Annalisa Hultberg, extension educator, food safety
Animal-based soil amendments such as composted manure and poultry litter can build the health, tilth, fertility and water hold capacity of your soil. They can also be a great way to use resources you might have on the farm such as manure. However, all animal-based soil amendments, especially those that include untreated (raw) manure pose microbial risks and should be used safely to reduce the potential for causing illness. Here are some guidelines to help you minimize any potential risk of contamination and foodborne illness as you use these soil amendments this season.
What are the risks with animal-based soil amendments?
All animal-based amendments carry a risk of microbial contamination, though many factors affect the level of risk in each. Different animals tend to be reservoirs for different pathogens. For example, poultry like chickens and turkey often shed Salmonella and Campylobacter and ruminants (cows and sheep) often shed toxigenic E. coli (STEC). It is not possible to know if an animal is shedding pathogenic bacteria in its feces by looking at the animal or observing its behavior. Therefore it is important to take care with all biological soil amendments, particularly raw (untreated) manure.
Treated Animal-Based Soil Amendments
First, it is important to think about the process for how the compost was made. Products like Sustane or other fully-treated compost that are purchased from a supplier and come with a certificate of conformance or an OMRI label have likely been processed using adequate turning and temperatures to reduce potential pathogens to very low levels.
Because these products have likely been treated they can be added to the soil at any time and do not need to have any application to harvest intervals, because they represent a very low risk to food safety. (Still take care with handling and storage to ensure the product does not get recontaminated in storage by animals or dirty equipment). Since all animal-based soil amendments carry some risk, it is still important to minimize contact between the crop and the compost, so do you best to keep these amendments from touching the edible portion of the crop.
Untreated Animal Based Soil Amendments - higher risk
However if the compost has not be treated using a validated treatment process such as those outlined in the NOP guidelines for compost making and it has simply aged in a pile, or if you got the manure from a neighbor who didn't use a validated treatment process or if you are unsure how the manure was treated, you must use caution when using this product.
The best practice when using compost that has not been fully treated is to follow the National Organic Program guidelines for "day to harvest" intervals. Wait 90 days between application and harvest if the produce is not grown in contact with the soil, and 120 days if the product is grown in contact with the soil. This will reduce the potential for contamination significantly, because if there are pathogenic microbes present in the compost the time interval will allow time for many to die off. Some might persist even after this time period though, so still continue to use caution and apply the compost so that it doesn't directly touch the edible portion of the crop.
On many farms the easiest way to follow the "days to harvest" intervals is to apply raw product in the fall months. This ensures adequate time in between application and harvest. If you want to apply raw, untreated product yet this spring, ensure that you keep good records and follow the "days to harvest" intervals as outlined above.
Storage and Handling
In your food safety plan, describe what kind of animal-based compost you use, where it is stored, and how and when it is applied. Keep the certificates of compliance from your supplier if you are purchasing fully-treated product from a supplier. Keep records to indicate when you applied the manure and to what fields. This will also be useful so that you can track fertility and determine how application is affecting your plant's health and growth.
For more information on compost and application, see previous post here from the manure management team at the University of Minnesota.
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