Can I safely use animal-based compost in my field or high tunnel this spring?

 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 can have pathogens that can cause human illness. Here are some guidelines to help you minimize any potential risk of contamination and foodborne illness as you use these products.

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 animal-based soil amendments, particularly raw (untreated) manure.

Fully Treated Animal-Based Soil Amendments = low risk

First, it is important to know how your soil amendment has been treated to determine its relative risk. If it comes from a store or supplier with a label, it is likely very low risk. Products like Sustane, composted poultry manure, feather meal or bone meal that are purchased from a supplier have likely been processed to reduce potential pathogens to very low levels. These are low risk, because the treatment lowers the number of pathogens to an acceptable level.

Fully vegetative compost like culls from the packshed are also likely to be very low risk and do not need to be treated to reduce the pathogen load.

This feathermeal is fully processed and can be used at any time in the fields

Fully treated products or 100% vegetative compost 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 re-contaminated in storage by animals or dirty equipment). 

Untreated Animal Based Soil Amendments or livestock = higher risk

However if the composted manure is:

• Not fully processed and it has simply aged in a pile
• From a neighbor who just scaped the manure out of their barn 
• From a supplier but they cannot tell you if they treated it fully

...then you must be careful when using these amendments in your vegetable fields, as they are much more likely to contain pathogens that can cause illness if ingested. You can use these products, but see below for strategies to reduce risks, since this raw product can contaminate your produce.

How to Safely Use Raw or Untreated Soil Amendments

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 at least 90 days between application of untreated soil amendments 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 pathogens 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 this spring, ensure that you keep good records and always follow the "days to harvest" intervals as outlined above at a minimum. 

More ways to reduce risk

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 how to make manure-based compost that is fully treated, follow these NOP guidelines for compost making.

For more information on compost and application, see previous post here from the manure management team at the University of Minnesota.