must be paid to the details of how the fumigation is done in order to guarantee excellent control of the targeted organisms and complete safety for workers and other persons close to the treated area, while also limiting the impact the product could have on our environment.
Types of Soil Fumigants
Soil fumigation uses pesticide formulations that volatilize from a liquid or solid into a gas state. Soil fumigants are applied to the soil as liquefied gases, volatile liquids, or granules. Due to the high volatility of these compounds, the fumigant must be incorporated into the soil during or immediately following application. At or shortly after application, these chemicals volatilize, allowing toxic molecules to move through the air pores in the soil. Soil pests are killed when they come in contact with a toxic concentration for a long enough exposure period. For all fumigants, enough concentration and contact time with target pests are required to obtain good results.
Methyl Bromide
Methyl bromide is a broad-spectrum fumigant that controls many weeds and soil-borne insects, nematodes, fungi, and bacteria. However, it does not adequately control all species. Methyl bromide is toxic to all stages of insect life. It is registered for use on a variety of crops, including ornamentals, vineyards, deciduous fruit and nuts, nursery sites, greenhouse soils, peppers, tomatoes, and strawberries.
Methyl Bromide and Chloropicrin Mixtures
Proprietary materials are available that contain both methyl bromide and chloropicrin. Such combinations are more effective than either material alone in controlling weeds, insects, nematodes, and soil-borne pathogens.
Metam Sodium
Metam sodium is recommended as a pre-planting treatment to control soil-borne pests that attack ornamentals and other crops. Do not apply to crops. It controls soil-borne fungal diseases (e.g., Fusarium, Pythium, Phytophthora, Sclerotinia, oak root fungus, Verticillium, clubroot of crucifers, and Rhizoctonia), nematodes, symphylids, and germinating weed seeds of annual grasses, chickweed, dandelion, ragweed, henbit, lamb’s-quarters, pigweed, purslane and suppression of perennial weeds such as quackgrass.
Chloropicrin
Chloropicrin is a broad-spectrum fumigant that controls some soil-borne insects, fungi, and bacteria. It provides limited control of some weed seeds and nematodes. Although chloropicrin is often added to other fumigants in low concentrations as a warning agent, it is also added at higher concentrations (up to 75%) to increase the overall spectrum of pest control. Chloropicrin is often formulated with methyl bromide, iodomethane, DMDS, and 1,3-D. It may be formulated as the sole active ingredient.
Methyl Iodide
The effectiveness of methyl iodide is similar to methyl bromide, rendering it a potential replacement. Methyl iodide offers broad-spectrum activity like methyl bromide, but there is not the concern on impacts to the ozone layer.
Dimethyl Disulfide
DMDS is toxic to some weeds, soilborne nematodes, bacteria, and fungi. DMDS is a widespread natural product and is labeled for use on vegetables (tomatoes, peppers, eggplants), cucurbit crops (cucumber, squash and melons), strawberries, blueberries, and field-grown ornamentals.
1,3-Dichloropropene
1,3-dichloropropene (1,3-D) provides nematode control, but does not provide broad spectrum weed control. The “C” formulations include chloropicrin for pathogen control.
Factors Influencing Soil Fumigation
Many factors affect soil fumigation and its effectiveness for pest control. The pest and its habits will affect fumigant selection, application rate, fumigant placement, and necessary length of exposure. Soil factors also play a key role in fumigation. Soil texture, soil condition, debris, soil moisture, and soil temperature may affect the volatility, movement, and availability of the fumigant once applied. Fumigant dosage is both pest- and soil-dependent. The following section discusses some of these factors in greater detail. After fumigation, aeration is important to make sure phytotoxicity does not occur.
Soil Texture
Soil texture influences fumigant movement and availability due to its effects on the amount of soil pore space (air spaces) and the number of adsorption (binding) sites. Fine-textured soils, such as clay, have many adsorption sites per unit area and many pore spaces.
Soil Tilth
Soil tilth is the physical condition of soil. It usually relates to the suitability of soil for planting or growing a crop. Factors include clods, moisture content, degree of aeration, rate of water infiltration, and drainage. The tilth of a soil can change rapidly, depending on environmental factors such as changes in moisture.
Soil Moisture
Soil moisture impacts the movement of a fumigant through the soil and off-gassing into the air. Improper soil moisture at the time of application can lead to poor control of target pests and could result in off-gassing. Soil moisture requirements vary depending on the fumigant. Additionally, these requirements may vary depending on a variety of factors, including soil texture, application method, and application depth.
Soil Temperature
Soil temperature correlates directly with fumigant volatility and movement. Soil temperature determines the fumigant state (solid, liquid or gas). As temperatures increase, fumigant volatility and diffusion increase.
Application Rates
Application rates depend on several factors. Higher pest densities and targeting multiple pest species may require using higher fumigant application rates noted on the label. Furthermore, certain difficult-to-control pests and those with high population densities require using the higher rates. Pest location is also important. For example, 1,3-D rates differ depending on whether the target pest is an insect, fungus, bacterium, or nematode, and even by species of organism.
Application Methods and Soil Sealing
Fumigants can be applied to soil in many different ways. The diverse chemical characteristics of soil fumigants largely determine how the products are applied. However, the application method is also determined by it formulation, the target pest, the cost and the area or site to be fumigated. (For example, fumigating a greenhouse soil versus fumigating a mound of potting soil.)
Shank or Spray Blade Application
For shank soil injection applications, knifelike blades called shanks or chisels are mounted vertically on a toolbar behind a tractor and pulled through the soil to deliver the fumigant. A tube carrying the fumigant runs down the back of each shank. Fumigant travels from the tank to the tubing through a pressurized system. Shank traces (the grooves the shanks make in the soil) are covered with soil.
Chemigation Application
Several fumigants can be applied through irrigation systems; however, some fumigants restrict their use only to drip irrigation (chloropicrin, iodomethane, DMDS, 1,3-D). To fumigate soil by chemigation, meter and inject a liquid fumigant into irrigation water. Fumigant chemigation is applied through several types of irrigation systems. Equipment includes an injection pump and nurse tank system.
Hot Gas Application
The hot gas, no-till application method is used for methyl bromide and chloropicrin mixtures. Heat the fumigant by passing it though a heat exchanger. Then, deliver it to the soil surface through a system of tubing or piping.
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