Is Manure Use on Organic Farms a Food Safety Threat?

One of the most frequent criticisms or concerns I hear expressed about organic food is the suggestion that organic farmers “rely” on animal manures for fertility, so therefore, organic food is at higher risk of microbial contamination. The statement contains a number of assumptions, and I’d like to take some time to explore them.

1) Reliance on Animal Manures
The idea that organic farmers “rely” on manure and/or compost for fertility is a common misconception: there are a wide range of soil amendments available to organic farmers, especially when it comes to high-value crops like fruits and vegetables. Many of these are raw, unprocessed minerals like calcium carbonate (lime), rock phosphates, or potassium sulfate. Others are derived from plant or animal sources (seed meals and fish emulsions are two good examples).

In fact, I’ve developed a fertility plan for my own farm’s vegetable acreage that does not rely on any manure or compost (I am applying the small amount we do generate here to build and maintain fertility on permanent hay ground and fields I may eventually transition to vegetable production.) Furthermore, there are “vegan” farms that guarantee their customers that no animal products are used in the production of food on their farms.

Whether or not we can have a truly sustainable agriculture without incorporating livestock somewhere into the system is certainly a debate worth having, but for the purposes of this blog post, let’s simply acknowledge that organic farmers can produce crops without relying on animal manure. The next question to ask is, “for the sake of food safety, should they?”

2) Managing Manure Risks

To read some of the more radical critics of organic agriculture, you may be led to believe that organic farmers can apply manure to their crops without any regard whatsoever for microbial contamination or food safety – nothing could be further from the truth. There are, in fact, a couple of different manure management strategies in place on organic farms.

The preferred method is composting. Organic standards define compost as, “The product of a carefully managed aerobic process by which non-synthetic materials are digested by microorganisms. Organic materials for compost shall be managed appropriately to reach temperatures for the duration necessary to effectively stabilize nutrients and kill human pathogens.” (from the Canadian Organic Standard)

How this is accomplished is further defined in similar terms in the organic standards for both the Canada and the United States (quoting from the Canadian Standard again):

Compost produced on the farm . . . if made from animal manures or other likely sources of human
pathogens, compost produced on the farm shall

a. reach a temperature of 55°C (130°F) for a period of four consecutive days or
more. The compost piles shall be mixed or managed to ensure that all of the
feedstock heats to the required temperature for the minimum time; or

b. meet limits for acceptable levels (MPN/g total solids) of human pathogens
specified in the Canadian Council for Ministers of the Environment publication
Guidelines for Compost Quality; or

c. be considered as aged or raw manure rather than compost (i.e. meet the
requirements specified in par. 5.5.2.5 of CAN/CGSB‑32.310, Organic
Production Systems — General Principles and Management Standards).

This last statement is particularly important: on most organic farms, it is practically impossible to meet the first of these requirements (which are actually taken from the requirements for commercial composting operations). Although it irks some organic farmers that the efforts they make to compost their manure (and all the associated benefits it brings) are not recognized by the certification bodies when it comes to the timing of application, it is an essential food safety consideration. Therefore, organic standards require that:

5.5.2.5 The non-composted solid or liquid manure shall be

a. incorporated into the soil at least 90 days before the harvesting of crops for human consumption that do not come into contact with soil,

b. incorporated into the soil at least 120 days before the harvesting of crops having an edible part that is directly in contact with the surface of the soil or with soil particles.

Organic farmers are required to keep records documenting manure application, and these records are reviewed during the annual inspection; farms can also be subject to random, unannounced inspections at any time. Interestingly, GAP (Good Agricultural Practices) food safety programs, which are quickly becoming a standard requirement for any grower wishing to supply large grocery chains, have the same 120-day standard for manure application.

3) Evidence of Contamination

I’ve done a fair bit of research and even challenged some critics to prove to me whether any microbial contamination issues in organic food have ever been traced back to manure applied to the growing area. The answer, to the very best of my knowledge, is no. Post-harvest handling appears to be the area of greatest risk, and whether the product is organic or not has little bearing at this stage in the process. However, organic standards require that any water used in post-harvest handling be routinely tested for potability and that these results are shared with the certification body.

 

Food safety and microbial contamination is a concern that every farmer, organic or not, needs to take very seriously, and I’ve never met a farmer who didn’t recognize this fact. As we can see, the standards for certified organic production also directly address these concerns in a manner consistent with recommended best management practices for all agricultural operations, and certification bodies verify and enforce these requirements. The bottom line is clear: consumers can trust the safety of organic produce.