Soil fumigation is an important agricultural technique to control pests in fields infested with plant diseases. Parasitic nematodes are one especially challenging pest that often requires these kinds of measures for its control or eradication. Several different pesticides are commonly used for soil fumigation; one of the most toxic, methyl bromide has at times proved controversial.
Parasitic nematodes--small worm-shaped organisms that live in the soil--can badly damage crops, as can a wide array of insect pests, fungi and bacteria. If the concentration of these pests in the soil exceeds certain levels, the yield will suffer accordingly. If carried out properly, soil fumigation can control or eliminate many of these common pests in the field prior to planting. Most of the pesticides used for fumigation have only limited success in killing weed seeds; methyl bromide is a notable exception.
Some soil types are more amenable to soil fumigation than others; clay soils, for instance, tend to slow the diffusion of the pesticide through the soil and permit some pests to survive unscathed. Excess organic matter like peat or mulch can absorb the pesticide and prevent it from working through the soil. The soil temperature must be over 50 degrees Fahrenheit for fumigation to work properly, although high temperatures can sometimes yield poorer results as well. Fumigation requires moderate soil moisture, although rain the day after fumigation can severely reduce the effect of the treatment.
Methyl bromide is generally considered one of the most effective pesticides for fumigation; it kills a wider range of pests and permits planting sooner after treatment than the alternatives. Other chemicals available include chloropicrin and 1,3 dichloropropene; although not as toxic as methyl bromide, like nearly all chemicals used for soil fumigation, these compounds are also potential carcinogens.
Before fumigating, it's best to ensure the soil is as free of clods as possible; generally it's preferable to plow prior to treatment to achieve these conditions. During fumigation, the pesticide is injected into the soil. Immediately afterward, it's sealed into the soil, either by rolling the field, irrigating or most commonly, covering it with plastic or a tarp. If methyl bromide is chosen as the pesticide, a plastic film seal is used. Inside the soil, the liquid chemical quickly vaporizes and as it seeps through the soil kills nematodes and other pests. The field cannot be planted for at least 3 weeks after treatment.
Methyl bromide and iodide
Methyl bromide is an ozone-depleting chemical according to the EPA, and as such it's regulated under the international Montreal Protocol. The US and some other countries, however, continue to use substantial quantities of methyl bromide under the Critical Use Exemption to the protocol, claiming that no equally effective alternative exists for many farmers. As of 2003, according to an article in The Scientist, methyl bromide remained the 4th most commonly used pesticide in California agriculture. Government agencies like the USDA have invested considerable sums in finding alternatives, however, and methyl bromide began to be phased out in some areas after methyl iodide was approved in 2007 as an alternative. This move has only provoked further controversy since methyl iodide is extremely toxic, according to a June 2nd 2010 article in the San Francisco Gate. Many environmental groups are critical and believe methyl iodide should not see use in agriculture.