Sean Westerveld, Janice LeBoeuf, Michael Celetti, Jason Deveau, Melanie Filotas, OMAF and MRA
From ONvegetables in The Grower, August 2013
For many vegetables and other high value crops, fumigation is a viable option for controlling soil-borne pathogens that are difficult to control using other management strategies. Fumigation is used to control or suppress plant parasitic nematodes, soil-borne pathogenic fungi and bacteria, soil-inhabiting insects, and weed seeds. However, the cost of fumigation can range from a few hundred dollars per acre for banded applications to a few thousand dollars for tarped and broadcast applications. With such a high cost, it is essential to ensure proper fumigation or a significant investment can be wasted.
Currently registered fumigants require careful attention to application procedures listed on the product labels in order to be effective. The available fumigants control different spectrums of organisms in the soil. Growers should find out what research has been conducted on their crop to determine which product is best for their crop and growing conditions.
Before the fumigant can be applied properly, it is important to know how the fumigant moves through the soil. Whether a fumigant is applied as a liquid or a solid, the active ingredients are converted to a gas and move through the soil in the air spaces between soil particles. A study conducted at Ridgetown looked at how far metam-sodium fumigant moved through the soil from the point of injection. The study basically showed that the fumigant rises from the point of injection moving laterally about 7.5 cm (3 in.) in each direction. Working the soil to seed-bed conditions prior to fumigation improved penetration of the fumigant.
There are 6 main things a grower needs to consider to get the most out of their fumigant application. Whether the product is a standard fumigant, a biofumigant, or nematode-suppressive cover crop, these general considerations still apply.
1. Soil Preparation. Preparing the soil before fumigation is very important for maximizing the efficacy of the product. As shown in the research at Ridgetown, working the soil prior to fumigation can result in better penetration and movement of the fumigant in the soil. Any large clods of soil or organic matter will result in large air spaces in the soil, which will allow the fumigant to escape to the surface faster. In addition, fumigants will not penetrate these clods very well and any pathogens contained within the clod may not be controlled by the fumigant. These pathogens will have an opportunity to re-colonize the soil after fumigation because they will no longer have any competition from beneficial soil organisms. Soil should be worked a few weeks or more before fumigation to allow organic matter to be broken down and then again prior to fumigation to break up soil clods.
2. Soil Moisture. Most fumigants work best when soil moisture is at 60-80% of field capacity. Too little soil moisture and the fumigant can move through the soil too quickly, and the soil cannot be sealed properly after fumigation. Too much moisture and the fumigant may not move through the soil, reducing the area treated. In addition, the fumigant may be slower to gas off after fumigation in wet soils, which could interfere with planting of the subsequent crop. If rainfall is not sufficient prior to fumigation, irrigation may be required. While this can be a costly and labour intensive, consider the money wasted on fumigation if it is not effective.
3. Soil Temperature. Most fumigants can be applied at soil temperatures ranging from 4 to 30oC. However, the fumigation may be more difficult to manage or less effective at the high and low end of that range. Soil temperatures above 20oC increase the volatility of the fumigant, meaning it can move through the soil more rapidly. This can lead to more rapid escape of the fumigant from the soil if it is not properly sealed. Soil fumigants should remain in the soil for 7 to 14 days for maximum efficacy, and the soil should be worked after fumigation to allow any remaining fumigant to escape so it will not damage the subsequent crop. Low soil temperatures can result in the fumigant remaining in the soil for longer than 14 days, which can delay planting. Generally soil temperatures around 10 to 20oC are ideal, which generally occurs in early May or late September in southern Ontario.
4. Weather Conditions. Weather conditions at the time of application are most important in determining the potential for escape of the fumigant and drift off of the field. Fumigants should not be applied if a temperature inversion is forecast within the first 48 hours. Temperature inversions occur on clear and calm nights when the soil radiates heat into the atmosphere. Cloud cover radiates heat back towards the surface and a breeze will mix the air preventing an inversion from occurring. In a temperature inversion, any fumigant that escapes the soil can be trapped in the cold layer at the surface and flow into low areas in or around the field. If houses or businesses are situated next to the field, people could be exposed to the fumigant, which can cause significant health effects and potentially significant fines.
5. Sealing. Regardless of the condition of the soil and the product being applied, it is essential to seal the product in the soil immediately after fumigation. Without a proper seal, the fumigant will rapidly escape from the soil and the money spent on fumigation will basically vaporize. With a poor seal, soil pathogens in the top layers of the soil may not be exposed to the fumigant long enough to be killed. At a minimum, a good soil seal is required after fumigation. For broadcast application, this usually involved rolling the soil with a roller that moves faster than the speed of the tractor. For banded applications, beds are often formed behind the injector to seal the product into the soil. Good soil moisture will be essential for forming a proper seal.
Any stubble remaining from an earlier crop can interfere with the seal and allow the fumigant to escape. A small amount of water applied after fumigation (e.g. 5 mm) can provide an even better seal. Tarping is by far the most effective method of sealing the soil after fumigation. Tarping is required for any broadcast applications of chloropicrin fumigants. A special tarp is used that is impenetrable by the fumigant. However, tarping adds significantly to the cost of fumigation, and it may only be practical for crops with a very high value or no viable alternatives.
6. Post-Fumigation Practices. Regardless of the methods used to ensure proper fumigation, it is highly unlikely that the fumigant will be 100% effective. Any organisms located below the injection shanks will not be treated, and banded applications only treat the area immediately around the shank. For broadcast applications, care should be taken to avoid digging up soil from below the treated zone or bringing in untreated soil from outside of the field. For banded applications, growers should ensure the plants are planted in the centre of the treated zone, and soil from outside of the row should not be introduced into the row through post-plant cultivation. Fumigated soil is virtually sterile and will be colonized by the first organisms introduced to the soil. If that first organism is a pathogen, significant disease issues may occur.