Fumigation: Materials and Risks

The practice of soil fumigation carries significant risks. These risks include the health and safety of agricultural workers and others who can be exposed to these materials, environmental risks from misapplication or accidents and other hazards.  Another risk is treatment failure from reintroducing pathogens on transplant material or farm equipment. This can cause a phenomenon called “the boomerang effect” in which a pathogen is (re)introduced in a partially sterilized soil and proliferates rapidly because checks and balances no longer exist in that soil. In such a case, the resulting epidemic is worse than if the soil had never been fumigated. So, it is very important to take care to plant very clean transplant material and to use only clean equipment when working in a newly fumigated field.

Fumigation is also a costly practice, one which a grower must carefully consider before using. The cost must be justified by the anticipated benefits. The benefits must be reliable and predictable. Moreover, availability of fumigants may decline in the future due to EPA restrictions and voluntary withdrawal by manufacturers. With this in mind, it is advisable to implement effective crop rotation plans and other soil management practices in anticipation of reduced availability of fumigants.

New Regulations Concerning Fumigation

Fumigants are very biologically active and produce gases that can readily move off site, so they can also be very dangerous to people and other organisms in the surrounding environment. Labels for most soil fumigants were extensively revised in 2011 to require additional steps of fumigant applicators (called risk mitigation measures) to safeguard the general public, the applicators and handlers, and the environment. These requirements are discussed on current fumigant labels. Reading and understanding the new soil fumigant labels is critically important. Additional revisions to these new soil fumigant labels are being developed, further increasing the importance for growers to study the labels and visit EPA’s website. Key changes already in effect include the following:

• All chemical fumigants are now “restricted use.” Previously, metam sodium, metam potassium, and dazomet were considered general use materials.
• Soil fumigant applicators must write a fumigant management plan (FMP) that outlines how the application will be made and describes plans to address problems should any arise. Custom applicators must provide growers with a copy of the FMP, which must be maintained for a period of two years.
• Practices previously recommended to improve efficacy and reduce off-gassing are now requirements, such as proper calibration, soil tillage before application, fumigating when soil temperature and moisture levels are within the proper ranges, etc.
• Maximum application rates are being reduced in some instances and untarped applications for some materials can no longer be made.
• Respiratory protection requirements for those involved with fumigant application or tarp perforation or removal have been significantly expanded and include medical evaluation and fit-testing for respirator use.
• The 48-hour “reentry interval” following soil fumigant application has been changed into a 5-day “entry restricted period.”
• Unless trained/licensed by their state for fumigation (fumigant category), people must now take EPA Fumigant Training.  See https://www.epa.gov/soil-fumigants/soil-fumigant-training-certified-applicators.

Additional new label requirements include the establishment and posting of restricted-entry buffer zones around application sites, among other changes.  Updates and templates of required forms are available on EPA’s website at https://www.epa.gov/soil-fumigants. 

Site Preparation For Chemical Fumigation and Treatment Guidelines

Fumigation must be done carefully in order to be effective. Soil fumigation treatments should be planned well in advance so that the site can be prepared properly. Several rules apply to most treatments.

1. Prepare the soil by deep plowing followed by disking. The purpose is to loosen the soil throughout what will be the crop rooting zone and to thoroughly incorporate all plant residues. Do this at least 3 weeks in advance of fumigation so that buried plant residues begin decomposing. Remove all woody or bulky accumulations of plant residues and large rocks from the site. These will foul the chisel applicators, decrease the effectiveness of the job, increase the hazard to workers who must clear them, and can cause a custom applicator to legally withdraw from a contract job.
2. If the soil is dry one week prior to treatment, thoroughly wet down the soil to at least 6 inches deep by sprinkler irrigation. Do not attempt to fumigate soil that is too wet or too dry. At the 6- to 8-inch level, a handful of soil should not clump tightly when squeezed, but it should have enough moisture to feel cool in the hand and remain in a loose clump when it is released. Soil that feels warm to the touch or that is crumbly and dusty is too dry. Some moisture in the soil encourages weed seed germination and is necessary for the fumigant to kill nematodes and fungi. Soil that is saturated will limit movement of fumigants through soil so that some of the soil to be treated may not be exposed to the product.
3. Soil temperatures at the time of treatment should be above 40°F at the 5- to 6-inch depth to allow for adequate volatilization of the fumigant but below 80°F to avoid too rapid an escape of the chemical. Optimal soil temperatures vary among different fumigants. Consult the label for the fumigant you are using for its specific temperature requirements.
4. Chisel fumigants in at least 10 to 12 inches deep with the shanks set 8 to 12 inches apart for broadcast treatments over the whole planting site. Because strip or row fumigation only treats a portion of the field, less chemical is used per field acre, and it is useful for annual strawberry production systems. However, this practice is not recommended for perennial systems where treated areas could be recolonized over time.
5. Soil should be sealed as stated on the product label. Leave treated sites undisturbed for at least 5 to 7 days.
6. Aerate treated sites to allow any residual fumigant and ammonia (a temporary side effect of fumigation) to escape before planting. Aeration times vary with the type of material used, soil type, temperature, and moisture level. Check the label for details. At least 14 to 21 days should pass between the application of most soil fumigants and the time a crop is planted. Details are available on the manufacturer’s label. A simple lettuce quick test can be done to determine whether planting in fumigated soil is safe. Collect a soil sample from the treated field (do not go below the treated depth). Place the sample in a glass jar with a screw-on lid. Firmly press numerous seeds of a small-seeded vegetable crop (lettuce, radish, etc.) on top of the soil (moisten if necessary) and tighten the lid securely. Repeat the process in another jar with nonfumigated soil to serve as a check. Observe the jars within 1 to 2 days. If the seeds have germinated, planting in the field is safe. If the seeds have not germinated in the fumigated sample and have germinated in the nontreated sample, then the field is not safe to plant. Wait and retest.
7. Fumigation kills most weed seeds, but it can also stimulate the germination of some species, such as Carolina geranium, velvetleaf, and morning glory. Use of chloropicrin has been shown to stimulate yellow and purple nutsedge emergence. Treat these problem weeds with herbicides before they become established.
8. For annual plasticulture strawberries, fumigation must be completed at least 21 days before planting. The optimal planting date varies widely within the region and also depends on plant type used. Thus, fumigation may need to be completed as early as early summer in cooler areas of New England when using dormant plants or as late as early fall for warmer areas of Virginia when using plug plants. The best timing for fumigation is early fall if planting matted-row strawberries, brambles, or blueberries in the spring, as soil conditions that satisfy the specific temperature and moisture requirements of fumigants are more likely to exist in the fall. The usually wet and often prolonged cool spring conditions in the region often cause delays in fumigation attempts in the spring. If fumigation is done in the fall prior to spring planting, a winter cover crop of small grains or a permanent between-row sod cover can be seeded after aeration.
9. Make sure to plant disease-free crops and use good management practices to avoid reintroducing pathogens.