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Get the Most From Your Fumigant!

As a Soil Management Specialist, fumigation is not my first choice as a soil management tool. However it is often necessary for the successful establishment of crops, particularly perennial crops. Fumigation is expensive and disrupts the soil ecology of a field so it is important to get the most out of the process when it is used.

Moist but not wet soil is critical for optimum fumigation activity. Dry soils have little water surrounding soil particles, really just thin films of tightly held water. More of the fumigant material will be adsorbed to the soil particles while in moist soils this adsorption is reduced. Wet soils have more pores filled with water which will block and slow the movement of the fumigant through the soil pore matrix. This can particularly interfere with fumigant penetration to depth as soils usually get wetter with depth. Check the soil moisture at the depth of application before applying. Optimum soil moisture for fumigant movement is usually described as -0.6 to -15 bars of water potential or half of field capacity. Another way to describe it would be the moisture level at which the soil would be fit to plant.

Soil structure plays a critical part in determining how well the fumigant moves and disperses in the soil and stays in the soil. Fumigants are applied to the soil and the materials generally vaporize and move through a process called diffusion from an area of greater concentration - the application zone, outward through soil pores. The general movement of the fumigant is up. Gravity does not play a significant role in pulling the material down into the soil profile. So the process relies upon connected pores that are not filled with water. Recently tilled soil is usually recommended for fumigant application to ensure consistency and to create a larger number of large pores. Compaction and other types of structural degradation have a direct effect on fumigation because of the impact on the number and size of soil pores. As soils compact or a compacted layer forms, pores are squished or reduced in size. Smaller pores are more likely to hold water. The squished pores may lose the connectedness; basically the route through the soil pores becomes more difficult, even for a vapour like fumigant. This has a direct impact on consistent pest control through soil profile.

Each fumigant material has a slightly different response to the soil conditions. It is important to read fully the product label before application to ensure that rates, timing, application and post application field management is correct.

Whether you use in-row or broadcast fumigation, after all the work and expense of fumigation, take steps to slow the re-infestation of the field. Wash tillage and planting equipment before field entry to remove any soil or crop residues that could be carrying soil pests.