While many growers are accustomed to fumigating at this time of year with no need for supplementary water, 2015 is a bit unusual. The winter of 2015 was drier than normal for most of southern Ontario, with most regions experiencing less than 80% of average precipitation from January through March. Additionally, the very cold temperatures, sunshine and dry air means that much of the snow pack sublimated instead of melting into the ground. As a result, southwestern Ontario and eastern Ontario are currently listed as abnormally dry.
At the Simcoe Research Station, soil moisture measurements taken on a recently-prepared sandy loam field yesterday were at 15% of available water capacity, which is much lower than the levels stipulated on fumigant labels. For metam sodium-based fumigants (e.g. Busan and Vapam), moisture levels in the top 15 cm of soil must be between 60-80% of available water capacity immediately prior to fumigant application. For chloropicrin-based fumigants (e.g. Chloropicrin 100, Pic Plus), soil moisture at a 20 cm depth must be equal or greater than 50% available water capacity. Fumigation in most areas of southwestern Ontario at this time will likely require adjustments to soil moisture prior to application.
Soil moisture recommendations on fumigant labels are not new, however they are now one of the mandatory Good Agricultural Practices required on the new fumigant labels and both soil moisture and method of measurement must be included in your Fumigant Management Plan. While this is now a legal requirement, ensuring adequate soil moisture is also critical to the effective application of fumigants. If soil is not adequately moist at the time of application, the fumigant will move too quickly out of the soil. Since the efficacy of fumigants depends on the length of time the active ingredient is in contact with the target pest, you need to make sure the soil is sufficiently moist to keep the product in the soil. Furthermore, fumigant gas escaping from the soil prematurely can be a hazard, particularly when occurring in association with temperature inversions.
It is also important to note that one or two rainfalls may not be sufficient to bring soil moisture up to 50 or 60% available water capacity, particularly if the soil is very dry. Even if a heavy rain has occurred recently, check your soil moisture prior to fumigant application. For more information on monitoring soil moisture levels, refer to the OMAFRA factsheet Monitoring Soil Moisture to Improve Irrigation Decisions (http://www.omafra.gov.on.ca/english/engineer/facts/11-037.htm ) or view a new series of videos on the subject on the OMAFRA website. To find them, use Google to search for “irrigation OMAFRA”.